Feedback Instrumentation

1. Feedback Instrumentation DOE Lehman Review Oct 24, 2006 Josef Frisch

2. What Is Covered In This Talk • Single Pulse instrumentation used for pulse to pulse beam feedback • Beam Position Monitors • Toroids • Beam energy monitoring • Beam Phase Monitors • Bunch Length Monitors • New devices – more discussion in this talk • Feedback Software • Diagnostic Instrumentation was covered in Henrik Loos’ talk “Injector Diagnostic Status”

3. Beam Position Monitors • 5 micron resolution and stability. • Requirements 5-20 microns depending on location • Pickups • Injector striplines nearly identical to old Linac BPMs. • LCLS LINAC BPMs ARE old LINAC BPMs. • Improved low loss cable (LMR-400) • Electronics • Direct digitization of the 140MHz component of the beam signal, 14MHz bandwidth • Use tone injected into strip to calibrate perpendicular strips • Same digitizer as LLRF (and other systems), but uses private Ethernet to send raw data to IOC. • Software • Initial calibration in Matlab, convert to EPICS

4. Electronics Block Diagram

5. Electronics Performance 5 Microns at 200pC demonstrated 7 micron pk-pk drift of front end measured over 3 days.

6. Calibration Scheme for DC stability • Calibration system not yet tested. • Software under development • For BPM status see Steve Smith, “Beam Position Monitors Status”, Oct 25, 09:30 in the Controls System Breakout

7. Toroid • Want 2% precision, 5% absolute accuracy • Fast pickups and integrators to allow background (or dark current) subtraction • In addition to beam diagnostics, are used for MPS (not discussed here) • Delivery / fabrication problems for ceramic gaps.

8. Toroid Electronoics (analog) SPI Bus SPI Bus EEPROM PIC uC ADC DAC buffer Ethernet Interface Chan_Clear Reset CPLD Int #1 x 4 Chan_Ready Int #2 Cal Pulse Cal Charge Level Control Bus MPS 24V Drivers EVR Triggers (4) IOC Calibration Synch (EVR) 119MHz Synch Hope to switch to digitizer / software solution in future

9. Beam Energy Monitoring • Energy monitored in DL1, BC1, BC2, DL2 • BPMs in bend or chicane with dispersion. • Both BC1 and BC2 have movable vacuum chambers to allow the use of small aperture BPMs. • This allows an independent calibration of the BPM gain. • Dispersion ~0.25-0.5M, combined with ~5 micron resolution BPM gives <<10-4 energy resolution • Upstream and downstream BPMs available to correct for incoming orbit errors

10. BC1 Optics

11. Beam Phase Monitors • RF cavity excited by beam passage • First cavity located between L0-a and L0-b • Used to control laser phase drift • Second cavity after BC1 • Can be used to directly measure compression, and to check drifts • 0.2ps single shot, +/- 1ps drift • Cavity at 2805MHz • 25 MHz below normal LLRF LO • Using lower sideband avoids signals due to 2850MHz dark current • Somewhat tighter requirements on LO phase drift (but probably OK) • Same electronics as LLRF system • Analysis algorithm looks a phase during cavity ring down to determine exact frequency – reduces sensitivity to cavity temperature variations • Cables in temperature stabilized runs • Expected drift is +/- 800 fsec, should meet requirements

12. Beam Phase Monitor Prototype

13. Bunch Length Measurement Requirements

14. Bunch Length Monitoring – Overall Scheme • Transverse Cavities are the Gold standard • Provide single shot energy vs. time, with excellent resolution (<5 micron bunches measured at TTF2/FLASH) • Invasive – can only measure at a low repetition rate • Used to calibrate other measurements • Described in Henrik Loos’ Injector Diagnostics Status • Coherent mm-wave radiation power detectors • Used a full rate for uncalibrated feedback measurement. • Other systems may be used to reduce need for transverse cavity based calibration – but not baseline • Electro-optical measurement • Optical spectrum statistical measurement

15. Millimeter Wave Gap Radiation • Single Shot (assuming single shot spectrometer, or multiple detectors) • Non-Invasive • Simple high rate readout – can use signal from single detector • Very simple, low cost • Low noise readout <1% RMS demonstrated • Diode detectors work to ~300GHz -> ~200 micron bunch length • Possibly can be extended to ~ 1THz, ~70 micron bunch length • Provides only relative measure of bunch length • To be installed after BC1.

16. Millimeter Wave Gap Monitor • Gap, with 2 pairs of millimeter wave detectors • 100GHz detectors • Very good intensity sensitivity • Initial tuning, ~1mm bunch length sensitivity • Same detectors used for End Station A run • 300GHz detectors • 200 micron bunch lengths for normal operation • New type of detectors • Detectors produce short (~100 picosecond) output pulse, set by dispersion in waveguide • Amplifiers • Best sensitivity with ~1KOhm input impedance amplifier • Best linearity with ~50 Ohm input impedance amplifier • Both types tested, will try both in actual operation. • Data acquisition similar to LLRF system

17. Millimeter-wave gap monitor tests in End Station A • Output of 100GHz detectors as phase (bunch length) is adjusted M. Woods SLAC • Comparison of 2, 100GHz detectors for a range of operating conditions • RMS difference 1.4% for 10,000 pulses

18. BL12 Gap Monitor

20. 300 GHz diode detector

21. Gap Monitor with Pyroelectric Detectors • Tested at End Station A • Just put a pyro detector with RF horn next to the ceramic gap. • Same detector as used for BL12 (discussed next) • Sensitivity too low for single shot measurements, but should extend measurement range to <100 microns • Limit not yet tested • Lower performance than the mm-wave CSR monitor, but simple to commission M. Woods • Pyro detector looking at gap in end station A during RF phase change

22. BL12 Gap Monitor Overall Status • Expect system to be ready at turn-on • Detector distance from gap must be adjusted • Damage threshold ~100X max normal operating range, but signal level is difficult to calculate. • Only see signals when bunch is fairly short (few mm) • Can find shortest bunch length without calibration, but for length measurement, Need calibration – use TCAV at end of linac. • 1Km of beam line away: this is probably our most serious problem • Usual commissioning problems – if no signal: • Long bunch? Bad alignment? Dead diode? Bad cables? Bad timing? Ceramic doesn’t transmit mm-waves?

