ClOVER: Measuring gravitational-waves from Inflation
Executive Summary
The existence of primordial gravitational waves in the Universe is a fundamental prediction of the inflationary cosmological paradigm, and determination of the level of this tensor contribution to primordial fluctuations is a uniquely powerful test of inflationary models.
We propose an experiment called ClOVER (ClObserVER) to measure this tensor contribution via its effect on the geometric properties (the so-called B-mode) of the polarization of the Cosmic Microwave Background (CMB) down to a sensitivity limited by the foreground contamination due to lensing. In order to achieve this sensitivity ClOVER is designed with an unprecedented degree of systematic control, and will be deployed in Antarctica.
The experiment will consist of three independent telescopes, operating at 90, 150 or 220 GHz respectively, and each of which consists of four separate optical assemblies feeding feedhorn arrays arrays of superconducting detectors with phase as well as intensity modulation allowing the measurement of all three Stokes parameters I, Q and U in every pixel.
This project is a combination of the extensive technical expertise and experience of CMB measurements in the Cardiff Instrumentation Group (Gear) and Cavendish Astrophysics Group (Lasenby) in UK, the Rome “La Sapienza” (de Bernardis and Masi) and Milan “Bicocca” (Sironi) CMB groups in Italy, and the Paris College de France Cosmology group (Giraud-Heraud) in France.
This document is based on the proposal submitted to PPARC by the UK groups (and funded with 4.6ML), integrated with additional information on the Dome-C site selected for the operations.
This document has been prepared to obtain an endorsement from the INAF (Istituto Nazionale di Astrofisica) on the scientific quality of the proposed experiment to be operated in the Italian-French base of Dome-C, and to be submitted to the Commissione Scientifica Nazionale Antartica and to the French INSU and IPEV.
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Index
1: Scientific Objective 3 1.1: Cosmic microwave background polarization observables 1.2: Current status of cosmic microwave background polarimetry 1.3 The scientific case for CLOVER 1.4: Foregrounds 1.5: Other Polarization Experiments 2: Technical specification 21 2.1: Overview of the experiment 2.2: Detailed description of key areas 2.2.1: Telescope and mount 2.2.2: Cryogenics 2.2.3: Array and polarimeter 2.3.3: Horns 2.3.4: Orthomode transducers (OMTs) 2.3.5: Waveguide hybrids 2.3.6: Phase shifters 2.3.7: Waveguide twists, transitions and bends 2.3.8: Superconducting detector array 2.4: Systematics 2.5: Scanning strategy 2.6: Site and Logistics 3: New technologies 41 4: Implementation of ClOVER/BRAIN at Dome-C 41 4.1: Introduction 4.2: Buidings/Construction 4.3: Electrical 4.4: Data Storage 4.5: Calibration 4.6: Timing and Personnel 5: Costs to UK 44 6: Costs to Italy and France 44 7: Draft Memorandum of Understanding between UK, France, Italy 45
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1: Scientific objective
1.1: Cosmic microwave background polarization observables
Thomson scattering of anisotropic radiation at last scattering gives rise to linear polarization in the cosmic microwave background (Rees 1968). The polarization signal is expected to have an r.m.s. ~5 mK, peaking at multipoles l~1000, corresponding to the angle ( q~p/l ) subtended by the photon mean free path at last scattering. The polarization signal depends sensitively on the fluctuations on the last scattering surface, and thus encodes a wealth of cosmological information, complementary to that contained in the temperature anisotropies. In addition, large-angle polarization is generated by subsequent re-scattering as the universe reionizes, providing a unique probe of the ionization history at high redshift. Linear polarization, characterised by Stokes parameters Q and U, can be decomposed into a curl-free part (electric; denoted E) and a divergence-free part (magnetic or B; Seljak & Zaldarriaga 1997; Kamionkowski, Kosowsky & Stebbins 1997). The electric and magnetic parts can be represented by their spherical multipoles on the full sky, Elm and Blm, which are related to the Stokes parameters by a spin-2 spherical harmonic expansion:
r m r (Q ± iU )(n) = å(Elm m Blm )m2 Yl (n) l,m Under the assumption of statistical isotropy, different l and m modes are uncorrelated, and the * E polarization power spectra are defined by e.g. Elm Elm = Cl . The cross-correlation between E and B vanishes if parity invariance holds in the mean. Rotationally- and parity-invariant T E B theories thus predict four non-vanishing power spectra, C l , C l , C l and the cross-correlation of TE E and T, the temperature anisotropies, Cl . Examples of pure electric and magnetic polarization patterns are given in Fig.1.1.
The cosmological importance of the E-B decomposition stems from the result that linear, scalar (density) perturbations do not produce magnetic polarization (Seljak & Zaldarriaga 1997; Kamionkowski et al. 1997) since they give rise to a spatial pattern of the polarization field at last scattering that is curl-free. However, a cosmological background of gravitational waves (tensor modes), such as that generated in most models of inflation, produces both E-and 3 B-mode polarization with similar power in each mode. The power is concentrated around l~100 since the amplitude of gravitational waves damps once inside the horizon. The polarization from gravitational waves adds incoherently with that from density perturbations, but the E-B decomposition allows the two contributions to be cleanly disentangled. Vortical motion of the primordial plasma, due to vector modes at last scattering, would also produce B-mode polarization, but vector modes are only expected to be significant in models with active generation of perturbations (e.g. defect models). Such models are definitively ruled out as the main source of structure formation by current CMB temperature anisotropy data. A detection of large-angle, primordial magnetic polarization in the CMB would thus be very strong evidence for a cosmological background of gravitational waves.
An additional source of magnetic polarization arises from weak gravitational lensing of CMB photons by large-scale structure. These deflections re-map the curl-free pattern of polarization from density perturbations, redistributing some E-mode power into B-mode, even in the absence of gravitational waves (Zaldarriaga & Seljak 1998). The various theoretical power spectra of the polarization and temperature anisotropies are shown in Figure 1.2 for the concordance flat, L-CDM model (Spergel et al. 2003) with optical depth through reionization t=0.148. The density fluctuations are adiabatic with spectral index 2 ns=0.96 , and the tensor-to-scalar ratio r =0.15 appropriate to chaotic (f ) inflation.
Only the temperature anisotropy spectrum has been mapped accurately to date, but the E and B TE power spectra, and the lensing contribution to C l , can be predicted with reasonable accuracy from current data.
However, the amplitude of the gravitational wave background is subject only to weak upper bounds by the anisotropy data. Although the amplitude of is not known, its shape is robust since it depends mainly on cosmological parameters that are already well constrained.
1.2: Current status of cosmic microwave background polarimetry
A detection of the polarization of the CMB was first announced in 2002 by the Degree Angular Scale Interferometer team (DASI; Kovac et al. 2002). They report a five-sigma detection of electric polarization, but no evidence for magnetic polarization in their data. These conclusions
4 have been reiforced but not changed significantly by the recently published analysis of 3 years of data from DASI (Leitch et al. 2004). The CBI experiment has also detected E-modes polarization at smaller scales, and with a similar significance (Readhead et al. 2004). At the time of writing the only other published detection is of the TE cross-correlation from the one- year WMAP data (Kogut et al. 2003).
The DASI, CBI and WMAP detections have already had considerable scientific impact:
•The presence of acoustic oscillations in the TE and E power spectra with precisely the phase relation and amplitude to the temperature spectrum that is predicted for passive, adiabatic fluctuations, lends further support to this paradigm for structure formation, and allows strong upper limits to be placed on the amplitude of isocurvature fluctuations (Peiris et al. 2003; Gordon & Lewis 2003).
•The detection of a correlation between polarization and temperature around l~150 is clear evidence for adiabatic, apparently super-horizon fluctuations at last scattering. Inflation is the simplest, causal mechanism for generating such fluctuations.
•The excess power on large scales in the cross-correlation of T and E, seen by WMAP, points to early (and complex) reionization with optical depth t˜0.17 (based on a fit to the TE power spectrum alone). As already noted, B-mode polarization of the CMB is currently subject only to weak upper limits. The best limit on large scales, l~10, is from the POLAR experiment (Keating et al. 2001), while the best limits on smaller scales are from DASI . These (95-per cent) upper limits are shown in Figure 1.4. Although the current upper limit on r from B-mode polarization is much weaker than that inferred from the CMB temperature anisotropies, the former route has the potential to detect much lower levels of r with improvements in instrumentation, but the latter route is limited by the inescapable cosmic variance of the dominant density- perturbation signal in the temperature anisotropies. Indeed, a perfect temperature-anisotropy experiment can only detect r>0.07, even assuming all other parameters are known. Supplementing the temperature anisotropies with electric polarization makes only a modest further improvement. .
5 1.3 The scientific case for CLOVER
1.3.1: Mapping the B-mode power spectrum
The main science goal of CLOVER is to measure the power spectrum of B -mode polarization in the multipole range 20–1000. We aim to make the measurement down to a sensitivity limited by the contamination due to foreground lensing of the E-mode signal for multipoles l<200.
The instrinsic sensitivity of the experiment and the foreground estimates are discussed later, but, in summary, a two-year experiment observing a near-circular survey region of radius 15 degrees results in a thermal noise level after subtraction of foregrounds of 0.24mK to the Stokes parameters Q and U per resolution element (15-arcmin by 15-arcmin). For comparison, the expected r.m.s. of Q and U is 2.1 mK at 15-arcmin resolution; 0.1 mK of this arises from the B- mode polarization generated by lensing, and 0.3 sqrt(r) mK from gravitational waves.
The high signal-to-noise (~9) of the total polarization signal per pixel achieved with ClOVER will be important for assessing the level of systematic errors in the final maps, and also for the lensing science. Given that the lensing B-mode background for l <200 is predicted to have an almost white power spectrum with rms around 1.3 nK, we find that a 15-deg radius survey is sample-variance limited on large scales, irrespective of the level of the gravitational-wave signal. BB We find that the one-sigma error on r, computed from the errors on Cl in the null hypothesis , is Dr =0 0037 . This sets the detection limit of gravitational waves from a measurement of B - mode polarization with CLOVER.
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1.3.2: Constraining inflation with gravitational waves
The inflationary paradigm proposes that the early universe went through a period of (near- )exponential expansion. Inflation is a testable theory, with the simplest models making three central predictions: (i) the universe should be spatially-flat to very high precision; (ii) there should be a nearly scale-invariant spectrum of Gaussian, adiabatic density perturbations; and (iii) there should be a stochastic background of gravitational waves with a nearly scale- invariant (but necessarily not blue) spectrum. The first two predictions have now been impressively verified, most notably with the advent of the WMAP data; the third awaits verification. Detection of gravitational waves would not only be an important verification of the inflationary paradigm, but would also provide unique information on the dynamics of inflation, greatly increasing our ability to select between the many models that have been proposed.
