LCLS_I Diagnostics Lessons Learned for LCLS_II

1. LCLS_I Diagnostics Lessons Learned for LCLS_II Josef Frisch

2. Diagnostics Functions • Provide data to beam feedbacks • Provide experiments with beam parameters • Allow beam tuning • Diagnose problems • Need to work when the accelerator is NOT operating correctly. • Need to be able to discover when the diagnostic IS the problem. • Support machine physics studies • Need to measure things we haven’t thought of yet.

3. LCLS_I • Seemed easy (if you weren’t there) • In reality things went wrong (COTR, Toroids, etc) but we had backups • Machine operating modes are very different from baseline (1nC, 800-8.3 KeV) • Now run 10pC to 250pC 480 – 11 KeV • We will probably have a different set of problems in LCLS_II. • Need to assume that LCLS_II may not operate with the parameters that we expect.

4. Stripline BPMs • LCLS striplines were in general very successful • Resolution ~5um at 250pC: Sufficient to tune machine down to 10pC. • Low power “coldfire” processor caused software difficulties. • LCLS_II has the same requirements with the additions: • Likely will want 2 bunches (~50ns separation) • May want to operate with lower charge for ultra-short bunches.

5. LCLS_II Stripline BPMs • Developing technology allows higher speed digitizers and higher bandwidth • Lower noise, or can tolerate cables with more loss • More powerful processors to allow multi-bunch decoding (electronics is OK). • Expect to package in uTCA, but design and performance are not dependant on form factor. • Will initially use existing cable plant in S10-20. Can upgrade later for lower charge operation if needed. • Try to find a common processing frequency for all stripline lengths – will simplify 2-bunch operation. • Try to find a processing frequency that is compatible with low transverse-wakes multi-bunch operation • BPMs are the only frequency in the system that are not harmonic with 2856MHz so they may set the multi-bunch frequency • Lots of technical issues with selecting the frequency. • Frequency selection has minimal impact on the rest of the design.

6. Cavity BPMs • LCLS design had acceptable resolution • Reliability and drift issues were caused by a manufacturing defect in the front end. • Waveguide seals were problematic • Problems with leaks. • Waveguide is lower loss than coax, but not needed – theoretical noise is only a few nm. • Waveguides transmit high harmonics which caused problems for front end electronics. • LCLS_II BPM requirements are the same but expect somewhat different design

7. LCLS_II Cavity BPMs • LCLS_I cavity design worked • Replace waveguide connections with coax. Additional loss is OK, we are far from theoretical noise limit. • Use single sideband PC board electronics similar to that used for ATF2 cavities (20nm resolution) • Lower cost and more compact • Easy to fabricate more boards if there are problems. • Can package as uTCA RTM module • Operate with unlocked digitizer clock and LO to simplify cable plant. • Need to demonstrate that this does not increase noise • Collaboration? Pohang FEL project has almost identical cavity BPM requirements. • Pohang built the cavity BPMs used for the ATF2 project – very sucessful! • Have Pohang build the cavities, SLAC does the electronics?

8. Charge Calibration • BPMs provide a sum “TMIT” signal that has good stability, but must be calibrated against an absolute measurement • Toroids and Faraday cups were provided for calibration but neither worked correctly. • LCLS now relies on an old SLC toroid as its absolute current calibration! • Why is absolute charge calibration important? • Need to cross-correlate LCLS_I and LCLS_II operating charge: If the machines behave differently is it because they are operating at a different bunch charge? • Important for understanding beam physics

9. Faraday Cups • Never worked properly for LCLS • They were not CUPS!!!! • Secondary and scattered electrons were not contained – can cause large errors in measured charge • Faraday cup design is well understood but requires significant space. • A single fixed faraday cup in the spectrometer line can provide charge calibration • Large space, does not need to move. • Beam losses are small through the accelerator • May also have a faraday cup at the gun spectrometer to calibrate charge before the beam is accelerated.

10. Toroids • Should provide absolute calibration • Both “by design”, and with a calibration loop • LCLS toroids were required to have high bandwidth to separate dark current from beam • Very difficult design – didn’t work properly • Can turn beam on / off to separate dark current • LCLS_II will use low bandwidth toroids to integrate beam current. • Will use a pair of (probably commercial) toroids to cross calibrate against LCLS_I • Mount both toroids in LCLS_1 after BC1 to cross calibrate • Move one to LCLS_2 after BC1 • Only need 1 toroidfor calibration, but keep BCS toroids to avoid re-design. • Can use low bandwidth toroids for BCS – but is it worth the change?