23. Millimeter Wave Coherent Synchrotron Radiation • Single Shot (assuming single shot spectrometer, or multiple detectors) • Non-Invasive • Measures from arbitrarily short to ~mm bunches (with appropriate filters). • Simple high rate readout – can use signal from single detector with input filter • Measures power spectrum (no phase information) – cannot reconstruct bunch shape • Variations on spectral response must be calibrated using external bunch length measurement – not practical to provide a calibrated signal • To be installed after BC1 and BC2.

24. BL11 Millimeter-wave CSR bunch length monitor • Mirror with hole after bend to collect synchrotron radiation stripe • Reflective optics (off-axis parabolas) to collect and transport light • Beam splitting filter for high pass / low pass to 2 mm-wave detectors • Different filters available • Compare power on detectors for (uncalibrated) bunch length measurement • Similar in concept to gap monitor, but bend and collecting optics give larger (>X10) signal, at cost of increased complexity • Need higher signal for short bunch measurements where diode detector do not work

26. Vacuum chamber • Minor modification of existing design used for screens. • Flat mirror with hole for beam, directs mm (and optical) radiation vertically through Z-axis quartz window. • No particular technical issues • Chamber is in fabrication, Expect complete in early February, install before commissioning. • Will install spool piece until ready.

28. Optical Path • Off-axis parabolas for transport / focusing • Millimeter-wave radiation awkward • Too small for waveguide • Too big for free space optics • Alignment not critical – mm, not micron wavelength. • Will be aligned with visible source • Filters not yet designed • In principal easy – pattern of squares, or wire grid on PC board to give low pass or high pass • Commercial solutions too sophisticated, very expensive (need multiple units for flexibility) • Difficult to test – millimeter wave test equipment expensive. • Do not need exact performance, only stability (calibrate system using TCAV) • Can be quickly installed • Most significant outstanding technical issue • Horns used to concentrate signal on detectors • Chamber purged with dry air / nitrogen • Choice is safety paperwork vs. performance

30. Detectors • Pyro-electric detectors used as good compromise on performance / cost • Wanted to avoid cryogenic detectors • Large area detectors needed to collect long wavelength signals. • Large area -> large capacitance ~700pf. Complicates amplifier design, but can use “physics style” charge amplifier Approx. requirement, comparison Of 9mm and 2mm detector energy collection

31. BL11 Millimeter Wave CSR monitor, overall status • Vacuum chamber slightly delayed, will probably not affect commissioning • Electronics / data acquisition not finished, but no significant problems foreseen • Filters designed but not yet fabricated or tested • Similar issues to gap monitor for calibration – need downstream TCAV. • Largest operational issue

32. Electro-optical Sampling • Non-invasive • Directly measures bunch longitudinal profile • Single Shot Measurement • Resolution down to ~100fsec • ~200 fsec demonstrated • Allows direct laser vs. beam measurements • Requires high peak current • LCLS BC1 current ~10X lower than at SPPS where some experiments were done. • Best at short pulses (BC2) which have higher current • Expensive and Complex – femtosecond laser, etc. • Probably will install after BC2 David Fritz

33. 1.5ps 4.5ps 1.5ps. 200-500pC, 44MeV beam using a spectrometer with a resolution of 0.05nm/pixel P. Catravas et al, Physical Review Letters 82 (1999) 5261 Optical Synchrotron Radiation Noise Measurement • RMS distribution measurement does not require calibration • Non-invasive • Not single shot • Will test after BC1 • Can upgrade to (near?) single shot measurement using optical spectrometer

34. Longitudinal Feedbacks (not shown) Laser phase controlled from phase cavity Energy in DL 1 controls L0 Amplitude Bunch length and energy in BC1 control L1 phase and amplitude Bunch length and energy in BC2 control L2 phase and amplitude Energy in DL2 controls amplitude in L3

35. Longitudinal Feedback Notes • Where multiple stations are controlled, klystrons are moved opposite directions in phase, rather than in amplitude to control overall RF amplitude • Better linearity • All feedbacks cascaded – use calculated / measured interaction between loops to allow higher bandwidth operation • Feedbacks will be commissioned at low repetition rate using Matlab • Tools already exist to allow Matlab code to read and write all measurements and actuators • Rapid development time, “physicist friendly” • EPICS based fast feedback system to be used in long term. • Uses private Ethernet connection for low latency • Transverse feedback uses same architecture (initially Matlab) as longitudinal. Sensors and actuators are more conventional.

36. Readiness and Issues • BPMs: Basic performance demonstrated, but still some engineering to be done • Toroids: No basic technical issues, but some fabrication problems • Beam Phase Monitor: Drift specification challenging, but a slight miss is a soft failure. • Bunch Length Monitor: New technology, but multiple parallel paths • Feedback: Production software not ready at commissioning, but most work can be done with Matlab.