The generation of gravitational waves during inflation arises from the parametric amplification of sub-horizon quantum fluctuations during accelerated expansion. As such, the amplitude of waves produced depends only on the rate of expansion of the universe (i.e. the Hubble parameter H) at the time the wavelength of the wave exceeds the Hubble length during inflation (often referred to as horizon crossing). The power spectrum of gravitational waves is given by (with c=h/2p=1 ) 2 n 16æ H ö æ k ö t P (k) = ç ÷ » A ç ÷ h ç ÷ t ç ÷ p è mPl øh=aH è k* ø where k is comoving wavenumber and mPl is the Planck mass. Since H necessarily decreases only slowly during inflation, the spectrum of gravitational waves is close to scale-invariant (but with nt<=1). In single-field, slow-roll models of inflation, a scalar field f (the inflaton) rolls down a shallow potential V(f) at a rate limited by the expansion of the universe. The Hubble parameter during slow-roll inflation is related to the energy scale of inflation V1/4 by 2 2 H =(8p/3)V/mPl , and hence the gravitational wave amplitude is a direct measure of the energy scale of inflation. In terms of the tensor-to-scalar ratio r=At/As, and of the amplitude As of the ns-1 scalar power spectrum, PR(k)=As(k/k*) where ns is the spectral index, the energy of inflation when the pivot scale k* left the horizon is
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1/ 4 16 1/ 4 -9 1/4 Vs = 3.33´10 r (As / 2.3´10 ) GeV
The current upper limit on r corresponds to an energy 2.6x1016 GeV, i.e. of the order of the GUT scale. CLOVER should detect gravitational waves from inflation if the energy scale exceeds 0.8x1016 GeV , or else constrain the energy to be below this level. It is instructive to compare this detection threshold with what might be achievable by a direct detector, such as the Laser Interferometer Space Antenna (LISA). Extrapolating to LISA frequencies, ~ 10-4 Hz, 2 -16 we find a dimensionless strain Wgw h ~10 . This is some three orders of magnitude below the estimated LISA sensitivity. An ultimate (three-sigma) limit of 0.8x1016 GeV on our ability to measure the energy-scale of inflation from B-mode polarization arises from imperfect subtraction of the lensing-induced B modes (Song & Knox 2003; see Section 1.3.3).
Inflation also produces a nearly-scale-invariant spectrum of density perturbations. The spectrum of the comoving curvature perturbation from slow-roll inflation is 1 æ H 2 ö P (k) = ç ÷ R 2 ç ÷ pmPl è e øh=aH which gives a scalar spectral index dependent on the curvature of the inflaton potential: ns »1- 4e + 2h where 2 m2 æV 'ö e = Pl ç ÷ 16p è V ø and
2 m2 éæV ''ö 1 æV 'ö ù h = Pl êç ÷ - ç ÷ ú
8p ëêè V ø 2 è V ø ûú
The amplitude As of the curvature spectrum thus depends on the energy scale of inflation (via H) and the gradient of the potential. Only by combining with the gravitational wave amplitude At can we separate these two contributions and start to reconstruct the dynamics of inflation. The tensor-to-scalar ratio r=16e, and so satisfies a consistency relation with the tensor spectral index in single-field slow-roll inflation: r=-8nt. Although this is an important test of these models, verifying the relation observationally is very challenging given the fundamental difficulties with determining the tensor spectral index (Song & Knox 2003). The dynamics of slow-roll inflation is characterised by the two small parameters e and h , which relate directly to the observables r and ns . The two-dimensional r-ns plane is thus a powerful means of constraining inflation (see Figure 1.6).
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The r-ns plane naturally divides into three regions: (i) r<8(1-ns)/3, which is inhabited by small- field models of inflation for which V’’'<0; (ii) 8(1-ns/3
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Significantly, Figure 1.7 shows that CLOVER is easily able to classify correctly parabolic small-field inflation as a small-field model, whereas BICEP plus QUaD is just on the verge of ruling out large-field models (although it can rule out hybrid models). There are many small- field models that only CLOVER would be able to classify correctly. The bottom panel of the figure shows further that CLOVER is close to having the sensitivity to distinguish between some typical small-field models at 95-per cent confidence. For potentials of the form V=1- (f/ f*)p with p an integer, an interesting distinction arises between p=2 and p>=3 models. In the former case, slow-roll requires that f* is much greater than mPl, which makes the model unnatural from the viewpoint of fundamental theory, while for p>=3 models, one can consistently take f< mPl, in which case the tensor-to-scalar ratio can be made arbitrarily small as f/mPl goes to 0. For our fiducial p=2 model, CLOVER cannot (quite) rule out models at 95- per cent confidence, but there are many parameter choices in the model (giving a larger r and ns) for which CLOVER would be able to distinguish between them.
Inflation may not be the causal agent that generated the fluctuations that seeded structure formation. Credible theoretical alternatives include the cyclic model (Steinhardt & Turok 2002), where fluctuations are generated during a slow contraction phase of the universe prior to a bounce. The gravitational waves produced in the cyclic model have negligible amplitude on scales relevant to CMB observations (Boyle, Steinhardt & Turok 2003), and so a positive detection from CMB polarization would definitively rule out this model.
10 1.3.3: Gravitational lensing and B-mode polarization
The generation of B-mode polarization by weak-gravitational lensing hinders the detection of the B-mode signal from gravitational waves. For CLOVER this should be the limiting factor. However, the lensing-induced B modes also carry useful cosmological information, and CLOVER will be able to extract this partially. Lensing also affects the temperature and E- mode polarization. Given the resolution Dl~30 that is achievable from the size of survey optimal for detecting gravitational waves, the best way to look for lensing with T and E-mode polarization is not with their power spectra, but to use the non-Gaussian imprints of lensing in the high signal-to-noise polarization images that CLOVER will produce (Hu & Okamoto 2002).
The amplitude and shape of the B-mode power spectrum produced by lensing can already be predicted with reasonable accuracy given current constraints on the cosmological parameters, and the amplitude As of the curvature perturbations. By mapping the B-mode power spectrum up to l~700 (see Figure 1.5), and comparing with the predicted lensing power, CLOVER will make an important check on the concordance of the L-CDM model.
In addition to checking concordance, the CLOVER measurement of the B-mode power spectrum on the scales where lensing dominates should bring further improvements in cosmological parameters. This is illustrated in Figure 1.8 where we compare our forecasts of the two-dimensional error contours obtainable with QUaD and WMAP, and QUaD, WMAP and CLOVER; one-dimensional marginalised errors are given in Table 1.1. The primary goal of QUaD is to map the E-mode power spectrum, and by doing this it is very effective in improving parameter constraints over WMAP alone. In the model considered, QUaD will only put constraints on the large-angle B -mode power spectrum, but these are good enough to account for nearly all the improvement in r. QUaD also has the sensitivity and resolution to detect the B-mode power from lensing (BICEP lacks the resolution), but this measurement will not be accurate enough to improve parameter constraints further. The situation is different when we add CLOVER data to that from WMAP and QUaD: while the further improvements 2 in Wbh , and the optical depth t come mainly from the E-mode spectrum, the majority of the 2 improvements in Wmh , the dark energy density WL and its equation of state w, and the amplitude of the curvature perturbation As comes from the lensing part of the B-mode spectrum. This is not surprising since lensing is sensitive to the growth rate of density fluctuations, determined by the matter density and the properties of the dark energy, and the fluctuation amplitude. Indeed, the lensing signal can lift the geometric degeneracy between the curvature and dark-energy density and equation of state that limits their determination from linear CMB anisotropies and polarization alone.
As a final example of the science from lensing that CLOVER may return, we note that high- quality polarization maps can greatly assist attempts to reconstruct the CMB lensing deflection field. The r.m.s. lensing deflection angle is only 2.6 arcmin, but the coherence length is of the order of one degree. Within a coherence patch of the deflection, features in the CMB are all sheared in the same way. This produces anisotropic correlations locally that can be used to reconstruct the deflection field. The temperature anisotropies have low intrinsic power at high l which limits the quality of the reconstruction, but this can be improved using E-mode polarization, which has more power at small scales, provided the latter can be imaged at high l. Furthermore, B-mode polarization above l~200 is expected only to be generated by lensing, and intrinsically probes the deflection field on smaller scales than electric polarization and temperature anisotropies. The local E-B correlations thus provide the best estimate of the small-scale deflection field (Hu & Okamoto 2002).
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The calculations of Hu & Okamoto suggest that CLOVER will be just on the limit of being able to reconstruct the deflection field on degree scales. Better resolution would improve the quality of the reconstruction, but the smaller instantaneous field of view would also place stronger demands on the scan strategy and optical design if the primary science goals of CLOVER are not to be compromised. Reconstructions of the deflection field can be used to map the (projected) mass distribution, measure the deflection power spectrum, and reduce the contribution from lensing to the variance of measurements of the B-mode power spectrum from gravitational waves.
1.3.4: Other physics
CLOVER has the clear primary science goals of mapping the B-mode power spectrum, and using this to constrain (or measure) the energy scale and dynamics of inflation. We have also argued that there is important secondary science that can be extracted from the effects of gravitational lensing. On a more speculative level, there are several other interesting tests of fundamental physics that might be accessible to a high-sensitivity polarization experiment. Searching for correlations between B- and E-mode polarization, or B modes and the temperature anisotropies T, may reveal evidence for statistically-significant violations of parity in our universe. A number of physical mechanisms have been proposed that give rise to such parity violations, including asymmetric parametric amplification of right- and left-handed gravitational waves during inflation (Lue, Wang & Kamionkowski 1999), and the generation of gravitational waves from the anisotropic stress of stochastic magnetic fields with helicity [Caprini, Durrer & Kahniashvili 2003]. Primordial magnetic fields have several interesting effects on the CMB and its polarization, although the theory is still rather under-developed. One such effect, more relevant to low-frequency observations, is Faraday rotation that occurs around last scattering in the presence of magnetic fields (Kosowsky & Loeb 1996; Scannapieco & Ferreira 1997). If the field is coherent over the scale of our last scattering surface, then global, parity-violating, anisotropic correlations will appear between the CMB fields due to a rotation of E-mode polarization into B. Although current CMB temperature anisotropy results definitively rule out topological defects as the dominant source of density fluctuations, a sub- dominant background of defects is a natural prediction of symmetry-breaking phase transitions
12 of the sort postulated to occur in the early universe. Defects seed vector and tensor modes, as well as scalar fluctuations, and so give rise to B-mode polarization. The resulting power spectrum is expected to peak on sub-degree scales (Seljak, Pen & Turok 1997). Pogosian et al. (2003) argue that a statistically-isotropic, but highly non-Gaussian, pattern of B-mode polarization with power peaking on small scales would be a smoking-gun signature for a background of cosmic strings. By constraining the parameters of a simple phenomenological string model with current data, Pogosian et al. derive an interesting upper bound on the B- mode polarization power due to global cosmic strings: they find roughly the same power at l=100 and 800 as the signal from gravitational lensing. If the string network contributes at the level of this upper bound, the signal should be detectable statistically with CLOVER. Note also that the signal-to-noise (the latter comprising thermal noise and the lensing signal) in the B- mode images will be much higher than for the temperature anisotropies, since the B-mode images are not confused by primary CMB fluctuations on small scales.
