Review of Diagnostics for Next Generation Linear Accelerators
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Proceedings DIPAC 2001 – ESRF, Grenoble, France REVIEW OF DIAGNOSTICS FOR NEXT GENERATION LINEAR ACCELERATORS M. Ross, Stanford Linear Accelerator Center, Stanford, CA 94309, USA Abstract will review ideas and tests of diagnostics for measuring electron beam position, profile (transverse and New electron linac designs incorporate substantial longitudinal) and loss. advances in critical beam parameters such as beam loading and bunch length and will require new levels of 2 POSITION performance in stability and phase space control. In the coming decade, e- (and e+) linacs will be built for a high 2.1 Purposes power linear collider (TESLA, CLIC, JLC/NLC), for fourth generation X-ray sources (TESLA FEL, LCLS, High peak current linacs require accurate, well Spring 8 FEL) and for basic accelerator research and referenced, beam position monitors (BPM’s) to suppress development (Orion). Each project assumes significant the interaction between the RF structure and the beam. In instrumentation performance advances across a wide addition, equally as important, the small beam must pass front. close to the center of each quadrupole magnet in order to This review will focus on basic diagnostics for beam avoid emittance dilution arising from the dispersion position and phase space monitoring. Research and generated from a small dipole kick. Some LC designs development efforts aimed at high precision multi-bunch include two separate BPM systems in each linac. Typical beam position monitors, transverse and longitudinal requirements are shown in table 2. profile monitors and timing systems will be described. Table 2: NLC Linac quadrupole BPM performance requirements 1 INTRODUCTION Parameter Value Conditions 10 - Next generation linacs have smaller beam sizes, Resolution 300 nm @ 10 e single increased stability and improved acceleration efficiency. rms bunch They will be used for single pass free electron lasers Position 1 µm over 24 hours (FEL) [1], linear colliders (LC) [2] and advanced Stability accelerator research and development [3]. Table 1 shows Position 200 µm wrt the quad the evolution of beam parameters from the SLAC Linear Accuracy magnetic center Collider (SLC) toward the next generation projects. x,y dynamic ±2 mm Performance improvements of a factor 10 are typical. If range 8 we consider older, more conventional linacs, the relative Q dynamic 5×10 to 10 - changes in performance parameters are more impressive, range (per 1.5×10 e often as much as a factor of 1000. In addition, there are bunch) special locations within the linac system where the beam requirements are much more stringent: for example at the The most challenging requirement is the long-term -4 LC interaction point or within the FEL undulator. position stability, ~ 2x10 r0 (r0=BPM radius). The -5 Table 1: Next generation linac parameter comparison planned resolution is ~ 6x10 r0; both are a factor of 50 for SLC (1985), the SLAC Linac Coherent Light Source improvement over BPMs used in the SLC. (LCLS) and the Next Linear Collider (NLC). The BPM’s are in continual use by an automated SLC LCLS NLC steering loop that keeps the beam centered in the σ_x (µm) 90 30 7 accelerating structure and the quadrupole magnets. The model for operation of the linac assumes that a second, σ_y (µm) 50 30 1 presumably more intrusive, automatic quadrupole beam σ_z (µm) 1300 30 100 centering procedure is implemented once per day, as peak I (A) 700 3400 1000 required by long term BPM drifts. power density 2e13 1e12 1e18 2 BPM system requirements for the FEL are tightest in (W/m ) the undulator itself. For full coherent emission saturation, It is evident that new technology and different physical the beam and the light it has generated must remain principles, such as diffraction radiation and Compton superimposed throughout the undulator. Surprisingly, scattering, are needed in order to extend the performance longitudinal considerations rather than transverse set the of diagnostics to meet the challenge [4]. In this paper we steering tolerances. The difference between the x-ray Invited Talks IT01 1 Proceedings DIPAC 2001 – ESRF, Grenoble, France Instrumentation and Diagnostics Using Schottky Signals F. Nolden, GSI, Darmstadt, Germany Abstract The frequency spectrum, i.e. the Fourier transform of this current is an infinite train of delta functions, as well, Schottky signal measurements are a widely used tool for the determination of longitudinal and transverse dynami- ( * .