Thomson scattering
Roberto Pasqualotto
11 February 2009
European Joint Ph.D Programme on Fusion Science and Engineering 2° Advanced Course in Lisboa, February 2009, On Diagnostics and Data Acquisition
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 1/181 OUTLINE
Theory: TS from single electron TS from plasma Æ Te & ne
TS measurement: experimental issues
TS diagnostic: main components
Examples of existing TS systems: RFX TCV Textor HRTS LIDAR JET
ITER core LIDAR issues & design
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 2/181 LASER-AIDED PLASMA DIAGNOSTICS
Laser-aided diagnostics are widely applied in the field of high-temperature plasma diagnostics for a large variety of measurements.
Various types of laser-aided plasma diagnostics exist, all based on different physical interactions between the electromagnetic wave from the laser and the plasma. In general one can distinguish interaction based on:
(a) absorption and/or reemission, (b) changes in the refractive index, (c) changes in the polarization ellipse, (d) scattering.
Incoherent Thomson scattering is used for highly localized measurements of the electron temperature and density in the plasma.
Coherent Thomson scattering yields information on the fast ion population in the plasma and/or depending on the geometry and wavelength chosen electron density fluctuations.
Interferometry and polarimetry are often combined in a single diagnostics setup to measure the electron density and the component of the magnetic field parallel to the laser chord.
Density fluctuations can be measured by means of phase contrast imaging, scattering, and various other laser-aided techniques.
A. J. H. DONNÉ, C. J. BARTH, H. WEISEN - FUSION SCIENCE AND TECHNOLOGY VOL. 53 FEB. 2008, p.397
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 3/181 LASER-AIDED PLASMA DIAGNOSTICS
Active diagnostics with lasers as the probing source have a number of distinct merits:
(a) the laser beam can be focused in the plasma, resulting in good spatial resolution; (b) the measurements do not perturb the plasma because of the relatively small interaction cross sections; (c) lasers have a high spectral brightness Æ good signals @ t,x,λ; (d) both with pulsed and continuous wave laser systems a good temporal resolution can be obtained; (e) the lasers (and in many applications also the detectors) can be positioned far from the plasma, where they can be more easily maintained.
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 4/181 Why Thomson Scattering?
• What is it? – Laser beam scatters off of electrons in the plasma – doppler effect gives wavelength shift
• Straightforward stand alone
measurement Laser beam ve (direct method: no models, assumptions,..) r to c te e • Electron velocity distribution D
directly observed (ne, Te)
• Accurate spatial location via imaging or time of flight
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 5/181 Thomson Scattering
• Scattering of electromagnetic radiation by a charged particle. • The electric and magnetic components of the incident wave accelerate the particle, which in turn emits radiation in all directions. • Phenomenon was first explained by J.J.Thomson. • It can be split into coherent and incoherent scattering (more later). • The experimental application of TS as a diagnostic tool had to wait for the development of high power light sources, e.g., the Q- switched ruby laser in the early 1960s. Since then, various plasma parameters have been measured by means of this technique. • The first demonstration of TS as a suitable diagnostic tool for hot plasmas was given by Peacock et al. in 1969 when they measured the electron temperature and density in the Russian T3 tokamak.
• Further developments: Te and ne along the full plasma diameter, resolving up to ~ 100 spatial elements with time separations of ~10 µs to 10 ms.
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 6/181 Role of Thomson scattering in
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 7/181 Thomson scattering spectrum
In this lesson the focus will be on:
Logic of steps to derive TS spectrum (less on math)
What can be measured
Under which conditions
John Sheffield, “Plasma scattering of electromagnetic radiation”, Academic Press 1975
S.E. Segre, “Thomson scattering from a plasma” Course on Plasma diagnostics and data acquisition systems, Varenna 1975,
P Nielsen, “Thomson scattering in high temperature devices” , Varenna 1986,
Some PhD Thesis: Rory Scannel (MAST), Alberto Alfier (RFX), R. Pasqualotto (RFX)
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 8/181 Thomson scattering from a single electron (classical limit of the Compton scattering)
Incident electric field electron ˆ - scattering of an incident photon r i Propagation by a moving electron (β=v/c) E & i ˆ scattering - electron energy is constant Incident θ k directions photon (Ee>>ħω) scattering sˆ angle Observer
The scattered radiation is frequency shifted as a double Doppler effect takes place, one in the reception, one in the emission of radiation by the electron:
1. the photon approaching the 2. the photon leaving the moving moving electron electron
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 9/181 TS as limit of Compton scattering
Conservation of energy and momentum
2 2 ħωi + mic = ħωs + msc ħki + mivi = ħks + msvs
2 ½ where: mi,s = m0/(1-βi,s ) The solution to these equations is: When incident wave has frequency ω such that 2 ħωi << mec 2 ωs = ωi (1-βi •êi) / (1-βi •ês + (1-cos(θ))ħωi/mic ) ÆThomson scattering, limit effect of Compton Scattering, 2 Ignoring the term ħωi/mec we get in which the quantum effect may be neglected: ωs – ωi = ∆ω = (ks -ki) • ve = k • ve the electron moves at same velocity as before
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 10/181 TS as limit of Compton scattering (some math)
Seen by the electron, initially stationary (vi =0):
With simple algebra:
In the TS limit
Transforming back to the lab reference system:
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 11/181 TS as limit of Compton scattering (some math)
Not relativistic Compton scattering
2 ħωi << mec 1 eV << 0.5 MeV 1 eV energy of a photon with λ = 1 µm
Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 12/181 TS from single electron
Incident wave electric field:
and associated magnetic field:
Force on the electron by the e.m wave:
with
Acceleration produced by this force
negligible if v< Such an accelerated electron produces an e.m. field. At an observation distance ρ large compared to electron displacement during measurement, electrostatic part (∝1/ρ2) is negligible. Radiative part (∝1/ρ) is the scattered e.m. wave: Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 13/181 TS from single electron distance electron – point of observation unit vector in propagation direction quantities in bracket evaluated at retarded time retarded time: delay between the photon emission and the moment at which it reaches the observer Phase of scattered field = phase of incident field (evaluated at ret- time) if v = const (influence of e.m. wave on electron is ignored and no static B field): ρ Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 14/181 TS from single electron The time dependent part of the phase indicates that the scattered wave is monochromatic, with a frequency ωs: Scattered radiation is still monochromatic Displacement in frequency proportional to the component of the e velocity in the k direction This expression is valid also at relativistic velocity. If we want to observe the drift velocity of a plasma, scattering geometry must be such that k ·vd ≠ 0 When Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 15/181 TS from single electron Only a flavour of full math formulation Incident wave electric field: and associated magnetic field: Equation of motion of the electron accelerated by the e.m wave: with Acceleration produced by this force Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 16/181 TS from single electron Only a flavour of full math formulation Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 17/181 TS from single electron Low electron velocity: non-relativistic approximation Standard geometry: 90° scattering ss the classical radius of electron Intensity of scattered wave does not depend on ψ and it is zero in the direction of the polarization (ξ=0) of the incident wave (not true if β finite) Max intensity when ξ = π/2 (s ┴ Ei) Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 18/181 TS from single electron Measured quantity is scattered power per unit solid angle: Is the Poynting vector Averaging over many periods, and using Incident intensity Is the Thomson scattering cross section Scattered power ∝ 1/m2 Æ in a plasma