11. Profile Monitors • Ce:YAG screens • Work well at gun energy, but saturate with the smaller spots at high energy. • OTR screens • COTR light makes OTR screens unusable after OTR2. • Screens in dispersive regions can provide some qualitative information • Wire Scanners • Only known way to measure high brightness beam profiles.

12. YAG Screens • LCLS Screens have good performance • Some mechanical problems with actuation system • Not clear if this warrants a re-design • Dump screen requires study • Large energy loss and energy spread from TW FEL operation.

13. OTR Screens • Coherent OTR prevents useful measurements • No known solution • Coherent enhancement is large >104 • OTR is partially coherent – can not assume fully coherent • Microbunching is not uniform – some parts of the beam are enhanced relative to others • Microbunching extends into the near UV, expected to reach 100nm • Problem expected to be worse for low charge beams. • With 1pC bunch FWHM is <100nm. • May be possible to use XUV OTR light, but this needs extensiveR&D.

14. Wire Scanners • Installed in LCLS as a backup, but now the primary profile diagnostic • Significant tuning time is spent doing wire scans • In some locations it is possible to steer the beam across the wire for high speed scans. • Two different high speed wire scanner designs under development • High speed external linear motor • In-vacuum mover • Can use LCLS_I design, but should switch to high speed scanners if available.

15. Spectrometers • Dispersive regions in DL1, BC1, BC2, DL2, Dump • All except dump used in beam feedback • Dump energy loss used to measure FEL pulse energy • BPMs used for centroid measurement • Wires used for profile measurement • LCLS_II changes: • Software should fit to beam orbit • Dump spectrometer may need modification (next slide)

16. Dump Spectrometer for TW operation • Long tapered undulator to generate TW beams is under consideration for LCLS_II • Energy spread and energy loss ~few %. • Can compensate for average energy loss with a current shunt on the dump bend • The shunt can be current limited to preserve BCS requirements • May still need to design the dump for larger energy acceptance. • Large energy acceptance may make low power energy loss measurements difficult • Can use thermal sensor for calibrated measurements.

17. Transverse Cavities • Only quantitative longitudinal profile measurement • TCAV0 works well, no changes required • TCAV3 works for high charge, but insufficient resolution for low charge • Use X-band for LCLS_II ? • Eliminate and use dump TCAVs • XTCAV being installed in the LCLS dump. • Will provide high resolution (~2fs) FEL temporal measurements • Should include XTCAVs for LCLS_II dumps

18. Bunch Length Monitors • Pyroelectric bunch length monitors work well for LCLS • May be possible to simplify the design – but may not be worth the engineering cost. • Want higher sensitivity detector for BC1 for operation with low charge • Need to investigate widow options – need wider spectral range for low charges. • Diamond will probably work – in use for THz system • Multi-bunch operation requires fast response detectors • Need to test pyro detector response time • Fast response and good sensitivity may not be compatible • R&D required

19. Phase Cavities • LCLS Injector and Linac phase cavities work, but are rarely used • Beam phase determined by accelerator structure phase scans • Gun and laser phase sufficiently stable that scans are only required 1/shift. • Phase cavity after each undulator required to provide precision timing to the dump • LCLS system works • Evaluate upgrading to X-band for better resolution • LCLS_II undulator cavity BPMs

20. Loss Monitors • Fiber loss monitors work well • Will not have position resolution when used in regions with multi-bunch beams • Undulator loss monitors • LANL “fork” design unnecessarily complex and has non-uniform response. • Can replace with Cherenkov radiator bars or scintillators– need to evaluate whether we need top and bottom or just one per segment • Need BSA readout system – link nodes not designed for this • Need integrating monitors (fiber, RADFET) for accurate total dose measurement. Remote readout is desirable. • Needs some R&D • BCS for LCLS_1 / LCLS_2 interaction needs to be considered • Trip both beams on any loss?

21. Decisions: • Stripline BPMs: • Study linac BPM frequency • Cavity BPMs: • Demonstrate low noise with unlocked clock • Collaborate with Pohang on cavity BPMs? • Charge Calibration: • Develop new toroid and Faraday cup design. • Profile monitors • Improved mechanical motion on YAG and OTR • Faster wire scanner design • Fix COTR (very difficult) • Spectrometer • Design broad band dump spectrometer • TCAV • X-band in linac? TCAV after each undulator? • Bunch length monitor • Develop fast mm-wave detector • Loss Monitors • Develop improved undulator loss monitor