1.4: Foregrounds
The dominant diffuse foregrounds in total intensity at the frequencies covered by CLOVER are Galactic synchrotron and vibrational (thermal) dust emission. Synchrotron radiation in a uniform magnetic field is linearly polarized. Thermal dust emission will also be linearly polarized if elongated grains are aligned in a magnetic field. Synchrotron radiation is expected to be the dominant polarized foreground up to 100 GHz. At the time of writing, the only well- surveyed part of the sky for which data is available is within the Galactic plane, and then only at frequencies below 2.7 GHz (e.g. Duncan et al. 1997). The only high-latitude observations available are very patchy (Brouw & Spoelstra 1976), and most are not useful for assessing foreground contamination in clean regions with low total-intensity emission. The exception is recent observations (Bernardi et al. 2003) at 1.4 GHz, made with the Australia Telescope Compact Array, of a 3ox3o patch centred on RA=5h, DEC=-49o in the BOOMERanG field, which is known to have low total-intensity CMB foreground emission. Extrapolating their data to the central frequency of CLOVER (150 GHz), Bernardi et al. (2003) predict an r.m.s. polarization of only 0 02 mK, although such an extrapolation from low-frequency data should be treated with some caution. The only detection of diffuse Galactic polarized dust emission in the sub-millimetre to date is from the 2002 flight of Archeops (Benoit et al. 2003) at 353 GHz; the data covers 17 per cent of the Northern Hemisphere at 13-arcmin resolution. While the Archeops data provides evidence for coherent, diffuse emission polarized at the five-per cent level in the Galactic plane, the instrument sensitivity does not allow a direct measurement of high-latitude dust polarization. We can shortly expect full-sky polarization maps at five frequencies below 90 GHz from WMAP, and deeper high-latitude maps at frequencies close to those of CLOVER from the 2002 flight of BOOMERanG. These will be valuable for assessing the likely foreground contamination from synchrotron and dust (in the case of BOOMERanG) in the CLOVER survey region.
1.4.1: Foreground subtraction
To estimate the level to which CLOVER will be able to subtract polarized foregrounds, we have constructed models of the polarized emission from synchrotron and dust at 15-arcmin resolution, with 6.8-arcmin pixels, in a 10-degree radius patch centred on RA=40 o and DEC=- 45o. This patch has low emission in total intensity, and is within the possible survey region for CLOVER For the synchrotron emission, we used the 100-GHz model map of Giardino et al. (2002), extrapolated to 90, 150 and 230 GHz with their spectral-index map. The polarization model weights the total-intensity data with the polarization fraction expected for a uniform magnetic field along the line of sight. The polarization directions are from a realisation of a random process with the same power spectra as the Galactic-plane data from Duncan et al.
13 (1997), and are assumed to be independent of frequency. Variations in field direction along the line of sight will tend to reduce the polarization fraction; indeed, in the clean region analysed by Bernardi et al. (2003), the model overestimates the r.m.s. polarization by a factor of ~10. For this reason, we also consider a synchrotron model where the polarized emission is smaller by a factor of ten. For vibrational dust emission we used the two-component model of Finkbeiner, Davis & Schlegel (1999) to estimate the total-intensity emission at the three CLOVER frequencies. We assigned a constant polarization fraction of 10 per cent, and aligned the polarization direction with that of the synchrotron emission. The assumed polarization fraction is a factor of two larger than that of the diffuse emission in the Galactic plane, determined by Archeops (Benoit et al. 2003), to allow for enhanced depolarization when integrating along lines of sight near the plane. The r.m.s. levels of dust and synchrotron polarization for these models are given in Table 1.2. For comparison, the r.m.s. level of CMB polarization expected at 15-arcmin resolution is 3.0mK, of which 0.16 mK is due to the B-mode polarization from weak lensing, and 0.40 r1/2 mK is due to primordial B modes from gravitational waves. For a total integration time of one year, distributed uniformly over a 10- degree radius patch, the noise levels on Q and U in 6.8-arcmin pixels that we estimate for CLOVER are 0.29 mK, 0.37 mK and 0.78 mK at 90, 150 and 230 GHz respectively. Maps of the total foreground are given in Figure 1.9.
Table 1.1: R.m.s. polarization in the synthetic synchrotron and dust maps at the three CLOVER frequencies Freq. (GHz) r.m.s. synch. (mK) r.m.s. dust (mK) 90 0.82 0.24 150 0.29 0.72 230 0.17 2.52
We adopt a simple foreground-removal technique in which we seek the linear combination of the three channel maps that preserves the CMB signal while minimising the mean-square polarization in this map. This is the obvious polarized extension of the technique used by WMAP (Bennett et al. 2003), and requires no knowledge of the foreground properties. Of course, our ability to remove foregrounds will increase if accurate prior information is available on the spectral and spatial behaviour of the foregrounds. For example, knowing the synchrotron and dust spectral index in each pixel would allow us to project out the two foregrounds exactly with three frequency channels. Here, we assume no such knowledge, so our estimates of foreground residuals are likely to be worse than may be achievable in practice.
14 Inevitably there is a trade-off when removing foregrounds between minimising foreground residuals and amplifying the noise. For this reason we consider a variant of the simple linear- combination technique where we subtract off a fraction f noise of the expected contribution to the mean-square polarization from the instrument noise before minimisation. For our baseline model, we find that the minimum foreground residuals (corresponding to f noise=1) have an r.m.s. of only 0.012 mK. However, the noise level on Q and U per pixel has then been amplified to 0.97 mK, which is equivalent to a single channel of detectors with sensitivity 565 mK s1/2. For a ten-degree patch, this effective instrument noise is approximately twice the sample variance from the gravitational lensing signal in a measurement of the B- mode power spectrum. The r.m.s. foreground residual is well below the level of 0.04 mK contributed by the B-modes from gravitational waves in the parabolic small-field inflation model described in Section 1.3.2. Since the polarized power spectra of the foregrounds in our model are dominated by the polarization direction, and so vary as l-1.7, (Giardino et al. 2002), while the CMB signal from gravitational waves peaks on degree scales, the peak in the primordial B-mode power spectrum is easily above the foreground level even when the r.m.s. of the fields are comparable (see Figure 1.10).
Repeating the analysis for the synchrotron model with polarized emission scaled down by a factor of ten, we find we can remove foregrounds down to an r.m.s. of 0.005 mK. However, we can now perturb the weights in the direction of the optimal noise-weighted solution (f noise goes to infinity) while still keeping the foreground residuals under control. A good compromise gives a residual r.m.s. of only 0.02 mK and an effective one-channel sensitivity of 200 mK s1/2. Our discussion of the scientific returns from CLOVER in this proposal are based on this effective sensitivity. The B-mode power spectrum of the residual map is shown in Figure 1.10, and is seen to be essentially negligible. Prior knowledge of the likely levels of the foreground components in the region CLOVER targets will come from other experiments, such as WMAP, QUaD and from the early phase of CLOVER observations. This will be valuable in optimising the balance between foreground removal and instrument noise level in the manner we have done here.
15 Finally, we note that the foreground removal technique adopted here turns out to be rather unstable against uncertainties in inter-channel gain calibration for the CLOVER thermal noise level and our assumed foreground models. (Calibration issues and other systematics are discussed later in this proposal.) The problem arises from the large contribution to the r.m.s. polarization coming from the dominant E modes. If the gains of the various channels are not calibrated perfectly, there is a danger that the linear combination that minimises the observed r.m.s. will be skewed towards that which suppresses the imperfectly-calibrated CMB signal. Fortunately, if we separate the E and B modes in each channel before attempting foreground removal, then the CMB signal in the B-mode variables is sufficiently small that we can still remove foregrounds to the levels discussed above in the presence of calibration uncertainties at the per cent level.
1.4.2: Atmospheric polarization
For a high, dry polar site, such as Dome C, the brightness temperature of the atmosphere between 50 and 300 GHz is dominated by strong oxygen lines at approximately 60 and 120 GHz and water lines near 180 and 325 GHz. In the absence of external fields, neither of these atmospheric components emits polarized radiation. However, the presence of the Earth’s magnetic field causes a Zeeman-splitting of the energy levels of oxygen. The intensity and orientation of the resulting polarized emission is dependent on the angle between the line of sight and the magnetic field direction, as well as the atmospheric brightness temperature. If the line of sight is perpendicular to the magnetic field, the emission will be linearly polarized; however, when the line of sight is parallel to the magnetic field the emission is wholly circularly polarized. In order to quantify possible atmospheric polarized emission we follow the method of Hanany & Rosenkranz (2003). Using the relevant parameters for the Dome-C site, where the atmospheric brightness temperature is ~5 K, we estimate that the linear and circular polarized atmospheric intensity at our observing frequencies will be no greater than 1 nK and 100 mK respectively. The predicted linear component is therefore negligible, but the circularly-polarized component must be considered more carefully. Although the CMB itself is not expected to be circularly polarized, there will be some conversion of the circularly- polarized atmospheric emission to linear polarization in the instrument. To ensure that this signal leakage is well below the expected CMB signal, we therefore require a suppression level of ~10-4. We predict that the intrinsic leakage between linear and circular polarization should be of the order of one per cent. However, unlike fluctuations in the brightness temperature of the atmosphere, which are primarily due to low-altitude water clouds, the polarized intensity due to oxygen is not expected to vary with time but will be fixed for a particular azimuth and elevation direction. Hence any scanning strategy, will modulate any residual atmospheric signal in a very predictable way. Also, these estimates are for the DC-level of the signal. Oxygen is extremely well mixed in the atmosphere and any fluctuations on the angular scales to which CLOVER is sensitive will be very much smaller. It should therefore be possible to separate this signal from the CMB polarization to well below the sensitivity required. Given the low level of the predicted atmospheric polarization signal, our low level of instrumental polarization leakage, and the ability to modulate any residual signals with our observing strategy, we do not expect polarized emission from the atmosphere to be a significant problem.
1.4.3: Extragalactic radio sources
We follow the recent approach taken by Tucci et al. (2003) who exploit the available polarization data at 1 4 .>.GHz to extrapolate the distribution of degrees of polarization of radio sources in the NVSS catalogue (Condon et al. 1998) to higher frequencies. In conjunction with high-frequency source count predictions provided by Toffolatti et al. (1998), an estimate of the polarization power spectrum of extragalactic radio sources is then achieved.
16 Point sources contribute equally, on average, to the E- and B-mode, so the angular power spectrum of polarized brightness fluctuations, produced by Poisson-distributed sources, can be written as 1 Sc C B = p 2 n(S)S 2 dS l ò 2 0 Here, n(S) is the differential number of sources per steradian, p2 is the mean-squared value of polarization degree at a given frequency and Sc is the minimum flux density of sources that can be individually detected and removed from the data. Our estimates of the B-mode power spectrum from extragalactic sources at 100 GHz are shown in Figure 1.11.
At 100 GHz, the integral source count is given by N > S ~ 30(S / Jy) -1.2 sr -1 (Toffolatti et al. 1998) and (Tucci et al. 2003). The flux sensitivity of CLOVER at 100 GHz after one year of observing is better than 2 mJy, so sources brighter than ~10 mJy should be easily detectable in the intensity maps. However, removing all such sources would be overkill for our primary goal of mapping the B-mode power spectrum of the CMB (see Figure 1.11), and would remove a significant fraction of pixels from the map. A better compromise is to cut at ~0.14 Jy, which reduces the point source B-mode power to one-tenth of the lensing power on large scales, while masking only one source per 10 deg2.