* )+* -/1032 %54 % §©¨ (2) , ¤-, cal properties of hadron beams in circular accelerators and &! storage rings [1]. When applied to coasting beams, it is $# possible to deduce properties as the momentum distribu- ¥ ¢¡¤£ i.e. the current spectrum consists of peaks at each harmonic 76©89 tion. the values and the average betatron amplitudes. of the revolution frequency with a random Scientific applications have been developed in the past few , phase. The mathematics of this Fourier correspondence can years, as well, namely nuclear Schottky mass spectrometry be found in most textbooks on signal theory. Because of ( )+* and lifetime measurements. the random phase, the expectation value of vanishes. Schottky signals from a coasting beam are random sig- However, the cancellation of the phases is never complete nals which appear at every revolution harmonic and the re- due to the finite number of particles in the beam. There- spective betatron sidebands. Their interpretation is more or fore the power spectrum of the random process can be less straightforward unless the signal is perturbed by col- detected, as we shall see. Before doing so, let us look at lective effects in the case of high phase space density. the distribution of the frequencies . The variation of the , Schottky signals from bunched beams reveal the syn- revolution frequencies around their mean value is pro- ,: chrotron oscillation frequency, from which the effective rf portional the the deviation of the particle momenta ; from voltage seen by the beam can be deduced. their mean ; : : >= The detection devices can be broad-band or narrow- ; , # # (3) band. The frequency range is usually in the range between ; ,<: : a few hundred kHz up to about 150 MHz. In connection with the frequency dispersion with stochastic cooling, Schottky signals are used at fre- !B =?A@ quencies up to 8 GHz. Narrow-band devices are needed %$CED (4) if signal-to-noise problems arise, e.g. in the case of an- @ tiproton beams. Heavy ion beams require less effort, it is Here, is the relativistic Lorentz factor, and CED is the mo- D relatively easy to detect single circulating highly charged C mentum compaction factor of the!B ring lattice. is related @GF = H@ D C ions. to the transition by F . Once is known, it is possible to infer from the beam current frequency spectrum the momentum width of the beam. It is sufficient to mea- 1 SCHOTTKY SPECTRA sure the spectrum at one given harmonic. This is one of the main applications of Schottky diagnosis. If the momentum ;KJ"LNM 1.1 Coasting beam current distribution is bounded by a momentum deviation I , # then there is a harmonic J"LM above which the correspon- *O Imagine a detector at some given location in the storage dence ; ceases to be unique, i.e. where the Schottky ring. The beam current at this given place is the sum of harmonics# begin to overlap: the currents from each charged particle passing by. Let us !E assume that the beam is composed of ¦ particles of a single = ;KJ"LNM 6UT T J"LNMQPSR §©¨ (5) species with charge . These particles are characterized # T T ; : T in the longitudinal phase plane by their revolution period T T T (which we assume to be constant). Secondly, they are The width of the frequency distribution is proportional to ordered randomly along the ring circumference. One way the harmonic number. to parameterize this random placement is to stop the time Signal detectors are linear devices which respond to the the particle passes by the detector during the revolution beam with a voltage . The beam current is the sum over all particles : ]\ _^` a^ a §©¨¤WYX cb 1%d % V 6 ¤< , "! &%' % [Z # §©¨ (1) "! $# (6) 6 IT02 Invited Talks Proceedings DIPAC 2001 – ESRF, Grenoble, France DIAGNOSTICS AND INSTURMENTATION FOR FEL M. E. Couprie, Service de Photons, Atomes et Molécules, Saclay, and Laboratoire d’Utilisation du Rayonnement Électromagnétique, Orsay, France Abstract accelerated and emit synchrotron radiation, at the resonant wavelength λr and its harmonics : Free Electron Laser are coherent sources of λ λ = 0 (1 +K 2 /2) (1) radiation based on the interaction of a relativistic r γ 2 electron beam in an undulator field. According to the 2 energy of the accelerator, they presently cover a wide with the deflection parameter K = 0.94 λo(cm) Bo(T) and spectral range, from the infra-red to the VUV. FELs γ the normalised energy of the electrons. The interaction combine the diagnostics of typical laser systems (for the between the optical wave and the electron bunch occurs measurement of spectral and temporal characteristics, along the undulator progression. Generally, an optical the transverse mode pattern, the polarisation) and the resonator, the length of which is adapted to the recurrence diagnostics of relativistic electron beams. The electron of the electron bunches, allows the radiation to be stored beam is characterised in order to evaluate and control and the interaction to take place at each passage. The the FEL performances, but also in order to measure the optical wave and the charged particles exchange energy, effect of the FEL on the electron beam. The FEL which can lead to a modulation of the electronic density at characteristics are monitored with various types of the wavelength of light (microbunching), phasing the detectors, depending mainly on the spectral range. emission and reinforcing the coherence of the produced Diagnostics for Linac based Infra Red FELs and radiation.