Jefferson Lab Experience with Beam Halo, Beam Loss and Related beam diagnostics

1. Jefferson Lab Experience with Beam Halo, Beam LossandRelated beam diagnostics Pavel Evtushenko, Steve Benson, Dave Douglas, Kevin Jordan, Carlos Hernandez-Garcia, Dan Sexton, Jay Benesch, Arne Freyberger

2. Outline • IR/UV Upgrade (JLab FEL) overview • Different sources of unwanted beam • Beam dynamics example • Drive Laser related remarks • Setting up for high current operation • Dynamic range of diagnostics • Recent LDRmeasurments • CEBAFoverview • RFtrip rate • Direct measurements • Vacuum “events” • Summary

3. JLab IR/UV Upgrade Ebeam135 MeV Bunch charge: 60 pC – UV FEL 135 pC – IR FEL Rep. rate up to 74.85 MHz 25 μJ/pulse in 250–700 nm UV-VIS 120 μJ/pulse in 1-10 μmIR

4. Flavors of Unwanted Beam Four sorts of the unwanted beams Fraction of the phase space distribution that is far away from the core (due to the beam dynamics) Low charge due to not well attenuated Cathode Laser (ERLs) – but real bunches that have proper timing for acceleration Due to the Cathode and Laser but not properly timed (scattered and reflected light on the cathode and in the DL transport) Field emission: Gun (can be DC or RF), LINAC itself(is accelerated in both directions) Actually, there is one more – ions that accumulate in are true CW electron beam, travel in both directions with thermal velocities in side the electron beam, reduce Q.E. of the cathode one really does not want this beam.

5. FEL Injector as an example of #1 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) non-linear beam dynamics can lead to formation of halo.

6. 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 4.678 MHz 5.7 uA (~0.9 %) 4.678 MHz “ghost” pulses

7. Wafer 25 mm diameter Active area 16 mm diameter Drive laser 8 mm diameter DL light scattered on photo cathode • a view of GaAs photo cathode when running beam (probably 6 % duty cycle or 1.5 %) • measured with simple vis. CCD camera • locations of the wafer and active area are knows from the same view, HV off and white light on • we are looking in to a gap between two non-flat mirrors • with a brand new wafer (no heat cleaning) one would not see any light from the DL spot • Visible (green) DL preferable over UV • Preserving cathode surface will be very helpful • Get rid of heat cleaning for GaAs (H – cleaning) • At least two processes contribute to the generation of scattering centers • Heat cleaning of the cathode (made periodically, every 4-5 re-Cs) • HV breakdowns can result in rather large pits – scattering and field emission Courtesy of C. Hernandez-Garcia

8. Cathode Laser pulse via streak camera • appears to be close to Gaussian on linear scale; tails not so much Gaussian • the difference from Gaussian distribution is obvious on log scale • realistic (measured) distribution must be used for realistic modeling • especially is the calculations are intended for large dynamic range effects

9. High current operation • JLab FEL driver is setup for high current operation in three steps/phases • Most of the measurements are made with low duty cycle beam beam • (this is Step 1 that establishes best RMS setup for FEL performance) • - setting up injector (RF phases and solenoids) • - transverse match • - longitudinal match • Step 2 is to increase the duty cycle, usually to 6 %, and look at • the beam loss, small adjustments in transverse and long. match often are • required; the adjustments must preserve the high performance of the FEL this is the reason the adjustments have to be small • When beam loss is small enough high average (9 mA) current can be • operatedand the long term trends in pressure (vacuum) are used for • Step 3 of machine adjustment, also very small.