1.4.4: Secondary scattering processes Re-scattering of CMB photons around reionization generates a secondary polarization signal by several mechanisms. First, the scattering of the primordial quadrupole of the radiation intensity will be modulated by the variation in electron density, producing a small-angle polarization signal on the sky that has the same frequency spectrum as the primary polarization. Second, a radiation quadrupole is generated from the 2.7-K monopole at second order in v/c in the rest frame of the scattering electron, where v is the peculiar velocity of the electron. This generates a net polarization proportional to the local peculiar velocity squared, but the frequency spectrum differs from the primary signal. Detailed calculations (Baumann, Cooray & Kamionkowski 2003) have shown that the polarization power spectra from these effects are safely below the level of the B-mode spectrum from lensing residuals that would result from imperfect lensing reconstruction with a noise-free experiment.
1.5: Other Polarization Experiments
17
In the coming years, numerous CMB experiments are planned to measure the power spectra of the CMB polarization anisotropies. The vast majority of these, including Planck, are designed primarily to measure E-mode polarization, and hence are not direct competitors to CLOVER. A summary of the current and upcoming E-mode experiments is presented below and summarized in Table 1.3. Upcoming B-mode experiments, planned for operation or launch within the coming decade, are summarised in Table 1.4. As can be seen from the table, CLOVER is well placed to provide a first detection of the B-mode gravitational wave signal . 1.5.1: Upcoming and proposed E-mode experiments
AMiBA. This is a ground-based interferometric array of 19 elements, operating at 90 GHz with full polarization capabilities and a 16 GHz correlation bandwidth, and will be sited on Mauna Loa, Hawaii. It will measure the small-scale CMB temperature and polarization anisotropies in the range l = 700–2000, and also make observations of the Sunyaev-Zel'dovich effect. First observations are planned to start in 2004. AMiBA will not usefully constrain the primordial B- mode polarization.
B2K. This balloon experiment is a follow-up to the successful BOOMERanG project. The instrument has been re-engineered primarily to detect polarization and is predicted to have good sensitivity out to approximately l = 1200. The balloon flew in early 2003 and took 10 days of good data before the flight was terminated due to loss of balloon altitude. It will not be able to detect primordial B-mode polarization.
CAPMAP. A ground-based experiment using sixteen correlation polarimeters (12 at 100 GHz and four at 40 GHz) on a 7-m off-axis Cassegrain telescope at Crawford Hill, New Jersey. It aims to measure E-mode polarization on small angular scales (l>1000). A prototype with four 100-GHz receivers operated during 2002-03 and results are expected shortly. Operation of the full instrument is expected in 2004.
CBI. This ground-based interferometer was initially designed to measure the CMB temperature anisotropy at high l, the results of which were released in 2002. Since then, the telescope has been re-commissioned to enable measurements of the E-mode polarization in the l-range 600–1800 over the frequency range 26–36 GHz. Results have been published recently, detecting E modes and TE at a few sigma. It is not able to detect B-mode polarization.
DASI. This interferometer made the first detection of E-mode signal (Kovac et al. 2003). But was not able to place useful constraints on the B mode. DASI has now been decommissioned.
MAXIPOL. This balloon experiment is a follow-up to the successful MAXIMA project. Similarly to B2K it has been re-engineered to be primarily a polarization-detection instrument. It was flown successfully in May 2003 collecting 14 hours of CMB data which is currently being analysed. It will not be able to detect B modes.
Planck. This satellite is scheduled for launch in February 2007. Despite its excellent technical specifications, results cannot be expected from the mission until at least 2010. In all but the most optimistic scenarios, Planck will only set upper limits on the gravitational-wave contribution to B-mode polarization. However, it should be able to detect the lensing signal.
18 SPOrt. Designed to measure the large-scale polarization (FWHM of 7°) at 22, 32 and 90 GHz, will be flown on the International Space Station starting in 2006. Primary science goals are a tentative detection of the E-mode signal and map Galactic synchrotron emission.
WMAP. The first-year data from this satellite project has measured the TE power spectrum out to angular scales of l ~ 500. Release of the E-mode power spectrum is expected shortly, but is being delayed by consideration of foreground contamination. Even with further data releases WMAP will not detect B-mode polarization.
1.5.2: Upcoming B-mode Experiments
QUEST and DASI (QUaD). This 31-element bolometric array telescope which will be sited at the South Pole on the old DASI mount. It is primarily designed to measure the E-mode polarization power spectrum, but it is also expected that QUaD will be able to detect the lensing B-mode spectrum at angular scales l > 200. QUaD is expected to start observations in the Austral Winter 2005.
BICEP Currently being built to attempt to measure the primordial gravitational-wave B-mode signal in the region 10 < l < 200 from the South Pole. The first season of observations with BICEP are planned to start at the South Pole winter 2005, but will only utilize a reduced focal plane of eight feeds at each of 100 and 150 GHz. The complete BICEP instrument is expected to come on line in 2006. Even with a fully populated focal plane array, the sensitivity of the BICEP instrument is expected to be a factor of ten less than CLOVER.
EBEX. This ambitious balloon project builds on the experience of the MAXIPOL instrument and aims to measure B-mode polarization over 5 < l < 2000. The final instrument will have 300 dual-polarization pixels at each of four frequency channels (150, 250, 350 and 450 GHz). This project is still being conceived, and little information is currently available. Here we report tentative plans for the project as reported by the EBEX team (B. Johnson and T. Renbarger, priv. comm.). A North-American test flight using only the 150-GHz channel is planned for 2006. Depending on the success of this flight, the instrument will then be taken to Antarctica for a long-duration balloon flight (maximum 200 hours) during the 2006/7 Antarctic season. This flight will again utilize only the 150-GHz channel and, like all balloon experiments, is subject to high risk.
PolarBeaR. This ground-based instrument will use a 2.5-m off-axis Gregorian telescope. The complete instrument will have a large array (~1000 pixels) of polarization-sensitive antenna- coupled bolometers and will aim to characterize both the E-mode and B-mode signals up to l ~1200. It is planned that PolarBeaR will eventually observe at 150, 250 and 350 GHz. At present, the project is only just beginning and little information about the proposed timescale is available. The PI, Adrian Lee, has however suggested that it will be deployed in two stages: (i) approximately 150 pixels distributed over 2–3 frequencies with deployment hoped for ~2006/7 at White Mountain, California; and (ii) PolarBeaR II hopefully be deployed two years after that (~2008/9) and will have ~1000 pixels, again distributed over 2–3 frequencies.
NASA Inflation Probe. This will be a dedicated all-sky CMB polarization satellite designed to conduct the definitive all-sky CMB polarization measurement. It is part of NASA's Beyond Einstein program, will cost in the region of $350–500M with launch possibly in the 2015–2020 timeframe and probably with some external collaboration (e.g. Europe).
19 1.5.3: Summary
There is a clear opportunity that CLOVER, if started now at Dome-C, will take a very significant scientific lead over the likely competition.
20 2: Technical specification
2.1: Overview of the experiment
Bolometric detectors are the most sensitive type of receiver available at millimetre wavelengths, and it is now possible to achieve performance limited by the sky background rather than the detector noise. For a ground-based experiment, the only way to improve sensitivity is thus to have more pixels. Recent experimental designs have therefore concentrated on building focal plane arrays with moderate numbers of elements (see Section 1.6 above). For very large numbers of pixels, however, the limited size of the unaberrated focal plane available in any reasonable optical design becomes a problem, particularly as the need to minimise systematic errors makes the constraints on optical distortions even more stringent. In order to combine the very high sensitivity and stringent control of systematics required to detect the CMB B-mode signal, we have developed a novel design that combines high throughput, clean low-aberration optics, a large number of background-limited detectors, and a low-systematics polarimeter.
CLOVER consists of three completely independent telescopes, operating at 90, 150 and 220 GHz from Dome C in Antarctica. Each telescope consists of four separate, co-pointed optical assemblies, each fed by an 8´8 array of feed horns. The signal from each horn is separated into the two independent linear polarization states, converted to circular polarization, phase modulated and then correlated. The two correlator outputs encode the Stokes parameters I, Q and U. The outputs from each corresponding pixel in the four telescopes are then summed incoherently before being detected by a TES bolometer. There are thus 256 horns per telescope but only 64 simultaneously observed pixels, since the optical assemblies are co-pointed. The sensitivity however is equivalent to 256 individually-detected pixels. Stokes parameters Q and U are measured instantaneously by the phase modulation in the polarimeter, while I is measured by scanning the telescope. The four optical assemblies are built around a single cryostat which houses all four horn/polarimeter arrays and the detector array, and are mounted on a common mount which allows altitude-azimuth tracking as well as rotation of the entire optical structure around the pointing axis. Figure 3.1 shows a block diagram of the signal path through the telescope, and Figure 3.2 shows the overall arrangement.
Figure 3.1: Block diagram of one CLOVER telescope. Only two of the four optical assemblies are shown.
21 The three telescopes are scaled to all give a beam on the sky of 15 arcmin. Each telescope will be azimuthally scanned over a single region of low-galactic foreground sky about 20 degrees across, the sky rotation during the day giving a good degree of cross-linking in the maps. The telescopes will also be periodically rotated about the pointing axis to calibrate out instrumental effects and improve the density and cross-linking of the sky coverage. Observations at the three frequencies will allow separation of CMB and galactic synchrotron and dust components. Table 3.1 show the overall specifications of CLOVER.
Figure 3.2: Schematic view of single observing platform (left) with four co-aligned telescopes each feeding an array of 64 feedhorns, all four co-aligned pixels at each frequency are then fed to a single detector in each of the two polarizations; a possible layout (right) of three such platforms at each of 90, 150 and 220 GHz.
Deployment of the experiment to the Antarctic site will be phased over three years in order to spread the logistical load over the short annual working seasons, and because of the schedule of delivering the detector arrays. The first season will see establishment of infrastructure and the commissioning of the first telescope structure. In the second season a 16-horn receiver at 90 GHz will be deployed, and the following year will be replaced by a full 256-horn system, along with deployment of the 150 GHz and 220 GHz systems with 16-horn receivers. In the fourth season the full 256-horn systems at 150 and 220 GHz will be deployed (see Table 3.2). Note that the installation season is from November to February, but the observing season is February to October. Infrastructure at the site will be provided by the French and Italian Antarctic agencies via collaborations with groups in Paris and Rome. Dome C is possibly the best site in the world for millimetre astronomy, surpassing even the South Pole, and has only recently become available for experiments of this kind.
In the following sections we describe each component of the experiment in more detail.
