Beam Diagnostic Needs and Challenges of some Future
Beam Diagnostic Needs and Challenges of some Future Light Sources Pavel Evtushenko, JLab This talk to a large extend based on discussion and comments: Storage Rings and USRs: Michael Borland, Glenn Decker, Fernando Sannibale Single Pass XFELs: Henrik Loos, Joe Frisch ERLs: Bruce Dunham Energy Recovery LINACs ERLs Outline Essence of and ERL based LS run high average current (~ 100 mA) of non equilibrium (Gaussian) beam with high peak brightness Injector: - generate and maintain high peak brightness beam - monitor when going to high average current
Full current beam vs. tune-up beam Two beams in the LINACs position and transverse profile Large Dynamic range understanding beam Halo origins and evolution Non Gaussian beams can be really difficult, hopefully better with well built injectors Drive Laser ghost pulses For machine tune up, beam studies, intercepting diagnostics a diagnostic beam with very low average current but nominal bunch charge is used (all beam can be lost without damaging machine) For example, JLab FEL: max rep. rate 74.85 MHz (CW) diagnostic mode: rep. rate 4.678125 MHz (16), 250 s / 2 Hz (2000)s / 2 Hz (2000) average current ~300 nA Most of the laser pulses are stopped by EO cell(s), but the extinction ratio of the an EO cell is about 200 (typical), two in series ~ 4104 Another example: want to reduce 1300 MHz (100 mA) to 300 nA (218) than for every bunch Qb we want
we also get 6.55Qb of ghost pulses (655 % !!!) we do not want ghost pulses overall intensity must be kept much lower than real pulses!!! for usual measurements ~ 1% might be fine much bigger problem if want to study halo, lets say 10-6 effects, than ghost pulses should be kept at 10-8 (???) Drive Laser ghost pulses Using a Log-amp is an easy way to diagnose presence of the ghost pulses Log-amps with dynamic range 100 dB are available 631 uA (100%) 135 pC x 4.678125 MHz 5.7 uA (~0.9 %) 37.425 MHz ghost pulses Injector emittance trans. phase space
Mask or a slit is used to cut out small emittance dominated beamlet(s) Beamlet profile measurements - Intensity (A) - width (w) - displacement (d) Dynamic range ~ 500 if gain is fixed Works for space charge dominated beams Measures emittance and the Twiss parameters in a single shot Destructive to the beam - works with diagnostics tune-up mode only (low duty cycle or average current) How to monitor emittance or just beam size, when going to high current???
Injector emittance trans. phase space Two fixed slits and beam scanning across them Faraday cup for current measurements Beam at several kHz - good measurements in a few seconds Turns injector into an analog computer for optimizations I. Bazarov ,et al PRSTAB 11, 100703 (2008) Slide courtesy of B. Dunham , ERL 2011 Injector: Drive Laser and Cathode Diagnostics In a real machine for understanding of the dynamics i.e. to see what it is and comparison with a model measured laser distributions (transverse and longitudinal) needs to be used.
The same is true for the cathode Q.E. distribution of the cathode since the emission profile is the product of this and the laser distribution. Drive Laser transverse profile - quite easy with the dynamic range ~ 500, probably, need much higher for high current systems. For longitudinal auto- and cross-correlation are used DR of this is limited to ~ 104 (scattered light) Time Correlated Single Photon Counting (TCSPC) can have a ps resolution and very large DR, well suited for high rep. rate sources, but takes time to measure If/When longitudinal pulse shaping is used must know that it is stable under high average power. 2-pass viewers JLab FEL LINAC OTR viewer
There are two beams in the LINAC When trying to measure decelerated beam with a viewer the accelerated one gets also intercepted Ultimately need non intercepting technique JLab FEL uses OTR viewers with 5 mm hole (first beam goes in to the hole) Difficult to make very thin and flat viewer with the hole 44% transparent mesh 5 micron thin SRF cavities see the radiation due to the intercepted beam (and does not like it) With the ultra bright beam OTR might be useless (OTR becomes COTR) Wire scanners is a solution (no 2D distribution measured) difficult near LINAC If the scanner measures radiation created by the wire, must take care of the
background. Need cheap Laser Wire scanner (take advantage of the high rep. rate i.e. ) 2-pass BPM Stripline BPM signal Motivation: For differential orbit measurements with both beams in the LINAC The decelerating beam gets adiabatically anti-dumped small errors corrections in the beginning leads to big orbit change at the end Orbit stabilization and feedback There are a few ideas in work now; Both time domain and frequency domain Solution can be very different for different machines
- long recirculation time vs. short; - every bucket filled vs. not The phase difference is not always 180 deg, especially when tuning machine this is some what a problem for both time domain and frequency domain Time domain approach requires very-very-very carefully built pickups (no ringing) On LINAC non Gaussian beams Measured at JLab FEL Obtained in a specially setup measurements to show how much beam is non Gaussian It in not how we have it during standard operation There is no Halo shown in this measurements in sense that all of it participates in FEL interaction (we think) and it is only Dynamic Range of ~ 500. The techniques we can borrow from rings assume Gaussian beam and therefore are concentrating on beam size (RMS) measurements Large dynamic range measurements
Measured in JLab FEL injector, local intensity difference of the core and halo is about 300. (500 would measure as well) 10-bit frame grabber & a CCD with 57 dB dynamic range PARMELA simulations of the same setup with 3e5 particles: X and Y phase spaces, beam profile and its projection show the halo around the core of about 3e-3. Even in idealized system (simulation) beam dynamics can lead to formation of halo. Single Pass FELs LCLS, FLASH, SACLA alike extreme peak beam brightness Transverse diagnostics COTR (big setback), wire scanners (need faster) Eventually very small transverse beams (diffraction limited resolution) Longitudinal diagnostics TCAV (great, but complex and expansive) Timing good for RF, main things is to sync the FEL to the user lasers