10. General FEL remarks • JLab FEL is a 9 mA average current machine, despite the fact that all four sorts of beam halo are present (9 mA and 135 MeV  1.2 MW) • Setting up for high current operation requires time, but has been done routinely • To properly (and quickly) deal with first kind of beam halo Large Dynamic Range diagnostics are needed; until then takes time and trail and error, and origins of problems are not well understood (not confirmed by measurements) • For Drive Laser transport Brewster angle windows (input and output); essentially light tight beam line; laser transport with spatial filter to mitigate scattering • Scattered DL light on the cathode is a reality one has to leave with, i.e., run beam when it is small enough and replace cathode when it is not. • Gradient in the LINAC is limited via requirements to keep dose rate below certain level (especially at the wiggler), but also due to other effects the same as at CEBAF (trip rate) • Instruments are: • Beam Loss Monitors (BLM) of the MPS • Rad.Con. calibrated ionization chambers • Radiation survey just after beam operation ended (for chronic losses) • LDR diagnostics have been developed

11. Why large dynamic range diagnostics? • Operation of JLab FEL with high average current requires a compromise (in terms of match) between high peak beam brightness (required by FEL) and very low beam loss • The match is iterative process and often does not converge easily (if at all…) • For the transverse beam profile measurements and transverse match JLab FEL relies heavily on beam imaging (2D distribution) large number of beam viewers • LINAC beams have neither the time nor the mechanism to come to equilibrium(unlike storage rings, which also run high current) • When setting up a high current accelerators with tune-up beam, halo is something invisible (due to the dynamic range of the measurements) during the setup, yet causing a lot of difficulties when trying to run high current • Increase the DR significantly to make the halo measurable visible with tune-up beam already; measure the phase space distribution with the LDR and use such information for the match. When DR is large enough no need to separate what is core and what is halo.

12. Imaging Sensor(s) Dynamic Range • The first issue to overcome is the DR of a single imaging sensor • The main principle is to use imaging with 2 or 3 sensors with different effective gain simultaneously and to combine data in one LDR image digitally(single sensor dynamic range 500..1000if cost is kept reasonable) • From experience (calculations tested by experiments) we know the safe level of beam current/power for a low duty cycle (tune-up) beam • With typical beam size of few hundred μm OTR signal is attenuated by ~ 10 to keep CCD from saturation. For phosphor or YAG:Ce viewers attenuation of at least 100 is used. • Using OTR there is enough intensityto measure 4 upper decades;lower two decades need gain ofabout 100 to be measured. • The key elements: • image intensifiers • alignment and linearity • combining algorithm(s) • understanding CCD saturation To be measured with imaging sensor #1 and attenuation ~ 10 Intensity range that can be measured without additional gain Intensity range where additional gain of ~ 100 is needed. (not high for an imageintensifier) To be measured with imaging sensor #2 and gain ~ 200

13. Raw images and combining algorithm • Two images (on the left) measured simultaneously with integration times 20 us and 400 us • Background measurements and subtraction is crucial!Made separately for two sensors and subtracted on-line. • Combining algorithm is efficient enough to provide 5 Hz rep. rate for 1024x768 images • At the time of measurements was limited by the flexibility of DLPC • Demonstrated dynamic range of ~ 5E+4 (factor of 100 increase) • Integration time is used for normalization and overlap (sufficient) • Averaging also improves SNR and therefore DR (beam stability) Data combining algorithm

14. linear & log; the “trouble” with the RMS • The two images show exactly the same data (beam profile - (x,y))but in linear and log scale • Next step is to use such measurements for beam characterization, emittance and Twiss parameters measurements (add x’ and y’) • Ultimately tomographic measurements are planned; but first just quad scan • For non-Gaussian beam RMS beam width is a tricky thing!It depends on how much of tails of the distribution function f(x)is taken in to account.

15. Quadrupole scan raw data Level of interest(LOI) more tails included less tails included

16. Emittance and Twiss parameters less tails included RMS emittance more tails included beta function(s) alpha function(s)

17. Diffraction limit and PSF • Imaging  measured distribution is a convolution of source distribution and so-called Point Spread Function (PSF) • PFS determined by optical system angular acceptance but also by the source angular distribution. Different beam viewers have different PSF. • Diffraction determines rather hard limits to the DR • Ways to mitigate: increase angular acceptance, use spatial filter, coronagraph-like optics