22
Table 3.1: Specification Table for Clover
Platform Three-axis mount Azimuth 0–360 deg. Elevation 0–90 deg.; pointing accuracy 7 arcsec Rotation ±90 deg.; accuracy 4 arcmin Operating Frequencies 3 bands at 90, 150 and 220 GHz with ~30% bandwidth Frequency band performance Transmission = 0.963 79–109GHz NEP = 2.70´10-17 W ÖHz, NET = 170 mK Ös Array NET = 10.5 mK Ös 130–175GHz Transmission = 0.967 NEP = 3.65´10-17 W ÖHz, NET = 215 mK Ös Array NET = 13.4 mK Ös 190–250GHz Transmission = 0.949 NEP = 5.82´10-17 W ÖHz, NET = 455 mK Ös Array NET = 28.5 mK Ös Horns 8´8 array per window One cryostat has four windows = 256 horns Total number = 3´256 = 768 horns FWHM: 10 deg System optical efficiency > 0.5 Cryogenics Three cryostats each with 4´300-mm window Pulse Tube Cooler (5W at 65 K and 0.5W at 2.5K) plus He3/4 fridge (335±0.1 mK) Optics Four telescopes per cryostat = 12 telescopes Compact Range Antenna, primary mirror 800 mm, secondary 700 mm (150 GHz) Main beam 15-arcmin FWHM Peak cross-pol -35dB worst case (edge pixel) l-range 20–1000 Ortho-mode transducer (OMT) One finline OMT per horn Isolation < -40 dB Return loss < -20 dB Phase switch Rotating 1/4 waveplate in waveguide Loss < -20dB; bandwidth 30% Phase range ±p/4 Frequency 0.1–5 Hz Detector chips TES bolometers at 335mK; NEP < 2x10-17 W/ÖHz Thermal time constant < 1ms 16 chips, total 128 bolometers per cryostat Read-out 8-channel SQUID mux reads 8 detectors (4 horns times 2 polarizations), hence 16 SQUIDs per cryostat
25 Table 3.2: Project deployment summary (as in the original UK proposal; see section 4 for an update) A, B, C refer to the three frequencies. Receivers (Rx) A1 and B1 refer to the 16-horn, single-detector chip systems whereas receivers A, B, C are the full 256-horn systems. Heavy equipment is shipped in August each year; light equipment and personnel travel in December for a three-month working season.
Activity in Europe Ship to site Site activity Year 1 (ship Aug Build optics A and B Enclosure Install infrastructure 2005) Build cryostat A Build Telescope A Year 2 (ship Aug Build Telescopes B and C Telescope A Install and commission 2006) Build Rx A1 Cryostat A (inc. Rx Telescope A Integrate Rx A1 in cryostat A A1) Install and commission Rx A1 Build cryostat B Year 3 (ship Aug Build Rx B1 Telescopes B and C Replace Rx A1 with Rx A 2007) Integrate Rx B1 in cryostat B Rx A Commission Rx B1 Build optics C Cryostat B (inc. Rx Build cryostat C B1) Build Rx A; test in cryostat C Cryostat C Year 4 (ship Aug Build Rx B and C; test in Rx B and C Replace Rx B1 with Rx B 2008) cryostat C Cryostat C Integrate Rx C, cryostat C Integrate cryostat C, telescope C
2.2: Detailed description of key areas
2.2.1: Telescope and mount
An optical assembly of CLOVER comprises a Compact Range Antenna (CRA), an offset-fed design that exhibits extremely low off-axis beam distortion and cross-polarization. The CRA consists of a concave hyperboloid sub-reflector and an offset parabolic main reflector. This antenna is more suitable for large-format array illumination than the offset-Cassegrain or Gregorian (Planck) antennas, since the beams generated by feeds located at the edges of the array suffer little aberration. Ray-trace and spot diagrams are shown in Figure 3.3 for a representative system with main reflector diameter 800 mm, sub-reflector dimensions 735 mm ´ 700 mm and ideal hybrid feeds with flare angle 10 degrees and aperture diameter 15 mm. Note that the rays originating from the horns do not deviate much from the centre of the sub-reflector, in order to satisfy the minimum cross-polarization condition. Figure 3.4 shows co-polar radiation patterns, and E- and H-plane cuts of co- and cross-polar patterns, both for the feed located at the centre of the array and also when the feed is offset by 50 mm in the focal plane. The cross polarization component for the central pixel is too low to be shown on the scale of the E-plane plot. The very low levels of cross-polarization and aberration of this optical system are a key advantage of the CLOVER design. The simulations below were made using the rigorous software package GRASP which has been used by Cambridge to design the optics for HARP and ALMA receivers.
26
Figure 3.3: Ray trace of the CRA optics (left) and full field spot diagram (right) showing the low level of aberration even at the edges of the focal plane array. The circle in the spot diagram is 15mm diameter and represents one horn.
The four CRAs are arranged around the central cryostat with four-fold symmetry, with a common pointing direction. The advantages of this design are: (a) the total optical throughput is increased relative to a single telescope design with the same size of focal-plane array; and (b) the rotational symmetry allows physically different arrays to be rotated into the same orientation, providing cross- checks on telescope-dependent systematics. Baffles separate the four assemblies, and the mirrors of each are extended to form a completely enclosed structure with no stray light paths to the ground. The whole telescope has a conical screen surrounding the reflectors which tracks and rotates with the structure. A further groundscreen surrounds the telescope providing a clean horizon with all surrounding buildings and other structures well below the top of the screen, and also provides protection from wind and drifting snow.
Figure 3.4: Beam patterns obtained when the feed is placed in the centre of the array (top) and moved 50 mm in the positive x-direction (bottom). The co-polar contour plots (left) are with the polarization in the plane of asymmetry of the antenna; the cuts (right) show E- and H-plane co-polar (blue and black), and cross-polar (green) responses.
The telescope mount is heavily based on the experience in Cardiff of designing a mount for the QUEST telescope. Full structural analysis has been conducted and a prototype assembled and tested
27 in the laboratory, including some of the drive software. For this reason a full sub-assembly costing is already known and is low-risk, and also the PDRA effort on the software development is small.
Figure 3.5: Schematic of the CLOVER cryostat Figure 3.6: Schematic of the CLOVER receiver block concept. The pulse-tube cooler sits on the showing the 64-element feed arrays and the sections outside and has cold stages at 65K and 2.5K. A behind containing the hybrids and phase switches. Note 0.3K He-3 fridge is mounted off the 2.5K stage. the twisted waveguide sections mating the horn arrays to the detector array (the central cube), allowing the same sky pixels to be brought to the same detector. 2.2.2: Cryogenics
The cryostats will be designed to operate with no requirement for liquid cryogens, for operational simplicity. Instead, cooling to 2.5 K will be achieved by using a pulse-tube cooler. A He3/He4 refrigerator will cool the detector block to 335 mK. This technology has been used in Cardiff for smaller cryostat systems and is also being used on the much larger SCUBA2 cryostat. The cryostat will have 4 windows and 4 horn arrays but otherwise will not be unusual for a sub-millimetre wavelength experiment. The window will be made from Zotefoam to give maximum transmission, and the blockers and bandpass filters will utilise the state-of-the-art technology developed by Prof. Ade in Cardiff. Conceptual drawings are shown in Figures 3.5 and 3.6. Technological development of liquids-free cryostats has been funded by the technology program of the Italian Programma Nazionale di Ricerche in Antartide (PI prof. P. de Bernardis).
2.2.3: Array and polarimeter
The pseudo-correlation polarimeter concept is shown in Figure 3.1. The outputs of the two detectors are
D1 = I + Qcosf + Usinf D2 = I - Qcosf - Usinf , where f is the phase introduced by the phase modulator. The detector outputs can thus clearly be used to determine both Q and U at the sky pixel by taking the difference of the detector outputs and phase-sensitively detecting. The effect is similar to that of a rotating wave plate, but is achieved without any moving optics. The intensity I of the pixel is also obtainable from the sum of the detector outputs, but is not modulated. Modulation of the intensity is achieved by scanning of the 28 array across the sky. This type of receiver has been widely used at lower frequencies (for example in the WMAP experiment).
2.3.3: Horns
Based on our previous experience for QUEST, Archeops, Planck and other projects, we have designed and tested horns for maximum sidelobe rejection and minimum cross-polarization using electromagnetic simulation packages and the antenna testing range in Cardiff. Several horn shapes have been considered, and a new shape based an a modified Winston cone equation is giving the best results in term of cross-polarization and sidelobe rejection, while having the phase centre located very close to the aperture of the horn (see Figures 3.7 and 3.8).
Central frequency 150GHz 200 H-plane Phase 0 E-plane phase H plane 150 -10 E plane 100 -20 Copolar -30 50 X polar -40 dB 0 0 5 10 15 20 25 30 -50
Phase (degrees) -50 -60 -100 -70
-150 -80 0 10 20 30 40 50 60 70 80 90 -200 Off axis angle (deg) Off axis angle (degrees) Figure 3.7: Horn beam pattern giving a 7.5 degrees FWHM. Figure 3.8: Phase at phase centre.
2.3.4: Orthomode transducers (OMTs)
An OMT separates two orthogonal linear polarizations in a circular or square waveguide. There are many kinds of OMTs but our requirements in terms of return loss (<-20 dB), isolation (<-40 dB), cross-polarization (<-20 dB) and band-width (40%) are easily achieved using OMTs based on the finline technique, as we have demonstrated with extensive electromagnetic simulations using the commercial HFSS software. An example of a finline OMT is shown in Figure 3.9. Cambridge has used finline transitions in high performance SIS mixers for many years.
Figure 3.9: Finite-element simulations of a finline OMT.
2.3.5: Waveguide hybrids
29 A ‘quadrature hybrid’ is a four-port directional coupler where the power incident on any port is divided equally between two other ports with a 90° phase difference, and the fourth port is isolated. We have investigated and simulated different kinds of directional couplers and intend to use ‘branch-line couplers’ in which the waveguides are interconnected by multiple branch waveguides between the walls (see Figure 3.10). These have been shown (Andoh 2003) to have excellent performance (S12, S13 = 3.0 ± 0.3 dB, S11, S14 < - 25 dB) across a 25-per cent bandwidth that fully meets our requirements. This geometry also has a symmetry that allows split-block manufacturing of the waveguides in two halves, without affecting performance, because no current flow is needed between them. Fabrication is by CNC machining techniques with the addition of spark erosion for the smallest structures.
1 2
4 3
Figure 3.10: Simulations and results of a branch-line coupler.
2.3.6: Phase shifters
We have investigated several different possibilities, from ferrite-loaded to mechanical, with and without dielectric loads. Our current baseline is a mechanical phase shifter based on a rotating ‘half- wave plate’. This works in circular waveguide and is composed of a rotating half-wave plate (HWP) between two fixed quarter-wave plates (QWP). An incoming rectangular waveguide TE10 mode is converted to a vertical TE11 circular waveguide mode with a transition section. The first QWP is oriented at 45 degrees from the vertical and converts the TE11 mode to a right-hand circularly- polarized mode. The rotating HWP phase-shifts the mode by twice its rotation angle. The second QWP oriented like the previous one converts the circular mode back to the vertical TE11. The final transition restores the TE10 mode. Between the QWPs and the HWP there are two l/4 chokes that minimise the effect of the waveguide discontuity giving a very small return loss. The HWP is made of two aligned QWPs. All the QWPs are broadband ‘iris polarizers’. Simulations of the performance of phase shifters are shown in Figure 3.11.
2.3.7: Waveguide twists, transitions and bends
30
After the OMT section the two rectangular waveguides are oriented differently. A 90-degree twist on the on-axis channel is necessary to connect the OMT with the first hybrid. Besides the usual commercial twists based on continuous and slow rotation there are other kinds designed for integrated waveguide systems that can be easily machined with CNC techniques. The one we use is the ‘compact stepped waveguide twist’ (Baralis 2002) that is based on the direct connections of rectangular waveguides sections with differently rotated cross-sections and lengths. This very compact twist is easy to fabricate, it has a very small return loss (<-40dB) and is broadband (30%).
Figure 3.11: Example of ‘rotating waveguide’ phase-shifter.