contribution from the ions is negligible : m_e = 10-32 kg m_p = 10-27 kg Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 19/181 TS from single electron non relativistic from Sheffield relativistic Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 20/181 TS from a plasma Total E given by contribution from each electron: Average scattered power First term: sum of power scattered by each electron independently of others Second term: contribution due to correlation between electron positions. = 0 if electrons randomly distributed For a plasma, typical correlation length is Phase very large and changes rapidly from electron to an other, when summing over distances of order λd Æ 2° term = 0, incoherent scattering Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 21/181 TS from a plasma • In (a) the phases do not add up incoherent coherent while in (b) the opposite is true • The scattering parameter is α = 1/kλd α >> 1 coherent scattering 2π 2π α << 1 incoherent scattering λ = λ = k k For hot plasma with medium density (Te = 1 keV; ne= 1019 –1020 m-3) using visible or NIR laser and with scattering angle 90°: α < 0.001 Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 22/181 Incoherent TS Electron velocity distribution function f(v) Electron contained in Contribute to total scattered power per unit volume, in frequency range If we define differential scattering cross section Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 23/181 Incoherent TS Thermal equilibrium: Maxwel distribution Assuming relativistic effects negligible Scattered spectrum is a gaussian centred on input frequency for a ruby laser Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 24/181 Incoherent TS - electrons are in thermal equilibrium (Maxwellian distribution) Te - relativistic effects are negligible the scattered spectrum has a Gaussian shape 3 2 ∝ ne 1 0 Visible or IR laser θ ~ 90° geometry -10 -5 0 5 10 λ -λ Laser The total power, integrated over frequency collected from volume ∆Ps_plasma = ∆Ps_e n ∆V where A area of beam cross section l length of scattering volume observed W0 = and the total collected energy ∆Es Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 25/181 Incoherent TS: relativistic effects - depolarization Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 26/181 Incoherent TS: relativistic effects – blue-shift Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 27/181 Incoherent TS: relativistic effects – blue-shift Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 28/181 Incoherent TS: relativistic spectrum Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 29/181 Incoherent TS: effect of B Spectrum scattered from single electron: series of lines centred on the line at frequency and separated by ωc From a maxwellian plasma: Modulation distinguishable Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 30/181 TS measurement: experimental issues TS attractive as diagnostic tool for plasmas: Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 31/181 TS measurement: experimental issues Incoherent TS: spectrum depends on 7 Scattered Spectra o •Electron density, n 6 Selden-Matoba, θ=180 e y 5 •Scattering angle, θscat 0.5keV •Laser wavelength, λ 4 5keV 0 10keV •Electron temperature, T 3 e 20keV 2 40keV Spectral Intensit 1 0 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 Normalised Wavelength Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 32/181 Imaging Thomson Scattering Laser Plasma • The scattered light is (pulse) imaged from the plasma • A ’spectrometer’ disperses the light Collection optics • A set of detectors collects the light • The data is digitised and analysed Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 33/181 Experimental issues Critical aspects: - low cross section Defines the time resolution - low collection angle (50Hz) - detection of the scattered radiation Define the spatial resolution TS (<1 cm) N photons −13 inc =10 N photons Background noise: - plasma light: broadband radiation - stray light from baffles and dumps: monochromatic radiation (at the laser wavelength) One of the most fundamental but critical diagnostics in fusion experiments Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 34/181 Detected signal Detected power, over entire spectrum: Photons entering the spectrometer 0.025 0.1 keV 0.5 keV 1 keV 0.02 2 keV 10 keV 0.015 a.u. l = 10 mm 0.01 # photoelectrons 0.005 detected/channel: 0 800 850 900 950 1000 1050 1100 1150 Fraction of spectrum detected by wavelength (nm) i-th spectral channel: 10-20 % 102 – 5x103 Balanced # of spectral channels: low to maximise signal, high to maximise resolution (min 2) Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 35/181 Signal errors Poisson statistic of photoelectrons (p.e) Detector noise: dark noise Æ noise equivalent number of p.e. multiplication noise Æ noise factor F Æ equivalent number of p.e. N* = N /F Background plasma light (most dangerous at high frequency: plasma fluctuations, ELMs): It can be 100-1000 x Bremmstrahlung contribution: Stray light: monochromatic λi from diffusion from inner wall, windows and optics rejection R = 104-5 usually required in spectrometers to sufficiently reduce it however it is quite reproducible Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 36/181 0.025 0.1 keV 0.5 keV 1 keV Signal simulation 0.02 2 keV 10 keV 0.015 a.u. 0.01 Npe Npe 0.005 0 800 850 900 950 1000 1050 1100 1150 ch.3 wavelength (nm) ch.2 ch.1 ch.4 EDGE CORE σ (%) σ (%) Te ch.4 3 2 1 Te ne ne keV keV Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 37/181 Te and ne from relative & absolute calibration Te: from the relative Ch 4 Nd:YLF sensitivity of the 4 spectral 1053n m Ch 3 channels Ch 2 Ch 1 relative spectral response of a spectrometer Normalized transmission TS ne: from the absolute spectrum sensitivity of the 4 spectral 3.5·1019m-3 channels 50 eV 3·1019m-3 Rotational Raman Scattering 1000 eV in N2 Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 38/181 Te: relative calibration Measure the relative transmission function of spectral channels in each spectrometer Standard technique: CW light source + monochromator + calibrated energy monitor Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 39/181 Te: relative calibration Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 40/181 ne: absolute calibration Rotational Raman Signal induced by the laser in N2 gas Torus filled with 50-500 mbar Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 41/181 ne: absolute calibration Two main issues affect its The dependence validity and make it very of signal on the difficult: pressure gives the abs. cal. - Laser misalignment - plasma deposition on collection window (may influence also relative calibration) Rayleigh scattering Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 42/181 Calculation of Te & ne Yi = measured signal of channel-i yi = theoretical signal of channel-i, given Te, ne σi = experimental errors B ∝ 1/Te Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 43/181 Calculation of Te & ne only depends on Te non linear minimization Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 44/181 Realising a TS System Lasers Spectral Calibration Plasma Measurement Density Calibration Scattering Collection of Light Spectral Analysis Data Acquisition Data Analysis Results Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 45/181 TS diagnostic: main components We’ll look now at main components of a TS diagnostic: Laser Collection optics Spectrometer Detector Data acquisition Analysis Then we’ll see examples of working systems Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 46/181 laser Most present TS experiments employ Q-switched ruby or Nd:YAG lasers as the source. The ruby laser operating at 694.3 nm produces outputs up to 25 J in 15 ns However, their repetition rate is usually rather low: 5 Hz (1 J / pulse). When more than several pulses per minute are required, an intracavity ruby laser can produce a burst of high-energy pulses (~15 J/pulse, ∆t~ µs) with a repetition rate of ~10 kHz (see Textor). Ruby lasers are usually employed in systems where good spatial resolution is preferred above a high time resolution. The most frequently used system for periodic TS measurements is based on the application of Nd:YAG lasers operating at 1064 nm, with outputs of ~1 J, 15 ns and a repetition rate of 20 to 50 Hz. Combining a set of lasers the repetition rate can be increased. The beam divergence of both types of lasers is ~0.3 to 1.0 mrad. The polarization of the laser beam is chosen perpendicular to the scattering plane. The high laser powers require special precautions for the used optics. Laser beam diameters should be kept large enough such that for 15-ns pulses the energy density is below the damage threshold of ~5-10 J/cm2. Transmitting surfaces need to be coated and tilted with respect to the beam propagation to Prevent losses and back-reflected light entering the laser system again. Furthermore, curved transmission optics should have concave entrance surfaces, to prevent focusing of the back-reflected beams (which might lead locally to very high power densities). Other types of pulsed lasers (e.g., Ti:Sapphire and Alexandrite lasers) have been proposed for TS (e.g., for ITER). Nevertheless, there are not yet applications of these sources to present devices. Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 47/181 Q-switched ruby laser in RFX Pockels cell Brewster angle TEM00 oscillator mode selection polarizer 8 x 1/4" ruby aperture 45° steering mirror 2 flashlamps etalon output R = 5 m rear mirror coupler R = E = 35 mJ (2 x 20 mJ) E = 1 J (2 x 0.75 J) spatial filter 7 mW HeNe E = 15 J (2 x 12 J) E = 10 J (2 x 7 J) 300 µm 45° steering mirror Amp. 3 Amp. 2 Amp. 1 pinhole 4" x 22.5 mm ruby 8 x 5/8" ruby 8 x 3/8" ruby 4 flashlamps 4 flashlamps 4 flashlamps F = –67 cm lens F = 30 cm lens • Ruby laser (λ = 694.3 nm) • TEM00 oscillator (35 mJ, 25 ns, single pulse) • 3 amplifiers (15 J, 25 ns, θ ≤ 0.4 mrad ) Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 48/181 laser • Modern TS systems use multiple lasers (typically up to 8) • These can be bunched to tackle different physics and provide redundancy Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 49/181 Stray light reduction The laser beam enters and leaves the plasma vessel through vacuum windows. Passing these windows— especially the entrance one—generates stray light, which can reach the collection optics. Without precautions this stray light level can be six to eight orders of magnitude larger than the TS light. Reduction of the vessel stray light can be achieved: by tilting the windows (placing them under the Brewster angle is very effective), by positioning them relatively far from the plasma, by using baffles in both entrance and exit ducts, and by mounting a viewing dump on the vessel wall opposite to the collection optics. A very effective light trap (reduction up to 100 times) can be made from a stack of knife-edge blades. Carbon tiles on the inner wall of the plasma vessel can give a reduction by a factor of ~ 20. Finally, the laser beam is dumped on e.g. a piece of absorbing glass placed under the Brewster angle. Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 50/181 Collection optics Scattered light is collected after passing a vacuum window and subsequently relayed to a spectrometer. Because of the low scattering yield, the transmission of the collection and relay optics should be of course as high as possible. In devices with hot plasmas, a shutter is required between the plasma and the window to reduce deposition of all kinds of materials on the inner window surface during the times the diagnostic is not in use. Various kinds of optics are used to collect the scattered light. These systems are used to guide the TS light to the spectrometer. Basically, there are two possible ways to guide the scattered light from the plasma to the detection system: via flexible fibers and via conventional optics (lenses and mirrors). The main advantage of fibers above conventional optics is that the linear etendue of the source can be matched to that of the detector, albeit at the cost of reduced spectral resolution. For this purpose the fiber array is rearranged such that the slit height is reduced and the slit width increased, for example, by a factor of 2. As a result, the usable solid angle of the collection lens increases by a factor of 4. However, for TS diagnostics at small-sized plasma devices where the detection system can be positioned relatively close to the plasma (≤10m), conventional optics gives a better transmission (up to a factor of 3) than fiber optics. For systems with a comparable length of the optical path from collection lens to spectrometer, e.g., the multiposition TS systems of JFT-2M and RTP, the overall transmission is larger for systems using conventional relay optics (RTP: 25%) than for those using fiber optics (JFT-2M: 7%). Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 51/181 fibres The major contributions to the losses in fiber-optic systems are the core-cladding ratio, the packing fraction, the attenuation, input and output reflection losses, and an increase of the exit cone. For fiber-optic arrays, transmissions of ~55% and even higher have been reported. Despite the lower transmission, fiber-optic systems have to be preferred when the scattered light needs to be relayed over longer distances (e.g., to get outside the biological shield of the plasma device). To bridge long distances with conventional optics would require many large-sized lenses and mirrors, resulting in a low transmission as well. NA = 0.37 Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 52/181 Spectral analysis Mainly two different techniques to disperse the scattered light are in use for TS systems: filter and grating spectrometers. In filter spectrometers, the scattered light is separated into different wavelength bands by means of a cascade of interference filters. The number of separate wavelength channels in these systems is usually rather limited (three to eight channels), and therefore, the interpretation of the data relies on the assumption of a Maxwellian electron velocity distribution in the plasma. In grating spectrometers a grating is used for dispersing the scattered light. Both mechanically and holographically ruled gratings are used for this purpose. In this case, the number of independent spectral channels can be quite large: up to 80 for the TVTS system on TEXTOR. In case of good photon statistics, this enables one to determine the shape of the Maxwellian distribution. To prevent the residual of the vessel stray light from disturbing the TS spectrum, the laser wavelength should be carefully filtered out after dispersion has taken place. This can be done by blocking the laser wavelength, by reflecting light at this wavelength away from the detector, or by focusing it onto a special detector. Both filter and grating spectrometers have typical stray light rejection ratios of 10-4 to 10-5. Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 53/181 Spectral analysis Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 54/181 grating Littrow spectrometer in RFX interference notch filter (30° incidence) achromatic doublet a.r. @ 694.3 nm holographic input fiber grating optic bundle (φ = 120 mm, f/3.4) entrance slit 7600 Å flat spectral interference notch filter 5400 Å plane (normal incidence) • input: 10 bundles of optical fibres • Concave holographic grating with flat field (F/3.4) • Interferential notch filters at λ = 694. 3 nm (R= 4x 10-3) • Stops on the focal plane at λ = 656.3 nm (Hα) and λ = 694.3 nm. -4 -6 • Rejection: RHα= 2x10 , RRuby = 10 , T = 30% Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 55/181 grating Littrow spectrometer in RTP & Textor Scattered light is collected by an F/19 achromatic doublet (item 3) and guided to a Littrow polychromator where the light is detected after dispersion. A field lens (item 4) and a spherical mirror (item 8) serve for pupil imaging. The Littrow lens ~F/12.5 (item 6) collimates the incoming light beams and focuses the dispersed light at the two-part mirror (item 8), giving a two dimensional image (λ, z). This image is projected onto the GaAsP cathode ~18% tube efficiency of a 25 mm image intensifier by means of a Canon 50 mm, F/0.95 TV objective. Finally, the phosphor screen of the intensifier is imaged at the cathode of two ICCD cameras (item 13) by a coupling lens system that consists of three F/1.2 Rodenstock objectives (item 11) and a beam splitter (item 12). Double pulse operation is feasible with this system. Light emitted by the phosphor screen of the GaAsP image intensifier (item 10) and generated by the first laser pulse is recorded by one ICCD camera by gating it open during 20 µs. The second ICCD camera is gated open at the moment of the second laser pulse, again during 20 µs. Time separation is typically 100–800 µs. The ICCD integration time is made 20 µs to keep the overlap of the two pulses as small as possibel (~ 7% for 100 µs separation). Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 56/181 Detection and data acquisition In general, two different types of detection systems can be distinguished: time-resolving single- element or multielement detectors, and signal-integrating multielement detectors. The first category includes avalanche photodiodes (APDs) employing the high quantum efficiency of Si between 500 and 1000 nm, photomultiplier tubes (PMTs), photodiode arrays, and multianode PMTs. These systems enable time resolutions of the order of the laser pulse duration of 15 ns. As a result plasma light can easily be sampled just before or after the laser pulse. TS systems using periodic Nd:YAGlasers mostly apply APDs for detection of scattered and plasma light. The signals of photodiode detectors are recorded with charge-integrating analogue-to-digital convertors (ADCs) or by means of fast transient recorders. Time-integrating detectors have a lower time resolution and are called TV systems because the detection principles are similar to those of a television camera able to receive a two-dimensional image.Vidicon, charge coupled device (CCD), complementary metal oxide semiconductor CMOS, and streak cameras belong to this category. These cameras have large numbers of image pixels, e.g., 106 to 107. The low readout time can vary for different types of cameras. For a 16-bit CCD camera, the readout time can be ~1 s, while ultrafast CMOS cameras sample with frame rates of 104 images/s at a 12-bit dynamic Range. The scattered light of the short laser pulse is captured with a gated image intensifier coupled to the TV- like recording system using a lens system. Data detected with TV systems are usually stored in internal memories and after termination of the plasma discharge are sent to a computer for analysis. Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 57/181 Detection and data acquisition Both PMT’s and TV detectors employ different kinds of photocathode materials to improve the photon to electron conversion process. PMT and image intensifiers are equipped with GaAsP, S25, extended S20 cathodes to reach high conversion efficiencies in the visible and near infrared wavelength ranges. The signal-noise S/N ratio of a detector directly depends on this conversion (quantum) efficiency: where Npe denotes the number of photoelectrons generated by the incoming photons (Nph) and η_conversion denotes the efficiency of the conversion from photons to electrons. However, the S/N ratio of the complete detector will be lower because of the noise added by the amplification and readout processes. More useful for evaluation of a detector is the effective detector efficiency, which includes the noise factor: η_det = η_conversion /noise factor. The noise factor refers to noise increase in the detector caused by the amplification process. The S/N ratio of a complete detection system is determined by the statistical noise, the dark current of the detector and background signals, as plasma light and stray light. Plasma light due to bremsstrahlung and line radiation can be easily corrected for when photodiode detectors are used. Using TV systems in combination with fiber optics for light relay offers the ability to sample plasma light from an area just next to the laser beam and guide this to the same detector for simultaneous recording. Alternatively, one can measure the plasma light just before and after each laser pulse. The contribution of plasma light can be kept negligibly low when the laser energy is high (>10 J) and the sampling interval is almost as short as the laser pulse (40 ns, for a pulse with 15 ns full width at half-maximum (FWHM)) . Grating spectrometers combined with TV systems result in a large number of spectral channels, which enables line radiation to be suppressed.. Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 58/181 Gated acquisition delay Plasma light + Plasma light only TS pulse + Stray pulse delay Plasma light fluct. + Plasma light TS pulse + Stray pulse Fluctuation only Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 59/181 Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 60/181 Multianod MCP photomultipliers in RFX position (1-10) spectral channel (1−10) VBias VMCP Vanode e- • 40 mm S20 photocathode (Q.E. = 7 % @700 nm). - e- e - 5 2 e- e • V-stack MCP (G= 10 @ 1800 V, j = 230 µA/cm , e- - s Photons e e- recovery time τ = 10 µs). e- e- e- • Array of 10 x 10 anodi e- • Insensitive to B (3mT in spectrometer). • 100 parallel channels in one detector. Photochathode MCP Anode Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 61/181 Photocathodes R&D Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 62/181 Examples of working systems: Details of experimental setup Measurements Physics studies Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 63/181 main TS at RFX Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 64/181 main TS at RFX: layout TS signal collected Fiber optic delay line multiplex through 3 ports (∆l=15m Æ ∆t=70ns): 3 positions/spectrometer 28 filters+APD polychormators 4 spectral channels ND:YLF laser (λ=1053nm): •E ~ 3J • Pulse length ~20ns FWHM • 10 pulses, 50 Hz 84 scatt. volumes on equatorial diameter (-0.95 Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 66/181 Polychromator Objects Field Z-pos adjustable Imaged on filters Imaged on detectors Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 67/181 Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 68/181 Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 69/181 Upgraded TS diagnostic TS su RFX TS su RFX-mod Improvement Time 1 profile per 10 profile per from Rb laser (693nm) to custom built resolution discharge discharge (<50Hz) Nd:YLF laser (1053nm) Spatial 10 points 84 points Optical delay lines resolution 8cm 1cm (3 points per spectrometer) from gated MCPs Acquisition raw signals gated signals to interference filters spectrometers system 0.5GHz with digitizers RFXRFX RFX-modRFX-mod Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 70/181 Results obtained on RFX-mod Te profile during different plasma states and in various scenarios: 800 T (eV) 19532 @ 95ms e 19532 @ 45ms 600 400 partially stochastic 200 ordered plasma core plasma core 0 -400 -200 0 200 400 Radius (mm) Tomographic reconstruction of SXR emissivity (pressure) profile in a poloidal section Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 71/181 RFX: Te from TS & double filter Double-foil Te 1. the entire profile is pumped up for Double foil and TS Te profiles all the QSH cycle; #22159 @ 199ms Double-foil Te in 2D Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 72/181 HRTS at JET Similartothe Main TS on RFX-mod -Te and ne profiles along external radius (R=2.9-3.9m); - maximum spatial resolution of 15 mm in 63 positions on 21 spectrometers with optical delay lines; - time resolution of 20Hz (Nd:YAG laser - 5J ); - partially share the laser path of the other TS system (red path) - interference filters spectrometers and digitizers Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 73/181 2. HRTS layout in torus hall HRTS system currently operational: - outer radius covered - 61 points, 1.5 cm sampling resolution - 20 Hz repetition rate, full JET pulse lens Vacuum Paraboloidal window mirrors & fibres (126) Laser beam Scattering volume Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 74/181 2. Imaging optics • double vacuum window, 192 mm diam, F/25 • imaging lens and 2 motorised mirrors • the lens images scattering volumes to West Wall first mirro • 5m optical bench on West Wall holds r 126 paraboloidal mirrors (3x4 cm) • each mirror images the lens onto one fiber Fiber Paraboloidal optic mirror • one fibre corresponds to 8 mm scattering volume second lens mirror fibres Vacuum window 192 mm diam Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 75/181 2. Laser Nd:YAG (λ=1064 nm) laser from Quantel (France): 2 beams vertically displaced E = 2.5 J / beam Repetition rate 20 Hz Full remote control Beams profile Burnspot on paper Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 76/181 2. Polychromators • 21 filter polychromators with avalanche photodiodes (APD) 7 fibers bundle APD + amplif from GA / PPPL • 4 spectral channels • Two sets of filters: 7 for the edge (Te = 30 eV - 3 keV) 14 for the core (Te = 0.2-15 keV) Lens + interf filter Amplifiers from PPPL: core • AC output for TS signal: - lower frequency cutoff (τ = 5 µs) filters plasma background light, - upper frequency limit allows to separate 650 750 850 950 1050 nm 3 time-delayed signals, 50 ns apart • DC signal for plasma background light edge 800 900 1000 1100 nm Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 77/181 2. Acquisition system Waveform recorders (oscilloscopes): AC output (TS) into 1 GS/s, 150 MHz, 8 bit. DC output (plasma background) recorded at 1kHz, 12 bits. Data acquired between laser shots : real time acquisition 3 positions / polychromator with optical delay lines: 2 fibres/position (15-20 mm spatial resolution) 20 m 20 + 30 m (150 ns) 20 + 60 m (300 ns) Fiber bundle into each polychromator