18. Objective Lens Pupil Apodization r Uniform pupil function • First a Lyot’s coronagraph was considered to improve the PSF, but this would not allow for simultaneous measurements of the beam core and halo, but it is a good exercise • Domain of Fourier optic, always Fresnel approximation – numerical calculations required for most of the interesting cases – becomes demanding on CPU and memory quickly due to large apertures and optical wavelength (~ 0.5 um) • Implemented and used quasi-discrete Hankel transform for optics modeling (allows to do 1D calculations vs. 2D) • Fourier optics  mage plane = Fourier transform of pupil function for a point source (this is the PSF) • Then it is easy to see that the uniform pupil function, i.e., the harp lens edgeis the problem (besides the uncertainty principal, which also adds to the problem) • Apodization – modification of the pupil function; First considered Gaussian amplitude apodization z Gaussian pupil with s=r0/3 optical field propagation by means of qDHT (false colors – intensity in log scale)

19. Objective Lens Pupil Apodization • First a Lyot’s coronagraph was considered to improve the PSF, but this would not allow for simultaneous measurements of the beam core and halo, but it is a good exercise • Domain of Fourier optic, always Fresnel approximation – numerical calculations required for most of the interesting cases – becomes demanding on CPU and memory quickly due to large apertures and optical wavelength (~ 0.5 um) • Implemented and used quasi-discrete Hankel transform for optics modeling (allows to do 1D calculations vs. 2D) • Fourier optics  mage plane = Fourier transform of pupil function for a point source (this is the PSF) • Then it is easy to see that the uniform pupil function, i.e., the harp lens edgeis the problem (besides the uncertainty principal, which also adds to the problem) • Apodization – modification of the pupil function; First considered Gaussian amplitude apodization Point Spread Functions Convolutions: PFS and 2D Gaussian

20. Wire scanner measurements Freyberger, in DIPAC05 proceedings, Measurements made at CEBAF • wire with diameter much smaller than the beam size interacts with beam as it is scanned across it • there is a number of interaction mechanisms: • beam capture • secondary emission • scattering • different ways to detect the signal • induced current • secondary particles (counting) • Only 1D projections (no 2D distribution) • Takes time (“patience limited”) • LINAC beams are non-equilibrium(non Gaussian) at JLab FEL

21. Wire-scanner viewer combination • Designed to allow measurements of OTR photons, not Bremsstrahlung X-rays, gammas • low Z wires for OTR; still can use high Z wires if Bremsstrahlung is better • Viewer can be OTR or YAG, or other scintillator (normal to the beam) • Mirrors must be very good in terms of scratch and dig spec.– minimize scattering • Must have impedance shields (at JLab FEL) • Two diagnostics at one location

22. Wire-scanner electronics etc. • One key problem is the required measurements timeshould keep as short as possible to make the diagnostics practical • Maximum useful counting rate is the important parameter • The quest - counting system with the max rate of 100 MHz • PMTs themselves are fast enough for this • PMT base (HV divider) is tricky though, high rate  high current  loading of HV power supply  changes HV  changes gain (passive divider are not good for high rate) • There are a few different options for the PMT base, we plat to use so-called transistorized bases (high beta, high voltage transistors) • Preamplifiers, discriminators, logic etc. – commercially available • Need time resolution !!! Options are: gated counting, Time Correlated Single Photon Counting (TCSPC) • Counter, Multi-channel counters and TCSPC: FPGA implementation

23. LDR measurements state • we have demonstrated beam imaging with DR increased by ~ 100 • applied the LDR imaging to beam characterization and have shown that for LINAC non-Gaussian beam the DR has strong impact on the measurements results • have modeled optics required to improve the DR range to reach 106 • new diagnostic station for LDR imaging and cross-check with wire scanner was designed and built • next1 - practical implementation of the apodization optics (manufacturing, error sensitivity study, optimization) • next2 - beam measurements with new diagnostics (tomographic phase space measurements based on LDR imaging)

24. CEBAF: overview Ebeam was 6 GeVis being upgrade to 12 GeV Bunch charge: 0.2 pC Repetition rate: 499 MHz (x3) Three independent beams (3 Halls) 1. Beam halo hitting beam pipe would create background in the NP detectors 2. FE in LINAC cavities affects the trip rate, which reduces up time and must be limited