2.3.8: Superconducting detector array
General considerations A detector may be considered suitable for CMB polarization measurements if it satisfies the following requirements:
· Large bandwidth · Background limited sensitivity · High dynamic range · High slew rate · Ease of fabrication and integration
The need for large bandwidth to enhance sensitivity makes bolometric detection appealing for CMB polarization measurements. We aim to achieve a bandwidth of 30%, which is possible through careful design of all components. The single-pixel sensitivity required for detecting CMB polarization is high, and therefore it is essential to ensure that the sensitivity is limited by the sky background rather than the detectors themselves. Assuming that the background level at Dome C limits the sensitivity at 150 GHz to s » 200 mK s1/2 and assuming an RF bandwidth of 50 GHz, the
31 required NEP » 4 ´10-17 W Hz which is within the capability of modern cryogenic bolometers. The specifications required for the CLOVER detectors are summarised in Table 3.3.
It is extremely important to be able to fabricate the individual elements of a large imaging array easily, which makes planar circuits fabricated using lithographic techniques highly desirable. Other advantages are: (a) the ability to feed the device with a microstrip transmission line terminated by a resistor. A 20 W Niobium superconducting microstrip line with a 400-nm SiO insulator will have a width of » 3 mm . This means that radiation is absorbed by a small component, which does not couple easily to stray radiation; (b) the beam received by the detector can be determined by a well-designed single-mode horn rather then by the size and geometry of a multi- mode absorber; and (c) lithographic fabrication allows many channels to be multiplexed onto a single output. Indeed, in our design we have two levels of multiplexing: first, a number of RF -moding’ we gain sensitivity by having several copies of the same mode brought to a single detector; and, second, the outputs from a number of TES are fed into a single SQUID.
Table 3.3: Specifications required for detectors
90 GHz 150 GHz 230 GHz Df 30 45 60
Photon NEP W Hz 2.7 ´10-17 3.7 ´10-17 5.8´10-17
NET mK s 170 215 455 Number of horns 256 256 256 Number of modes 512 512 512 Detector NEP W Hz < 2´10 -17 Operating temperature 300 mK Thermal Response time < 1 ms Number of SQUIDs 16 16 16
Transition Edge Sensor Voltage-biased Transition Edge Sensors (TES) can achieve all of the objectives listed above. A TES consists of a thin superconducting film deposited on a silicon nitride membrane. The device is biased at the middle of the transition region between the normal and superconducting states. Absorbed photons heat the quasiparticle gas, causing a sharp increase in the electrical resistance, and therefore a decrease in bias current. This electro-thermal feedback keeps the TES on the transition point and, as a by-product, decreases the effective time constant, making the devices very fast. The change in electrical current is read out by a SQUID. A typical Mo/Cu TES has an area of 50´50 m and a film thickness of around 100 nm.
Detector configuration The detector chip architecture is chosen to suit the horn configuration. One possible layout is a rectangular wafer loaded with finline substrate transitions from four sides, each side fed by one of the four horn arrays. Each side of the wafer will therefore have 16 finline channels corresponding to eight horns with two polarizations. In total, the wafer will have 64 microstrip channels and 16 TES
32 detectors. Alternatively the finline channels can be re-directed to two sides of the rectangular substrate thus avoiding microstrip crossings. This is the arrangement described below. Each group of eight TESs is frequency multiplexed and read by a single wideband, low noise SQUID.
Figure 3.12: Read-out schematic of a detector module. 32 finlines are coupled to eight TES and a single eight-channel multiplexer. Read-out is by a single wideband, low noise SQUID.
Detector and readout fabrication The detector can be fabricated in units of 32 finlines terminated by thin-film resistors that are thermally coupled in groups of 4–8 TESs, these in turn are fabricated as part of an eight-channel frequency division multiplexer (see Figure 3.12). Finlines, microstrip lines, termination resistors, TES, thermal isolation and the multiplexer can be fabricated as a single integrated chip. Fabrication on 100-mm wafers allows up to 12 such units per wafer (see Figure 3.13), so that the full instrument can in principle be fabricated from two such wafers.
The TES will be fabricated from a Mo/Cu bilayer with Tc ~ 400 mKwith Nb bias lines. The bias lines also form one-half of the finline and microstrip. Selective anodization of the Nb forms the dielectric layer of the capacitors required for the individual band-pass filters of the multiplexer. The same anodization step can also be used for addition of the multi-turn inductors. Isolation of the top
Nb films of the finline and microstrip is provided by SiO 2 . Selective etching of the SiO 2 allows inter-layer connects and connection to the termination resistors. Thin film inductors for the multiplexer filters, identical in design to those used for SQUID coils are also fabricated from the same Nb/SiO 2 /Nb layers. Termination resistors matching the microstrip impedance are fabricated n 33 from the same Au/Cu alloy films used for SQUID shunt. The proposed layout of a single pixel is shown in Figure 3.14. The TES itself is formed from a thermally-isolated Si3N4 island on which is fabricated resistive termination for the four microstrips. The continuous microstrip lines are routed along the Si3N4 bridges.
Figure 3.13: Scale layout of 12 detector units on a 100-mm wafer. The dotted cicle shows the size of the wafer. Each unit comprises 32 finlines (red), eight TES and membrane (yellow) and a single eight- channel multiplexer (blue capacitors, green inductors).
Figure 3.14: Scale layout of a single pixel with TES (red), Nb bias lines (blue), microstrip line (white on blue) and termination resistors (green). The nitride membrane is removed in the regions shaded yellow. The central TES is 50´50 mm2.
RF design To reach background-limited sensitivity we need to achieve efficient RF coupling to the device, a high quality device and low-noise read out. Power is coupled from the waveguide to the TES planar circuit using an antipodal finline taper consisting of two superconducting fins of 300-nm Nb separated by 400 nm of SiO (Yassin & Withington 1995). The whole structure is deposited on one side of a 200-mm substrate. Before the fins overlap, the thickness of the SiO is much less than that of the silicon and behaves as a unilateral finline. As the fins overlap, the structure behaves like a parallel-plate waveguide with an effective width equal to the overlap region. When the width becomes large enough that fringing effects can be ignored, a transition to microstrip is performed, then tapered to the required width.
Thermal design The detector operates from a bath temperature of 300 mK, and is modelled as a 50´50-mm 2 Mo/Cu TES each of 50-nm thickness with 300-nm thick Cu edges (to ensure fully-normal edges of the TES), with a total Nb volume of 200 ´ 200 ´0.5 mm 3 representing the microstrip and TES bias lines. The total heat capacity for Tc = 400 mKis estimated as 84 fJ K . Thermal isolation is provided by eight-patterning the0.5-mm thick Si3N4 into eight bridges each of 20-mm width giving 34 a total thermal conductance of G = 58 pW K , an intrinsic time constant of 1.4 ms and a limiting NEP determined by the thermal conductance of 1.7´10 -17 W Hz .
Photon
Total 10.0 Phonon 1/2
Johnson in pA/Hz noise I 1.0
SQUID
0.1 100 101 102 103 104 105 f in Hz
Figure 3.15: (above) Calculated contributions to the input current noise assuming a photon NEP of 3.8´10-17 W/ÖHz.
Figure 3.16: represented by the variable resistors are fabricated as part of an LRC resonant circuit. At resonance, each TES is voltage biased. The ac bias sources are represented as a sum of bias voltages SV . All of the structures within the dotted box are fabricated on-chip.
Detector readout and multiplexing Direct readout of a single TES requires a minimum of five wires from room temperature to the cold-stage. For these large arrays some form of multiplexing is therefore required. Here a frequency division mux is described which meets specification and has the advantage of being readily integrated onto the same chip as the finlines, microstrip and TESs. The multiplexer is
fabricated from superconducting Nb with SiO2 isolation that is also required for the microstrip and finline. Only one additional processing stage is required to form the thin film capacitor by anodization.
Eight resonant LRC series circuits multiplex eight TES detectors into one SQUID (see Figure 3.16). All of the structures within the dotted box in the figure can be fabricated on-chip. Eight TESs are biased and read-out with only two connecting wires. The circuit is biased by a comb generator, SV , which is a sum of bias voltages at the resonant = p frequencies, f0,ii12( LC ) , which voltage bias individual TESs. The LRC circuits filter the (mainly white) noise from the TESs. Off-resonance the TESs are current biased and the current flowing in each of the branches is given by - 2 1/2 I Rféùæöf i ==êú1,+-Q 2 ç÷0,i IZf êúi ç÷ff 0( ) ëûèø0,i
where the quality factor Qi = 2pf0,iL R . Signal crosstalk between pixels arises because of the finite currents flowing in the off-resonance channels. The signal crosstalk is the sum of the power
RC3C4C1C2C5C8C6C735 LRbSV (P P ) = (I I )2 . 0 i å j 0,i j ¹i
The half-width of the resonance Dfi = f0,i 2Qi needs to be chosen to exceed both the detector thermal bandwidth f thermal =12( pt ) , where the time constant includes the effect of ETF, and the signal bandwidth Dfsig .
We assume a frequency spacing of 20 kHz and a minimum resonant frequency of 100 kHz.
With a signal bandwidth of 1 kHz, the maximum required Qmax=ffmax(2D=sig ) 120 .
Assuming RTES =W0.5 at the operating point, requires capacitor values in the range 11–64 nF. These can be achieved by anodization of thin film Nb with dimensions in the range 1.9– 4.6 mm2. Estimated power cross-talk is about - 25 dBin this example.
All the eight-channel multiplexers fabricated on a single wafer can be biased from the same eight signal generators, although this requires high uniformity in individual components of the filters. A linear variation of up to ± 2.6 mm is acceptable for the component dimensions used in this example. This is well within the capability of our high-quality optical lithography which achieves better than ± 0.2 mm across a 100-mm wafer.
SQUID and slew rate requirements Some improvements in SQUID system performance will be necessary to optimize the multiplexer. The dc-SQUID is stable for flux variations of DF=±F0 4about the bias point. The SuperSQUID has a slightly reduced linear flux working range arising from a desired sharpening of the V -f curve. Stable operation of the read-out with large signals requires flux feedback into the SQUID to linearise the response and avoid losing lock. This means the SQUID flux feedback needs to follow the total flux change due to current in the input coil. A changing current DI in the input inductance Lin causes a flux change DF=DMI in the input coil where M= kLLinsquid and k ~ 0.8 is a coupling efficiency. In our present design for SuperSQUID, M = 2.5 nH.
The maximum rate of change of current at the input coil assuming random phases for the bias & voltages is of order Imax ~ 8pfmax I0 , where the bias power I0Vb = Pmax 2 ~ 2 pW. For eight & channels with a maximum bias frequency of 240 kHz, Imax ~ 12 A s equivalent to a rate of & -1 change of flux FFmax0~14 ms. The SQUID has a linear range F=lin VVpp f where Vpp is the peak-to-peak value of the voltage-flux V -f curve and Vf is the slope at the working point. & The maximum slew-rate of the SQUID system is Fmax=F2p linf3dB . Present SuperSQUID electronics has a f3dB = 2.5 MHz and a linear range Flin0»F0.2 . This gives a maximum flux & -1 slew-rate of FFmax0~3.2 ms. This can be increased by reducing the input inductance (at the expense of increasing the input current noise), by reducing Vf , or by modifying the feedback components. Published electronic designs indicate f3dB =15 MHz is possible, giving & -1 -1 FFmax0~19 ms with the present geometry, and rates up to ~100 F0 ms have been achieved. Figure 3.17 shows the measured flux noise for SuperSQUID.