ns TS pulses from 3 positions into same channel Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 78/181 4. Project schedule 2001 2002 2003 2004 2005 2006 2007 2008 Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 79/181 4. Project evolution: 2005 First TS measurements on 4th October 2005 (JPN 63804) Green: with plasma; blu: without plasma Spectrometer 16 (core) JPN 63863 (dry run) & 63865 Spectrometer 7 (edge) ch1 Raman in ch1 air White stray ch2 TS signals ch2 TS signals from inner wall (dump) ch3 ch3 ch4 ch4 Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 80/181 4. Project evolution: 2006 Improvements in 2006 End 2005: good sensitivity demonstrated, but spurious pulses pollute TS signal During 2006, general upgrade of the system (operation restarted in October) - Broadband stray light nearly cancelled by enlarging the laser beam on the dump - Monochromatic stray light nearly cancelled by tilting last filter in spectrometers - Edge spectrometers realigned and all recalibrated with more accurate procedure - Fiber optic delay lines installed with long delays to avoid problems with spurious pulses, but with smaller number of positions (37 instead of 60) - Raman calibration improved - Analysis programs finalised - Protection system for laser beam risking to damage optics: burst max duration 20/10 pulses Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 81/181 4. Project evolution: 2006 TS improved measurements in November 2006 (1) Main improvement: both monochromatic and broadband stray light nearly Spectrometercancelled 7 blu: with plasma; green: without plasma Spectrometer 5 ch1 ch1 ch2 ch2 ch3 ch3 ch4 ch4 20 m 20 m Delay lines 20 + 30 m (150 ns delay) 20 + 30 m (150 ns delay) configurations 20 + 60 m (300 ns delay) Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 82/181 4. Project evolution: 2007 Te and ne profiles in March 2007 The new HRTS system compared to existing electron diagnostics at JET - The systems are in fair agreement - All systems show single profiles, but core LIDAR averaged over 5 profiles (1 s) ne Te Nucl. Fusion 48 (2008) 115006 Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 83/181 4. Damaged optics Laser produces 2° lens, damaged 2 beams, vertically displaced. If last amplifier doesn’t work: Burnspots: beam divergence is changed and beam focuses on 2nd lens because each amplifier has a thermal lensing effect on the beam Laser NORMAL Laser FAULTY Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 84/181 5. Position calibration • Scaled ruler positioned along laser beam path by remote handling • Each fibre is back-illuminated: an image of collection mirror is produced • Circular spot with diameter: average separation=8 mm Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 85/181 5. HRTS single profile measurements LIDAR HRTS Single profiles are now of good quality 1.5 cm sampling resolution across full profile Temperature LIDAR HRTS Æ Pedestals are very steep! Æ Only one point in barrier Density Spatial profile Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 86/181 5. Time evolution Time evolution of Te and ne by KE11 at fixed position (R = 3.2m), compared with core LIDAR (ne) and ECE (Te) Pulse #73634 HRTS LIDAR Electron Density HRTS Electron Temperature LIDAR ECE Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 87/181 5. Profile comparison ELM mitigation through impurity seeding: the pedestal pressure stays about constant during the type I ELM phase and then collapses after Frad ~ 50 % during the type III ELMs Average over 3-4 measured profiles Nucl. Fusion 48 (2008) 095004, M.Beurskens et al. Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 88/181 5. ROG sweep to improve spatial sampling 1.5 cm ROG sweep ROG sweep and pre-ELM data selection Increases data sampling: Æ Pedestal width analysis is possible Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 89/181 5. Diagnostics comparison ne: HRTS vs reflectometer Te: HRTS vs ECE Agreement with ECE is often very good Agreement with preliminary data from KG8A However a shift of the ECE profile is required is also good 1.7MA/1.8T r/a KK3 pre-ELM (shifted 8 cm) (see movie) HRTS pre-ELM Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 90/181 5. RLCFS from HRTS A problem we encountered is that the absolute position calibration got lost (presumably the position stepper motors moved during shutdown) Æ Before position loss a good agreement was found between EFIT LCFS and HRTS pedestal foot in 2007. (Nucl. Fusion 48 (2008) 115006, A.Alfier et al.) Æ Use EFIT LCF position as reference for absolute calibration EFIT and HRTS agree in LCFS within +/- 0.5 cm Pedestal fit: R =C+1/2δ LCF 3.84 3.83 R = C+1/2δ EFIT 20 lcfs 3.82 3.81 R_LCFS (m) ROG sweep HRTS (kPa) 3.8 e 10 1.0 (single P 0.5 73344 profile fits) 0 0 3.7 3.8 3.9 -0.5 discrepancy (cm) -1.0 55 56 57 58 59 60 61 62 63 64 65 Time (s) At start of campaign the position was not right and the profile was shifted to match LCFS. But technique was validated when we had absolute calibration in 2007 Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 91/181 5. Edge filaments Filaments in the edge during the ELMy H mode phase. Evidence confirmed by the fast visible camera paper IAEA-CN- EX/P3-4 (2008), M.Beurskens et al. Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 92/181 Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 93/181 The TS diagnostic(s) on TCV Main TS (blue): - 25 spatial channels (3 cm) - 40Hz time resolution (2 x 20Hz 1J Nd:YAG) Edge TS on loan from Consorzio RFX: - 9 spatial channels (1cm) -sharing the same laser Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 94/181 H-mode and ELMs H-mode (high confinement regime): a transport barrier develops at the edge, called “pedestal” Æ enhanced confinement properties ELMs: MHD instabilities appear at the edge when the edge pedestal gradient overcome a stability threshold (Hα lines) TS laser ∆tEL Released energy and particle M may damage plasma facing components Æ their control is crucial Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 95/181 Analitical fit of the edge profile The edge profiles is fitted with a modified tanh fit (5 parameters) X ⋅e− X F(X ) = a(5) − a(1)⋅ tanh X − a(1)⋅a(4)⋅ e X + e− X where X is the normalized coord.: R − a(2) X = a(3) Pedestal parameters a(1)+a(5) : height 2·a(3) : width a(1)/a(3) : slope Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 96/181 Type-III ELM on TCV In the narrow time window ±150µs around the ELM peak : - Relaxed : monotonic slope, no clear sign of a pedestal a significantly smaller gradient than normal profiles; - Bumpy : bumps at the LCFS; - Normal. normal relaxed bumpy Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 97/181 Results from single profile fit method Time evolution of pedestal height and width during an ELM phase: - drop of Te(15%) and ne(35%) – Te drop 500µs after the ELM peak - sub-ms recovery time scale - transient region not available, profile deviates from tanh fit Transient phase of the ELM Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 98/181 Time evolution during ELM cycle from coherent averaging 1. Single profiles are grouped in bins with respect to their ∆tELM (quasi-stationari condition); 2. the tanh-fit is then avereged out From two bins Results obtained with the single fit method are confirmed From several bins Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 99/181 Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 100/181 Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 101/181 Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 102/181 Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 103/181 Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 104/181 RFX: edge TS Why an edge TS on RFX-mod? Main TS Outer edge region: Edge TS - edge physics is influenced by the active MHD control system - not covered by the main TS - Time resolution: 1 shot per pulse with a Ruby laser (7J @ 694nm, 30ns at FWHM); - Spatial resolution: 1 cm resolution on 12 measuring points; - Dispersion system: Intensified CCD spectrometer measuring from few eV to 500eV; - Novelty: the input system and collection window are on the same mechanical structure: easier alignment Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 105/181 RFX-mod: TS systems (1/5) Main TS RFX-mod - Profiles @ 50Hz; RFX - 84 spatial points @ 10mm; 19 -3 -Te = 20 – 1500eV and ne > 10 m . Plasma Edge TS (is being commissioned) -Single profile; Vessel wall - 12 spatial points @ 10mm resolution; 19 -3 -Te = 3 – 300eV and ne > 0.3·10 m . Ruby laser Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 106/181 Input system The entrance port hosts the A ruby laser is focused on a input system & the collection 3mm pin-hole in vacuum. window Æ stable alignment. 