25. CEBAF: trip rate, statistics JLAB-TN-05-57 J. Benesch,Field Emission in CEBAF's Superconducting RF Cavities and Implications for Future Accelerators JLAB-TN-10-008 J. Benesch,Comparison of arc models from March 2003/Nov 2004 and December 2009 JLAB-TN-12-049 J. Benesch, A. Freyberger,CEBAF Energy Reach and Gradient Maintenance Needs • Uses “accounting” and statistical analysis of the trip rate and its dependence on the cavities gradient • For 12 GeV CEBAF; 400 cavities + each cavity trips 1/(2 days) would result in on average 8 RF trips per hour • Original C25 design / unfortunate feature / RF window has a direct line of sight to the beam – charges up / eventually break down • With time performance of cavities degrades i.e. at the same gradient trip rate goes upexact mechanism is not known (speculated that # of FEs goes up) • Conclusion – gradient maintenance is needed (reprocessing cavities and refurbishing the cryo modules)

26. CEBAF: trip rate Initial distribution of gradients of C50 cavities Distribution of gradients in C25 cavities that resulted in 1/(2 days) trip par cavity Distribution of gradients of the same C50 cavities at the end of 6 GeV operation (~ 4 years later) Courtesy of J. Benesch

27. CEBAF: no Halo ? • One ways to make large dynamic range measurement is to arrange it to be frequency measurement • Then make it work for 1 Hz and for 100 MHz and this is 108 dynamic range. • For instance use PMT and keep them working in counting mode Courtesy of A. Freyberger

28. CEBAF: vacuum • Despite the idea/claim that CEBAF beam is quite Gaussian and has no or very little large amplitude non Gaussian tails,there are vacuum “events” • Two types of events:1. Burn through that require a new piece of beam pipe to befabricated as it hasa hole drilled into it.2. Low current, very low intensity lose (chronic lose) that heatsup a flange. This requires Rad. Con. to identify the hot spot, and then the flange is tighten up and the region recoversquickly. • Frequency of such events is 1-2 per year (35 weeks of operation) • Type2 is due to some kind of beam that is not seen (not looked for) • Type1 (some of them) related to rapid energy change due to RF changes • Fortunately it did not happen close to the SRF LINAC

29. Conclusion / Summary • JLab FEL (IR/UV Upgrade) • RF gradients in LINAC always require attention, set radiation background level (F.E.) • Drive Laser transport if made very carefully, seems to be not a problem • Drive Laser rep. rate control (EO cells) always need attention (extinction ration drifts) • Cathode suffers when conditioning and from breakdowns, still makes beam as needed, but scatters DL light – generates some halo • Non-linear beam dynamics is responsible for some fraction of the halo. When setting up for high current operation, a lot of effort and time goes in to “fitting” the halo through the recirculator, such that peak beam brightness does not suffer. • Radiation monitors, BLMs and vacuum are used as tuning diagnostics • CEBAF • NP detectors (background) require essentially no beam halo • Large statistics of cavity performance and its evolution (F.E.) • Direct effects of F.E. – RF trip rate, reduction of max. possible energy • Vacuum events related to beam loss (both high and very low current)

30. Backup slides

31. FEL Injector as an example of #1 (1/6) downstream ofthe gun

32. FEL Injector as an example of #1 (2/6) upstream of the buncher cavity

33. FEL Injector as an example of #1 (3/6) downstream of the buncher cavity

34. FEL Injector as an example of #1 (4/6) upstream of the SRF cavity 1

35. FEL Injector as an example of #1 (5/6) downstream of the SRF cavity 1

36. FEL Injector as an example of #1 (6/6) downstream of theSRF cavity 2

37. Beam Loss Monitors • The primary BLM at the JLab is a 931B Hamamatsu photo-multiplier tube,operated with a fixed integrator and individually variable HV power supply • The BLM electronics are 12 channel VME boards. • PMTs are used in current (analog) mode • There is a single FSD fiber output to the MPS for each VME board • All 12 channels have analog monitors that are connectedto the Analog Monitoring System (AMS) • These are used as tune-up diagnostics in the control room • Calibration procedure • machine is locked into 1 uA CW operation • beam is driven into chamber and detector gain is varied by changing HV • the HV is adjusted until the system trips • this new “gain” setting is saved in EPICS and accounts for aging of tube. Ch3: Fault Trigger H = No Fault L = FSD Fault CH1: Drive Laser Intensity Courtesy of K. Jordan, D. Sexton