Orbit stability (not too bad, RF cavity BPMs are very good!) LINACs non Gaussian beam - does not seem to be a big problem (very well made injector/beam, but also low rep. rate) For seeded systems the overlap between the seed and the beam (phase/time measurements of the beam and seed laser) OTR turns COTR an old working horse that tells you you will have a different transportation in the future The Optical Transition Radiation has been a true working horse for the transverse beam profile measurements, from 10 keV to many GeV with resolution down to few microns Main advantage - simplicity, resolution - diffraction limited (energy independent), enough yield for single bunch measurements COTR first observed at LCLS with gain ~ 10, then at other facilities too Now gain up to ~105 at LCLS at the optical wavelength Attributed to micro bunching instability that has gain at the optical key parameter is the small slice energy spread (longitudinally bright beams)
The are several mitigation scheme proposed but 105 gain !!! The replacement with best results so far YAG:Ce plus gaited CCD this is expansive and eventually scintillators get to saturation Catastrophe equals opportunity COTR Also good overview by S. Wesch, DIPAC 2011 Another problem with OTR OTR image of a beam ~ 10 m 10 m before (up) and after (down) the OTR radiator was exposed to 51010 e-/train; rep. rate of the bunch trains 1.5 Hz for 5 minutes OTR radiator (initially) optically polished 500 m Be With ~ 10 time less charge per train for 30 min no degradation Suggested explanation radiator deformation beyond elastic limit 51010 e 10077 pC bunches
Radiators with small thermal expansion and large elasticity modulus might be the solution. Si is a good candidate, already used as OTR radiators canbe optically polished. Optics for such beams is essentially a microscope (must collect light in a very large angle) From SLAC-PUB-9280, courtesy of M. Ross Bunch Length Measurements Now at LCLS bunch is so short the measurements are resolution limited Transverse deflecting cavity gold standard; direct, time domain, self calibrating measurements. Going to X-band 1 fs resolution. But expansive and complex. Not every facility can afford it. However, provide absolute measurements
which can be used to calibrate spectral (frequency) domain diagnostics Frequency domain techniques: Compact and affordable, Used with CTR and CSR in THz range, few ps to ~50 fs as is Going to the shorter buncher is shifting the CTR to visible and UV diapason (already now) Simplest Martin-Puplett interferometer, multiple shots Single shot spectrometers DESY demonstrated, LCLS another under development Main issue phase information is lost JLab FEL bunch compression diagnostics JLab IR/UV Upgrade FEL operates with bunch compression ration of 90-135 (cathode to wiggler); 17-25 (LINAC entrance to wiggler). To achieve this compression ratio nonlinear compression is used compensating for LINAC RF
curvature (up to 2nd order). The RF curvature compensation is made with multipoles installed in dispersive locations of 180 Bates bend with separate function magnets - D. Douglas design (no harmonic RF) Operationally longitudinal match relies on: a. Bunch length measurements at full compression (Martin-Puplett Interferometer) b. Longitudinal transfer function measurements R55, T555, U5555 c. Energy spread measurements in injector and exit of the LINAC Trim quads (BdL) 740 G Sextupoles
(BdL) 12730 G Trim quads (BdL) 700 G Sextupoles (BdL) 10730 G Trim quads (BdL) 660 G Sextupoles (BdL) 8730 G Martin-Puplett Interferometer data in frequency domain give upper limit on the RMS bunch length
Storage Rings Ultimate storage rings full transverse coherence Small transverse beam size (is the point of the USRs) Small source size requires corresponding transverse stability X-ray BPM Sort Pulse X-rays (SPX system) For a proper overview of the SR diagnostics status see: G. Decker at FLS2010 workshop C. Steier at the ERL09 workshop Transverse Beam Stability Pointed out by C. Steier at ERL2009 and M. Borland now again. USRs and high-energy ERLs will converge toward requirements that are similar to those that "leading" present-day storage rings must meet in the vertical plane. Emittance sets the scale for beam stability requirements. The best present vertical-plane emittance in third generation rings (5~10
pm) is similar to what's projected in the both planes for ERLs and USRs. Also, as the beam gets smaller going below the diffraction limit e- beam will be some what smaller than the optical mode size of the x-ray beam and, thus the latter one will care less about variations of the former one. The difference at ERLs will be the sources of the beam instability. Starting from the Cathode Drive Laser, Gun, Injector. So, one will have to have additional feedback/forward for the new sources of the instabilities. Transverse Beam Size When SR is used to measure the transverse beam size the resolution is diffraction limited Therefore one went now to x-ray wavelength for the measurements However, if below diffraction limit of xray beam is smaller than what can be measured with the help of xray (if doing imaging) i.e. if this diagnostics works the USR is not succeeding The way around the diffraction limit was found by astronomers when measuring size of the stars two slit interferometer. It also has been adopted by T. Mitsuhashi for beam size measurements at optical .
T. Mitsuhashi, PAC97, 766, (1997); Phys. Rev. ST Accel. Beams 9, 122802 (2006) Courtesy of F. Sannibale Point source g=1 Extended source 0 < gg < 1 By measuring the visibility and first minimum position (phase) vs. the distance between the two slits the full beam distribution projection can be reconstructed. If the beam distribution is symmetric, the phase measurement is not required.
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