2 Vacuum 3 chamber Pin-hole 4 1 Ruby laser beam Sapphire prism deflects the beam by 30°; a sapphire lens images the pin-hole in the vacuum vessel. Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 107/181 Collection system Schematic top view Image of the back illuminated fibers with the extracted structure during the alignement process 4 points for measuring BKG (13-16) 12 points for measuring He-Ne laser to TS spectrum (1-12) trace the path Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 108/181 The spectrometer E D F I.I. K G A 8° CCD B Fiber C bundle Energy I.I. CCD monitor fiber controller controller A: 4x4 fiber pattern D: three square spherical mirrors f = 200mm B –G –K: camera lenses F: one square lens f = 400mm C: four square lenses f = 400mm J: energy monitor fiber I.I: Image Intensifier, ∅25mm CCD : 578x385 pixels, 22µm x 22µm pixel size, 1.5ms frame transfer Transmission 2eV functions 500eV Expected accuracy of Te and ne measurements Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 109/181 10 kHz Repetitive High-Resolution TV Thomson Scattering System for TEXTOR * Intracavity laser with three bursts of 50 – 100 pulses with 15 J each * Ultra fast detector with CMOS camera * expected performance: errors on Te ~ 8% and ne ~ 4% @ 0.05 ≤ Te ≤ 2 keV & 19 -3 ne = 2.5×10 m Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 110/181 • Double-pass system Achieved • Number of bursts 1 • Number of pulses 10-12 • Number of back and forth passes per pulse 10 • Lens-spher. mirror space 8.5 m • Effective cavity length 18 m • Pulse probing energy 12-23 J • Pulse probing power 6-12 MW • Total probing energy 150-220 J • Pulse duration 0.002 ms • Pulse interval 0.200 ms • Beam divergence 0.7 mrad • Beam chord in plasma 900 mm • Probing region diameter 2-5 mm Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 111/181 Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 112/181 plasma light image Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 113/181 Plasma light and TS spectra Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 114/181 TS spectrum Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 115/181 Sequence of profiles in a burst Temperature profiles Density profiles Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 116/181 Temperature profiles through 2 phases of an m=2 island Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 117/181 Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 118/181 LIDAR •LIght Detection And Ranging Plasma • ‘Point and shoot’ method er Las lse) • So required access minimised rt pu (sho • Short pulse of light transmitted to Mirror labyrinth the JET - ITER plasma. • The back-scattered light is collected and analysed. • Note the spatial extent is recovered by the time delay. Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 119/181 LIDAR • Spatial resolution means short pulse lasers and fast detectors are required e.g. ITER requires ~7cm Laser Pulse Plasma, Length L Scattered Light Scattered Light Note that the profile length in time is dt=2L/C. Effectively 15cm/ns! Detector and laser response defines spatial resolution •7cmis equivalent to ~460ps combined laser and detector response time (so det/laser response ~300ps FWHM) Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 120/181 Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 121/181 LIDAR Thomson Scattering Principle Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 122/181 Scattered signals at different times Gives Te and ne at different positions Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 123/181 Advantages over more conventional 90 degree scattering geometry • One set of (6) detectors for all spatial positions – easy calibration • 180 degree geometry makes alignment simple – easy to maintain • All sensitive components can be outside biological shield – easy access • Because of time localisation, stray laser light pulses can be traced to particular objects – easy (ish) to remove • Fast detectors means background plasma light level is low – (can’t think of anything easy about that – easy to subtract perhaps?) •BUT • Spatial resolution? – not so easy Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 124/181 LIDAR r ne ase Te L c.t x= λ Space resolution: Collection 2 2 2 2 2 optics ∆x = c .( tlaser + tdet + tdaq )/4 tlaser=300ps tdet= 600ps tdaq= 400ps x=c.t/2 ∆x= 12 cm Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 125/181 LIDAR at JET KE1 was the first TS on JET, ready nearly from the start of operation in 1983. The temperature measurement on JET were based mostly on ECE. TS was required to keep it honest. The KE1 system was designed with this in mind and the fact it was only single point improved S/N in any case as the laser could be more focussed. First time that all essential components were outside Torus Hall. Limited access. Long distances. Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 126/181 LIDAR at JET The idea of LIDAR was around at the Varenne meeting i 1982 Fortuitously it required only minor changes of KE1 to implement the system LIDAR improves S/N by a factor >100 from the shorter integration time. This factor is reduced by ~ 10 due to a larger etendue required. Weakness: limited resolution Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 127/181 JET LIDAR laser Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 128/181 JET LIDAR polychromators Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 129/181 Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 130/181 JET LIDAR detectors Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 131/181 JET LIDAR raw data Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 132/181 JET LIDAR profiles Pedestals as measured with ECE, Li beam, LIDAR and CXS Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 133/181 Divertor LIDAR at JET Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 134/181 Mapping on flux surfaces If we can assume that Te and ne are constant on a flux surface and If we can align our LIDAR system so that the angle it’s line of sight makes with the flux surfaces, instead of being perpendicular, is much closer to tangential then Although the spatial resolution is still 12 cm along the line of sight, perpendicular to the flux surfaces it can be ~4 - 5 times better giving ∆L = 2 –3 cm However, the radial extent over which this resolution is achieved is limited Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 135/181 Divertor LIDAR at JET: polychromator 4 channel filter spectrometer • Optical path lengths to detectors are the same. • Three filters at 12 degree incidence (F1 – F3) are shown • A fourth filter limits the channel nearest the laser line. Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 136/181 Divertor LIDAR at JET: raw data & profiles Flux surfaces 10 ) -3 m Mapping 19 increases spatial (x10 resolution e n 0 Rmid r se La t 9D igh KE f-s e-o 10 lin ) 3 - m 9 1 0 1 x ( e 0 n JET boundary Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 137/181 ITER requirements for Te & ne Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 138/181 ITER requirements for Te & ne Electron temperatures in ITER of up to 40keV and densities of up to several times 1020 m-3 are expected. Thomson Scattering is a proven technique for making these measurements. Successful deployment of such a system requires that all components maintain adequate performance throughout the lifetime of the experiment. The parameters accessed by ITER lead to very different operating conditions from existing devices. These range from a high dose neutron environment to in-vacuum mirrors and the extremely long plasma discharges. Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 139/181 ITER LIDAR Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 140/181 Laser Beam dump Mirrors Large mirrors collect suitable amount of light Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 141/181 Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 142/181 The Relay Mirror A possible solution for ITER LIDAR 2007, ~92inch Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 143/181 The Relay Mirror Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 144/181 Bird’s eye view Laser diagnosis unit New proposed laser beam test area Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 145/181 The Neutron/Radiation Challenge •Influence of optical labyrinth •High level of detail obtainable Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 146/181 Laser Options/Requirements • Needs reasonable energy and short pulse simultaneously • Options to chose from: – Nd:YAG (1064nm) – Nd:YAG second harmonic (532nm) – Ruby(694nm) – Ti:Sapphire (~800nm) – Nd:YLF (1056nm) • Wide temperature range • Time repetition expected from laser(s) – 100Hz • Also need to consider – Space envelope/ Maintainability/ Power consumption/ Data quality Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 147/181 Laser System • Laser specifications – wavelength~ ~1.06microns (1ω +2ω +cal ) – laser energy ~5J/pulse – laser pulse ~250-300ps • Proposing 7 lasers at ~15Hz – More achievable technology – Compact footprint – Measurement capability maintained even if 1,2,3... lasers malfunction – Burst mode available to exploit plasma physics e.g. very fast MHD events Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 148/181 Options to combine lasers • Hexagonally pack lasers – no moving parts • Use a scanning mirror – all beams can overlap • Rotating wheel with encoder – all beams can overlap Above shows a 7 laser hexagon pack at machine vacuum boundary In this option beams are expanded as they go to the machine area to minimise risk of damage to windows Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 149/181 Scattered Spectrum 7 Scattered Spectra 6 Selden-Matoba, θ=180 o y 5 0.5keV 5keV 4 10keV 20keV 3 40keV Spectral Densit 2 1 0 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.80.5 2 GaAsP Normalised Wavelength 0.4 NIR region Note getting to / ~0.35 gets past the peak for (λ > 850 nm) λ λ0 GaAs 40keV 0.3 For 532nm laser, this means getting to 186nm 0.2 S-25 0.1 For 800nm laser, this means getting to 280nm Quantum Efficiency 0 For 1064nm laser, this means getting to 370nm Nd:YAG 200 400 600 800 1000 1200 Wavelength [nm] Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 150/181 Core detectors Upper: The Thomson scattering spectrum for an input wavelength of 1064 nm and a scattering angle q = 180˚, calculated at 5 different plasma temperatures. Lower: examples of the spectral quantum efficiency of visible photo-cathodes available for LIDAR TS. 12 10 0.2 keV •Long wavelength laser (e.g. NdYAG) 8 •Wide spectral range •Shorter wavelengths efficient fast 6 detectors exist 4 1 keV Recently proven at JET (GaAsP) 5 keV 2 40 keV 10 keV Modest improvement required Photon spectral density (a.u.) Photon spectral density 0 Nd:YAG •Detectors in the >850nm required 200 400 600 800 1000 1200 0.5 Ternary alloy InxGa1-xAs could GaAsP produce a QE of the order of 5% up 0.4 NIR region to a cut-off wavelength of l~ 1000 (λ > 850 nm) nm. GaAs 0.3 Transferred electron (TE) detector. Externally biased, 0.2 InGaAsP/InP photocathode with a S-25 possible QE in excess of 25% up to 0.1 λ=1.33 µm Quantum Efficiency 0 Nd:YAG 200 400 600 800 1000 1200 Wavelength [nm] Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 151/181 Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 152/181 Attenuation in LIDAR Windows on ITER, due to ionising dose Light Collection Window (Double – total thickness 3 cm) Over ITER lifetime, equivalent ionising dose is 1.7 x 10-2 MGy λ (nm) Absorption (%) 400 – 800 0.5 350 0.9 300 2.9 250 8.3 Laser Input Window (Double – total thickness 3 cm) Over ITER lifetime, equivalent ionising dose is 1.0 MGy λ (nm) Absorption (%) 400 – 800 20 350 35 300 76 250 98.5 Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 153/181 Radio-luminescence in KU1 Silica Windows The value for the radio-luminescence intensity from the LIDAR light-collection window, falling within the étendue of the detection optics, is around seven orders of magnitude lower than that due to plasma bremsstrahlung collected within the same étendue. Consequently, the radio-luminescence signal can be ignored in the assessment of the parasitic light that will be collected along with the laser light scattered from the ITER plasma. Variation of Luminescence with Wavelength for Various Glasses Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 154/181 Motivation First mirror 100 90 Cu Rhodium is a very attractive 80 Mo option for first mirror material: Rh 70 SS ¾ Good reflectivity 60 ¾ High melting point (1966 °C) 50 W Reflectivity (%) Reflectivity ¾ Low sputtering yield (high Z) 40 Calculated with (n, k) from [1] 30 500 1000 1500 2000 Wavelength (nm) High price of the raw material calls for developing thin film technology: Magnetron sputtering (Vacuum deposition technique) 1Handbook of optical constants of solids, ed. E.D. Palik, Acad. Press, 1985 and 1991 Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 155/181 Dielectric mirrors Broadband Dielectric Max size now <100mm Protected Aluminium to compare Laser Mirror, max size 530mm! Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 156/181 Lasers • Study of short pulse high rep rate Nd:YAG lasers for scattering needs to be carried out (both 1st and 2nd harmonic) •2nd Harmonic will generally have half the energy and half the photons! • Ruby has been demonstrated to work but the repetition rate is a problem • Now to have a brief look at the TiS optionÆ Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 157/181 Use of TiS lasers • TiS lasers have never been used for LIDAR TS in fusion experiments • Potential issues need to be studied and analysed: bandwidth, ASE (amplified stimulated emission), maintenance, stability, functionality. • May be desirable to set up a test experiment (perhaps look at scattering off a gas) using an existing TiS facility after a feasibility study Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 158/181 LIDAR Detector sensitivity? • For LIDAR, presently use gated 20 mm photocathode dual chevron-MCP photomultipliers (10-12mm may be acceptable) • Photocathodes such as S20, Gallium Arsenide phosphide, Gallium Arsenide, etc can cover the region to ~850nm • What about a detector in the 850-1060nm region? (sensitive detectors in this region would beneficial) Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 159/181 Detector Response time? • Spatial resolution in the LIDAR system is directly related to the system response with 400ps (complete response when convoluted with detector, digitiser and laser) corresponding to 6 cm (ITER specification) • Laser pulse can be ~200ps • Detectors currently in use have about 650ps response time (800ps when response of complete system is included) • But very recent detector developments are encouraging: Detector ~10mm diameter(between 10 and 20mm required): response time between 110 and 133ps. Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 160/181 Laser coupling efficiency • To optimise laser energy coupling efficiency, one should make use of high reflectivity mirrors where possible. • This is not possible if broadband metal mirrors are used for simultaneously transporting-in and collection-of the scattered light. For example, if 5 rhodium mirrors were used in the duct area, then immediately a transmission of 20% could be the result (mirrors and windows) • Can we get around this? Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 161/181 Can use separate laser and collection First laser mirror could be here Bio Shield Port Plug F/18 F/12 F/6 Separate Laser path (Fold not shown) Detector ø18mm Schematic straight through Vacuum window ø110mm optical path shown for clarity 4.2m 6.2m 8.2m 11.2m 14.8m Using this approach, one can optimise the mirrors for the laser and collection separately. This would require a small hole in the back of the First mirror (<5cm diameter) Can we have dual laser dielectric mirrors situated at the back of the port duct that will be robust? Note: Expect 6x1011neutrons/cm2/s First mirror, this would be down by at least a factor of 5 at first laser mirror (window position is being studied) Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 162/181 High Temperature Thomson Scattering Theory Review • Reliable electron temperature and density diagnostic for modern tokamaks • All across the operating range • Current devices have temperatures exceeding 10 keV • But up to 40 keV predicted for ITER. • Then electron velocities are a substantial fraction of the velocity of light • Large blue shift’ in the scattered spectrum • Change in the polarisation of the scattered light • The incoherent Thomson scattered power per unit solid angle per unit angular frequencyβ can be written ω β β 2 2 β δ 2 θ 2 β d P ()1− cos β 1− 2 3 = r 2 S d 3r 1− β e × i ()1− f ()(k ⋅ v − ω d ) e ∫ i ∫ ()()1− dΩs d s 1− i 1− βs s Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 163/181 • Depolarisation term important at high temperatures • Theory is solid but experiments challenged this due to fact TeTS can be up to 15-20% lower than TeECE in some experiments • However the presence of high energy electron tail would cause TS to overestimate • Urgency to investigate the cause of the temperature discrepancy Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 164/181 Laser Reliability •For the TS systems, reliability is of paramount importance and redundancy in design must be incorporated where possible. •Typical operation of a multi-laser system on the MAST device has shown that out of 4 lasers, at least 1 laser was available almost 100% of the time while all 4 lasers were available >70% of the time (this corresponds to an individual laser availability of about 92% per plasma shot). •Translating this simply to ITER for a 7 laser system would give 5 or more lasers available more than 98% of the time, and all 7 lasers > 56% of the time. Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 165/181 EU-Core TS (LIDAR) Spatial Parameter range Time Space Accuracy range resolution resolution Electron r/a<0.9 0.5-40keV 10ms a/30 10% Temperature Electron Density r/a<0.9 3x1019 to 3x1020m-3 10ms a/30 5% 35 30 25 20 15 10 •An example scenario (5n) that is expected in ITER. 5 •The required measurement resolution is a/30. ElectronElectron Temperature Temperature (keV) (keV) 0 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 •This is equivalent to approximately 7 cm in real R (m) space. 3.5 ))) •Note: the full profile from -0.9r/a to 0.9r/a is required -3-3-3 3.0 mmm 191919 2.5 2.0 1.5 1.0 Assumes IR detectors are possible 0.5 ElectronElectronElectron Density Density Density (10(10(10 0.0 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 R(m) Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 166/181 High Importance Generic Topics to be Addressed • First/second Mirror surface recovery (MSR) techniques • Deposition prevention • First dielectric laser mirror • Background light calculations (need to get much better modelling) • Wide-band in-situ calibrations • Detectors (previously discussed at ITPA-need more physics assessment) • Laser development • Shutter/calibration combination specification/outline • Alignment systems • Beam Dumps (common issue) • Reliability (should define what is expected) • TS/ECE issue resolution • Measurement requirements-still consistent- (detailed physics case) • Diagnostic exploitation Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 167/181 Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 168/181 ITERCore LIDAR vignetting The ITER TS design is based principally on the design of the JET LIDAR. JET has the only LIDAR systems in the world. It was generally acknowledges that LIDAR was the only way of introducing TS on ITER. To a large extent the ITER system is a copy of KE3. • The reliability is very high > 90% • Alignment is stable, no components on the Vacuum vessel • BUT, The Jet systems are not using one window but a cluster of 7 windows with the laser in the center Vignetting! Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 169/181 ITER Core LIDAR vignetting (old design) • Eliminate the central window, i.e. use the full aperture for collection. • Use the collection window/mirror for laser beam as well • Signal from inside double cone has no vignetting • Laser beam can be anywhere in this cone • Simple calculation of solid angle if the two apertures are relayed to the detectors Ddet / Fdet = Dblanket / Fblanket = Dblanket x Dmirror/(L2 -L1) Dmirror/Dblanket = L2/L1 Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 170/181 ITERCore LIDAR optical design The aim is to re-image the entrance pupil on the detector surface. The advantage of this idea is a field-independent image diameter. The entrance pupil size is given by the size of the first surface of the Collection Optics, M1. The diameter of M1 follows from the field definitions and F/#’s. M1 and M4 are spherical mirrors, M2 and M3 are identical toroidals. M1 is imaged onto M4 Field position [m] + 2100 [m] before M1 Field [mm] at field position 0F/6 10 2100 50 4200 F/17 110 For the relay-optics there is a balance between size-of-the-components and the total number-of- components. Task is to re-image M4 by a relay and keeping every link in the chain identical. Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 171/181 ITERCore LIDAR detectors The TS spectrum in ITER will range from NIR (low Te) down to UV (high Te). IR laser will be used only if IR detection is available Æ this influences also calibration techniques. •Long wavelength laser (e.g. NdYAG) •Wide spectral range •Shorter wavelengths efficient fast detectors exist Recently proven at JET (GaAsP) Modest improvement required •Detectors in the >850nm required Ternary alloy InxGa1-xAs could produce a QE of the order of 5% up to a cut-off wavelength of λ~ 1000 nm. Transferred electron (TE) detector. Externally biased, InGaAsP/InP photocathode with a possible QE in excess of 25% up to λ=1.33 µm Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 172/181 ITERCore LIDAR detectors The main requirements for the ITER LIDAR TS detectors are : Active area diameter D ≥ 11 mm Equivalent quantum efficiency EQE ≥ 6% Pulse response time t ≤ 330 ps FWHM.[1] gating shutter ratio ~ 106. gating on-off time ≤ 5ns. EQE = QE/kF kF is the excess noise factor that accounts for any additional noise introduced after the primary detection. At present these specifications can be met only by photoemissive detectors. The above specifications are at the limit of the present technology for the detectors operating in the visible. To extend them to the NIR is a real challenge. Two types of detectors available for the above spectral range: the transferred electron (TE) hybrid photodiode and the InxGa1-xAs microchannel plate (MCP) image intensifier Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 173/181 NIR detectors Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 174/181 NIR detectors Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 175/181 NIR detectors Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 176/181 Signal simulation with NIR detectors Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 177/181 ITER core LIDAR project Work Breakdown Structure Thomson Scattering Core (LIDAR) 5.5.C.1.0.0.0.0 LIDAR Project LIDAR System Laser Collection Laser Path Control & LIDAR LIDAR Interfaces & Management Concepts Systems Optics Optics Acquisition Port Engineering Services Integrated Testing 1.1.0.0.0 1.2.0.0.0 1.3.0.0.0 1.4.0.0.0 1.5.0.0.0 1.6.0.0.0 1.7.0.0.0 1.8.0.0.0 1.9.0.0.0 Key Project Overall Cluster Lasers Collection Laser Path Control System Shutters Water LIDAR Milestones Co-ordination 1.3.1.0.0 Optical Design Optical Design Interface Definition 1.7.1.0.0 Services Interfaces 1.1.1.0.0 1.2.1.0.0 1.4.1.0.0 1.5.1.0.0 1.6.1.0.0 1.8.1.0.0 1.9.1.0.0 Key Project Performance Laser Collection Laser Control Labyrinth Interspace Mock-up Deliverables Analysis Layout Windows Windows System 1.7.2.0.0 Vacuum Facility 1.1.2.0.0 1.2.2.0.0 1.3.2.0.0 1.4.2.0.0 1.5.2.0.0 1.6.2.0.0 1.8.2.0.0 1.9.2.0.0 Key ITER LIDAR Laser Beam In-Vacuum Plasma Facing Acquisition Extension Tubes & LIDAR Basic Mock-up Milestones & IPL Neutronics Combiner Collection Mirrors Laser Mirrors System Mirror Mounting Power Tests 1.1.3.0.0 1.2.3.0.0 1.3.3.0.0 1.4.3.0.0 1.5.3.0.0 1.6.3.0.0 1.7.3.0.0 1.8.3.0.0 1.9.3.0.0 Overall Scattering Ex-Vacuum Other Laser LIDAR External Port Optics Spectrometer Tokamak Management Theory Collection Optics Mirrors Instrumentation Mounting Area Tests 1.1.4.0.0 1.2.4.0.0 1.4.4.0.0 1.5.4.0.0 1.6.4.0.0 1.7.4.0.0 1.8.4.0.0 1.9.4.0.0 Safety & HP R&D Collection Optics Laser Path Safety Bioshield Laser Final System Management Tasks Mechanical Design Mechanical Design Interlocks 1.7.5.0.0 Room Testing 1.1.5.0.0 1.2.5.0.0 1.4.5.0.0 1.5.5.0.0 1.6.5.0.0 1.8.5.0.0 1.9.5.0.0 Risk Radiation Spectrometer Beam Safety BSM Port Cell/ System Assembly Management Effects Data System Dump System Penetrations Interspace & Dis-assembly 1.1.6.0.0 1.2.6.0.0 1.4.6.0.0 1.5.6.0.0 1.6.6.0.0 1.7.6.0.0 1.8.6.0.0 1.9.6.0.0 Quality Remote Detectors Alignment EM Analysis for Management Handling 1.4.7.0.0 System In-Port Comp. 1.1.7.0.0 1.2.7.0.0 1.5.7.0.0 1.7.7.0.0 Item Alignment Calibration Test Unit System System 1.2.8.0.0 1.4.8.0.0 1.5.8.0.0 Engineering Calibration Analysis System 1.2.9.0.0 1.4.9.0.0 Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 178/181 ITER divertor TS’s Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 179/181 ITER divertor TS’s Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 180/181 ITER divertor TS’s The life-time of optical components is expected to be limited due to contamination with carbon and beryllium-based material eroded from the beryllium wall and carbon tiles. As well as significantly reduced optical transmission, thin layers can dramatically change the slope of the spectral reflectivity of rather low reflectivity mirrors, especially like W or Mo. Roberto Pasqualotto Eur.PhD Fusion: Thomson scattering - 11th February 2009 181/181