36 Figure 3.17: Magnetic flux noise measured for SuperSQUID.
Electronics and data processing The resonant-bias AC sources for the TESs also provide reference signals for the demodulation, cancelling phase noise to first order. Demodulated signals are low-pass filtered so that the Nyquist sampling rate for each individual A to D is of the order of 2 kHz. The AMI read-out electronics uses a low-cost 20-bit A to D with a data rate of up to 40 kbit/s. A 14-bit A to D (equivalent to 42-dB dynamic range) with 2.5 kHz per sample is readily achievable.
Figure 3.18: Front-end electronics and data processing.
2.4: Systematics
Attempts to measure signals as weak as the expected B-mode signal are likely to be dominated by systematic effects rather than raw sensitivity. The keys are to minimise contaminating signals to begin with, then to remove them with switching or differencing schemes, and finally to be able to calibrate any residuals. CLOVER has multiple switching schemes to separate genuine sky signals from spurious ones. A key feature of the design is that nearly all spurious polarization effects produce signals that are fixed in orientation either to the telescope or to the environment, but not to the sky. The freedom to rotate the telescope 1i SAn37 LPF about the pointing axis independently of the tracking means that these effects can be suppressed.
Effects of systematic errors on B-mode measurements have been considered by Hu, Hedman & Zaldiarraga (2002; hereafter HHZ). They classify errors into the following types:
· Calibration (gain differences between polarization channels of a pixel) · Rotation (of the polarization axes relative to the nominal axes) · Pointing (relative to the nominal pointing) · Spin flip (leakage between the ‘spin states’ Q±iU) · Monopole leakage (intensity to polarization) · Dipole leakage (intensity gradient to polarization) · Quadrupole leakage (trace-free curvature of intensity to polarization)
In all cases it is the level of instability of an error parameter that more important rather than its absolute value (assuming the absolute value is reasonably small, of course). For example, in the measurement of CMB polarization by DASI, the instrumental polarization was of order 1 % but was stable to better than 10-4. We will consider each type of error in turn.
· Gain errors. For a simple differencing scheme in which the two orthogonal electric fields are detected individually, differential gain variations in the two channels lead directly to a leakage of intensity into polarization. For a correlation receiver such as we are proposing, however, gain differences have no such effect, and the mean gain simply multiplies both spin states Q ± iU. The gain is thus only required to be stable on the timescale of switching rate between these states, i.e. the phase switching frequency. From HHZ, an rms gain instability of 1%, which should be easily achievable, results in a spurious B-mode signal an order of magnitude lower than the B-mode lensing signal.
· Pointing and rotation. Drift between the pointing directions of the orthogonal polarization states of a given pixel results in a mixing of E and B. Uncalibrated rotation of the telescope about the pointing axis mixes Q and U. For designs such as CLOVER in which the orthogonal polarizations are fed through the same optics, differential pointing is not a significant problem . For balloon experiments absolute pointing and rotation jitter is a serious potential problem, since the telescope is free to swing and pointing must be carefully tracked and modelled. CLOVER should easily achieve sub-arcmin accuracy.
· Spin flip is leakage between the ‘spin states’ Q±iU. This is mainly due to phase errors in the hybrids and phase switch that make up the correlation receiver. Although it is difficult to ensure phase accuracies of better than a few degrees, these should be extremely stable as they are fixed by the machined features of the waveguide components, and can thus be calibrated by observations in which the instrument is rotated relative to the sky.
· Monopole leakage (intensity to polarization) This arises from the non-orthogonality of the polarization beams, i.e. the overlap integral of the vector beam patterns for the two
38 states derived from the OMT. Calculations of the beam patterns show that the absolute value of this is at worst -30dB and since it derives from the geometric arrangement of the optics, its stability depends on the stiffness of the optical assembly. For large beam sizes, the effect is to ‘copy’ the temperature anisotropy spectrum to the B-mode spectrum at the level of the leakage; however as the beam size is reduced the spurious B-mode signal is suppressed on angular scales much larger than the beam. For a 20- arcmin beam, a leakage level of around –30 dB will reduce the contamination from the temperature anisotropies to an order of magnitude below the lensing signal near the peak of the primordial B-mode spectrum at l~90.
· Dipole leakage (intensity gradient to polarization) This results from a differential pointing between the two polarizations (‘squint’), and couples the gradient of the intensity into the polarization. For a system in which polarization is measured by differencing the intensity in two linearly-polarized detectors, this effect enters at the level of the beam difference times the intensity gradient. However, for a correlation system such as CLOVER there is no direct comparison of the intensity in the two beams, and the effect is suppressed by the degree of cross-polarization rejection in the system. Again, it is the uncalibrated variation in this term that is important rather than its absolute value.
· Quadrupole leakage. This effect arises from the uncalibrated differential ellipticity of the polarization beams, and couples intensity to polarization via the trace-free curvature of the total intensity . Like the dipole leakage, the effect is zero in an ideal correlation polarimeter and is thus suppressed by the degree of cross-polarization rejection. It is invariant under rotation of the instrument, but if stable it does not contribute to the B-mode (it is pure E-mode).
Table 3.4: Systematic specifications Calibration 5% 0.5% Rotation 40 arcmin 4 arcmin Pointing 1.2 arcmin 7 arcsec Spin-flip (Q-U leakage) 5% (-13dB) 0.5% (-23dB) Monopole (I-Q/U leakage) 0.1% (-30dB) 0.01% (-40dB) Dipole (squint) 0.3% (-15dB) 0.03% (-25dB) Quadrupole (differential 0.4% (-14dB) 0.04% (-24dB) ellipticity)
Table 3.4 shows the levels to which each error type has to be stable and calibrated in order to achieve useful sensitivity to primordial B-modes, defined as the quadrature sum of the effects (in temperature) being less than one-quarter of the expected primordial B-mode signal if the 16 energy scale of inflation Einf = 10 GeV. The centre column gives the level to which each error type has to be stable/calibrated in order for the spurious B-mode signal so generated to 16 be equal to the true signal for Einf = 10 GeV; the right column gives the level required for the total spurious signal to be negligible for the same Einf. The most stringent constraints are for 0.5% relative calibration error, and –40-dB leakage of I to Q and U. We believe these constraints can be readily met, however, given the inherently low cross-polarization of the design and the stable operating environment at Dome C.
39
2.5: Scanning strategy
It is important to make accurate maps of Stokes parameter I as well as Q and U since instrumental cross-polarization will inevitably result in leakage between the Stokes maps. Although Q and U are extracted directly by the phase switching, I can only be derived by scanning the telescopes. This is necessary anyway, of course, in order to have sufficient sky coverage. The scanning strategy should also provide for a high degree of cross-linking i.e. each pixel should be observed many times in scans of different direction, and each scan should also ideally be made at constant elevation, in order to minimise differential atmospheric and ground-spill signals.
Figure 3.19: Noise per sky pixel including all horns for the MULTI- CROSS strategy per 24 hours.
These requirements have lead to the following strategy, which we call the multi-cross scan, consisting of observing a patch at a given right ascension and declination range, scanning over a fixed azimuth range while keeping the elevation constant for a 2-hour period. Once this time has elapsed, a point with the starting RA but slightly higher declination is retrieved and used as a starting point, and the procedure repeated. The azimuth range differs between patches but is constant for a given patch. The RA and dec range are chosen such that a clean portion of the sky is observed (see Section 1.4). The advantage of this scanning strategy is that it enables us to scan a clear portion of the sky while keeping the elevation changes minimal. Figure 3.19 shows an example sky coverage pattern around declination -55 degrees, where there are known to be areas of very low Galactic foreground emission in total intensity.
2.6: Site and Logistics
We propose to install CLOVER on one of the best mm and sub-mm observing sites in the world: Dome Concordia (Dome C) on the Antarctic Plateau. This choice was driven by the requirement that, because of the high frequencies and high sensitivity, we need the driest, most atmospherically-stable site, with good logistical backup. Four sites easily satisfy these needs: Mauna Kea, Hawaii; Chajnantor, Chile (the ALMA site); the South Pole; and Dome C. Mauna Kea and Chajnantor are high sites (4,100 and 5,000 m respectively) at low latitudes. The South Pole and Dome C are lower altitude (2,800 and 3,200 m respectively) but high latitude. Peterson et al. (2003) have compared Mauna Kea, Chajnantor and the South Pole and conclude that the atmosphere is more stable at the Pole than at the other two sites with a
40 lower mean opacity as well. There are scientific advantages in a polar site in terms of scanning strategies as well as the sensitivity gains. Of the Antarctic sites the choice is between the US-run South Pole and French/Italian Dome Concordia. The comparison in opacity between South Pole and Dome C is shown in Figure 3.20. This shows that Dome C is at least as good and arguably a better site than the Pole.
Figure 3.20: A comparison of 860-GHz (350-mm) opacity at South Pole (brown line) and Dome C(green) over a single season.
The final point in favour of Dome C over the South Pole is that the South Pole is a US-run operation whilst Dome C is a European one, and for this experiment we have already established a collaboration with Italian and French groups who have access to the site. In Figure 3.21 we show the position of Dome C with contours of altitude, slope, average snow accumulation and visibility, which all overlap there. The site is operated as a French/Italian collaboration funded directly by their respective governments. Access is from either the French or Italian coastal bases by 3-h flight from Terra-Nova for people and light equipment or 2-week overland tractor from Dumont-d’Urville for heavy equipment. It has been operational as a summer base since 1995 and the winter-over facilities are now fully operational as of this year.
Weather protection is clearly essential in such a harsh environment and our present concept, shown in Figure 3.22, is for the telescopes to be mounted on large concrete bases on the snow with a sliding cover for both the experiment, if required, and staff working on it. During operations very little maintenance is required since no liquid cryogens will be used, and it is intended that the experiments will run over the Antarctic winter. However the logistical support provided by the base will include not just power but a winter-over scientifically- trained support person for the experiment.
41
Figure 3.21: The position of Dome C on the Antarctic continent (left), and the recently-commissioned and operational accommodation facilities at Dome C (right).
We already have preliminary permission to begin a programme of CMB polarization observations at Dome C with our collaborators in France and Italy. An application is being made on a similar timescale to this one to expand that permission into the full CLOVER experiment. Once approved, all logistical costs such as transfer of equipment and people from New Zealand to Antarctica, on-site accommodation, equipment and consumables will be covered by the French/Italian Antarctic Agencies.
Figure 3.22: Possible arrangement of telescopes with sliding cover for weather protection and to allow sheltered work on the instrument.
42 3: New technologies
The major new technologies in this project are the detectors, which will define the state-of- the-art and the pseudo-correlators, which, to the best of our knowledge, have never been made in such large blocks before, and certainly not at such high frequencies. The liquid-free cryogenic system developed in Italy is also a new technology with wider application range.
4: Implementation of ClOVER/BRAIN at Dome-C
4.1: Introduction
We describe here a preliminary plan to implement the experiments BRAIN and ClOVER in DOME-C. Both the experiments are devoted to ultra-sensitive measurements of the polarization of the CMB. ClOVER is an imager, funded mainly in UK, while BRAIN is an interferometer, mainly funded in France and Italy. The two experiments make use of a number of common parts, and provide the necessary complementarity to produce a precise, robust detection of the B-modes of the CMB, allowing a deep investigation of cosmology in the first split-second after the Big Bang and of the related ultra-high energy physics.
A pathfinder experiment is being assembled and will run in Dome-C for the forthcoming seasons to assess a number of technical and scientific issues related to the implementation of ClOVER and BRAIN. During the first 3 seasons, the pathfinder will take atmospheric and calibration data.
ClOVER/BRAIN consist of three telescopes and the ancillary thermal, electronic, atmospheric control systems. During the installation phase we will also need a small electronics-lab and a small machine-shop.
In the following we list the Italian/French contributions needed to implement the experiments in Dome-C, in addition to our contributions to the hardware of the experiments.
4.2: Buidings/Construction
We plan to install the three telescopes on three towers, about 5 m high, at the verteces of an equilateral triangle with a side of about 8-10 m. The experiment should be located at a distance from the MZ station (at least 1 km, see below) and upwind in order to avoid contamination from the exhausts of the generators.
At the center of the triangle there will be - a shelter for the cryocoolers - a container for the electronics for telescope control, data acquisition/telemetry At small distance from the triangle there will be - a container for the electronics lab - a container for the small machine-shop - a shelter for the atmospheric monitor
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4.3: Electrical
Each of the 4 cryocoolers needs about 10kW, and additional 10kW are needed for the other electronics. This requires a dedicated generator, to be run at a distance from the telescopes. Our best suggestion is to install an additional generator of the same kind as the others already powering the MZ station, so that additional power can be used by the station itself or in emergency. A power line should run (Vutron 5 poles cable) from the MZ station to the experiment site.
4.4) Data Storage
The instrument will produce about 200 MBytes of data per day. In addition to local storage, we would request parallel storage in the station computers. Real-time Transmission of selected data to the laboratories in Europe will be done through IRIDIUM. We forecast to be able to transmit (at a non-negligible cost) between 1 and 5 MBytes per day, enough to monitor the operation of the experiments.
4.5) Calibration
A thermal calibration source will be lifted by a balloon at an altitude of 2 Km, at a distance of about 2 Km from the telescopes site. The position of the source will be monitored by means of a CCD camera.
4.6) Timing and Personnel
The pathfinder instrument will operate in 2005-2006. The first instrument of ClOVER will be ready to start operations in Dome-C in Nov.2006, the second in Nov. 2007 and the third in Nov. 2008. This means that in Nov.2005 we should install the pathfinder, the first tower and the containers, following with one new tower in 2006 and 2007. In 2008 the full system will start operations, at least till 2010.
This is a large experiment prepared by a large international collaboration, and needs adequate technical and scientific manpower on site during the Antarctic Summer. For the experiment itself we forecast that every campaign we will need 1 machinist and one electronics technician, plus a set of three scientists (telescope expert, cryogenic expert, data acquisition expert) for each telescope operated in that campaign. One additional person will be in charge of the atmospheric monitor.
In summary, for the summer campaigns we forecast campaign personnel activities 2005-2006 4 persons for 3 months (in mount pathfinder shelter common with BRAIN and test it; leave it operating personnel) during the winter setup the containers, the test tower, operate the test cryosystem
44 and take atmospheric data 2006-2007 6 persons for 3 months setup the second tower and first cryosystem, start data acquistion with the lowest frequency channel, and take atmospheric data s2007-2008 9 persons for 3 months setup the third tower and second cryosystem, start data acquistion with the second frequency channel, continue acquisition with the first channel and mantain it, take atmospheric data 2008-2009 12 persons for 3 months setup the third cryosystem, start data acquistion with the third frequency channel, continue acquisition with the first and second channels and maintain them, take atmospheric data 2009-2010 4 persons for 3 months mantain and recalibrate the three telescopes 2010-2011 4 persons for 3 months mantain and recalibrate the three telescopes
Logistic personnel support is very much needed in the setup phase: operations like moving the containers and the equipment and building the towers will require significant help from logistics operators. In the winter, during data taking, the personnel can be reduced to 1 scientist and 1 technician wintering-over.
45
5: Costs to PPARC
The costs of each workpackage are detailed in Annexes A and B and summaries of each of Staff, Equipment, Consumables, Exceptionals and Travel are also provided on a workpackage basis and also by institute. The summary costs and profile are given here as a cash planned total cost- using real increments plus a salary inflation of 3% - including working allowance but excluding contingency.
FY04/05 FY05/06 FY06/07 FY07/08 FY08/09 TOTAL £1292k £1438k £1260k £537k £76k £4.60M
6: Costs to Italy and France
The following is a rough draft, to be detailed with the PNRA and French institutions. The purpose of this table is to show that the contributions of Italy + France will be comparable to the already funded UK constribution.
46 7: Draft Memorandum of Understanding between UK, France, Italy
Memorandum of Understanding
1. This document establishes a Memorandum of Understanding for entry of the Italian and French institutions listed below into the ClOVER collaboration, currently involving the Universities of Cambridge, Cardiff and Oxford. The objective is to build a B-mode polarization experiment to be fielded in Antarctica at the French-Italian Concordia Station located at Dome C, Antarctica. Due to the high atmospheric stability and transmission at millimeter wavelengths, good meteorological conditions and potential for long, stable integrations, this site is recognized as being the best suited for the experiment.
2. The agreement provides for a 50-50 contribution (UK)-(France/Italy) to all the aspects of the experiment: funding, hardware, data/science analysis; Italy/France will have 10 PIs and Co-Is in total, a number equal to that from the UK. The 50-50 division will be reflected in all ``Steering'' and ``Management'' committees.
3. Italy and France are responsible for assuring access to the base. They will coordinate logistics with their respective polar institutions (PNRA-Italy, IPEV-France) in agreement with the requirements set by the “Project Management Board” (PMB), consisting of all the PIs and CO-Is . The primary budget contribution of France and Italy to the project will be the development, logistics and operation at Concordia Station; in addition, each of the two countries is responsible for funding their respective hardware contributions as listed below (items 4 & 5).
4. The French hardware contribution to ClOVER comprises
a. on-site data-processing and control electronics for the hardware developed by French ,
b. Arrangement of a broadband satellite link to Concordia Station, available to all the involved institution for support of winter operation and summer maintenance operations. Procedures for the support of the operation in the field will be arranged and a “Point of Contact” nominated before the starting of the first campaign.
c. In addition, France will engage in R&D effort on the superconducting phase switches (part of WP06) and, in parallel to efforts in Cambridge, on the TES+SQUID detection chain (WP07+WP08).
5. The Italian contribution to the hardware comprises:
a. R&D for the cryogenics systems in collaboration with Cardiff,
b. a stand-alone atmospheric monitor system and water vapour monitor
47 c. the annexed laboratories and the mount, that will be designed as a cooperative effort between all the parties, in order to allow the best integration of the experiment.
6. Italy and France will take full part in all science preparation (including simulations) and data analysis for the experiment.
7. The PMB will oversee project progress. The “Instrument Team” will coordinate hardware contributions, while the “Science Team” will coordinate data analysis efforts and scientific interpretation. A Site Manager will be nominated by the PMB, to act as liaison between the project and the polar institutes (PNRA-Italy, IPEV-France). Overall responsibility for the project resides with the Project Principle Investigator.
8. The minimal logistic requirements to be granted by the Concordia Steering Committee for the first 3 years of operation at the site are described in Annex 1: “Logistic Requirements for the Clover Experiment: 2005-2008”, see annex 1.
9. This MoU will only take effect only if and when the access to Concordia Station will be approved, in written form, by the Concordia Steering Committee, in any case not later than the 1st of June 2005.
List of Italian and French Institutions
Italian Institutions French Institutions Paolo de Bernardis, Silvia Masi Yannick Geraud Heraud, Michel Piat, Jim University of Rome, La Sapienza Bartlett – CdF Massimo Gervasi, Giorgio Sironi … University of Milan Bicocca
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References
Andoh H. et al., 2003, Int. J. Infrared and Kosowsky A., Loeb A., 1996, ApJ., 469, 1 Millimeter Waves, 24, 773 Kovac J. et al., 2002, Nature, 420, 763 Baralis M. et al., 2002, in Proc. XIV Riunione Lue A., Wang L., Kamionkowski M., 1999, Nazionale di Elettromagnetismo, Ancona, p. Phys. Rev. Lett., 83, 1506 318 Peiris H. V. et al., 2003, ApJ. Suppl., 148, 213 Baumann D., Cooray A., Kamionkowski M., Peterson J. B. et al., 2003, PASP, 115, 383 2003, New Astron., 8, 565 Pogosian L. et al., 2003, Phys. Rev. D, 68, Bennett C. et al., 2003, ApJ. Suppl., 148, 97 023506 Benoit A. et al., 2003, astro-ph/0306222 Polushkin V., Gu E., Glowacka D., Goldie D., Bernardi G. et al., 2003, ApJ. Lett., 594, 5 Lumley J., 2002, Physica C, 367,273 Bowden M. et al., 2003, astro-ph/0309610 Rees M. J., 1968, ApJ. Lett., 153, 1 Boyle L. A., Steinhardt P. J., Turok N., 2003, Rosenkranz P. W., Staelin D. H., 1998, Radio hep-th/0307170 Science, 23, 721 Brouw W. N., Spoelstra T. A. T., 1976, Scannapieco E. S., Ferreira P. G., 1997, Phys. Astron. Ap. Suppl., 26, 129 Rev. D, 56, 7493 Caprini C., Durrer R., Kahniashvili T., 2003, Seljak U., Zaldarriaga M., 1997, Phys. Rev. astro-ph/0304556 Lett., 78, 2054 Condon J. J. et al., 1998, ApJ., 115, 1693 Seljak U., Pen U.-L., Turok N., 1997, Phys. Duncan A. R. et al., 1997, MNRAS 291, 279 Rev. Lett., 79, 1615 Finkbeiner D. P., Davis M., Schlegel D. J., Smith K. M., Hu W., Kaplinghat M., 2004, 1999, ApJ., 524, 867 astro-ph/0402442 Giardino G. et al., 2002, Astron. Ap., 387, 82 Smith S. et al., 2004, Nucl. Instrumen. Meth, Gordon C., Lewis A., 2003, Phys. Rev. D, 67, in press 123513 Song Y.-S., Knox L., 2003, Phys. Rev. D, 68, Hanany S., Rosenkranz P., 2003, astro- 043518 ph/0307052 Spergel D. et al., 2003, ApJ. Suppl., 148, 175 Hu W., Okamoto T., 2003, ApJ., 574, 566 Steinhardt P. J., Turok N., 2002, Science, 296, Leach S. M., Liddle A. R., 2003, Phys. Rev. D, 1436 68, 123508 Tegmark M. et al., 2003, astro-ph/0310723 Lewis A., Challinor A., Turok N., 2002, Phys. Toffolatti L. et al., 1998, MNRAS, 297,117 Rev. D, 65, 023505 Tucci M. et al., 2003, astro-ph/0307073 Kamionkowski M., Kosowsky A., Stebbins A., Yassin G., Withington S., 1995, J. Phys. D: 1997, Phys. Rev. Lett., 78, 2058 Appl. Phys., 28, 1983 Keating B. G. et al., 2001, ApJ Lett., 560, 1 Zaldarriaga M., Seljak U., 1998, Phys. Rev. D, Kogut A. et al., 2003, ApJ. Suppl., 148, 161 58, 023003
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