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LCLS Cavity Beam Position Monitor

LCLS Cavity Beam Position Monitor. Steve Smith BPM Workshop CERN 16 January 2012. Linac Coherent Light Source (LCLS). X-Ray Free-Electron Laser Tunable coherent X-rays Mechanism: Send compact single bunch of electrons Through 130 m long undulator Coherent synchrotron radiation.

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LCLS Cavity Beam Position Monitor

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  1. LCLS Cavity Beam Position Monitor Steve Smith BPM Workshop CERN 16 January 2012

  2. Linac Coherent Light Source (LCLS) • X-Ray Free-Electron Laser • Tunable coherent X-rays • Mechanism: • Send compact single bunch of electrons • Through 130 m long undulator • Coherent synchrotron radiation Peak Power > 30 GW

  3. LCLS Injector at 2-km point Existing 1/3 Linac (1 km) New e- Transfer Line (340 m) X-ray Transport Line (200 m) Undulator (130 m) Near Experiment Hall Far Experiment Hall Linac Coherent Light Source at SLAC X-FEL based on last 1-km of existing linac 1.2 - 25 Å

  4. LCLS Layout/Diagnostics • 2 Transverse RF cavities (135 MeV & 5 GeV) • 180 BPMs and 13 toroids • 7 YAG screens (at E135 MeV, one at 14 GeV) • 13 OTR screens at E 135 MeV • 17 wire scanners (each with x & y wires) • CSR/CER pyroelectric bunch length monitors at BC1 & BC2 • 5 beam phase monitors (2856 – 51 MHz) • Gun spectrometer line + injector spectrometer line gun • YAG screens • OTR screens • Wire scanners • Phase monitors old screen L1X L0 3 wires 2 OTR 5 wire scanners TCAV0 4 wire scanners 3 OTR vert. dump L1S heater m wall 3 wires 3 OTR L2-linac DL1 135 MeV BC1 220 MeV BC2 4.7 GeV TCAV3 5.4 GeV BSY 14 GeV DL2 14 GeV undulator 14 GeV L3-linac

  5. Cavity BPM Requirements • The undulator orbit is critical • Must keep electrons and photons coincident • to fraction of beam size • over distance > gain length (m)

  6. Design Concepts • Avoid monopole mode • Cavity-waveguide coupler rejects monopole mode by symmetry • Zenghai Li (PAC 2003) • T. Shintake, “Comm-free BPM” • V. Balakin (PAC 1999) • Predecessor at KEK’s ATF • 16 nm resolution • Walston, (NIM 2007) Choices • Single, degenerate X&Y cavity • Reference cavity per BPM • Normalize to reference • amplitude and phase Dipole Cavity Reference Cavity Output waveguide Beampipe

  7. Cavity Parameters

  8. Prototype cavity

  9. Cold Test Set-Up for Pre-Braze test • All BPMs are tuned and cold tested before brazing • Tuning accomplished by micro-machining end-caps • Good correlation between cold test data before and after braze • Position and reference cavities machined in common block • Closed with endcaps

  10. BPM & Receiver BPM System

  11. Undulator

  12. Receiver • Downconverts X-band to ~40 MHz IF • Mounted on Undulator stand

  13. Receiver Chassis • Three channel receiver • X, Y, Reference • Downconvert 11.4 GHz RF to 40 MHz IF • Waveguide in • Coax out • Located underneath undulator

  14. Data Acquisition 4 Channel VME ADC (1 of 36) Undulator Readout Racks (1 of 2)

  15. Algorithms • Reduce each waveform to amplitude & phase (I,Q) • Normalize position signal to reference (amplitude and phase) • X’ = X/Ref (complex normalized amplitude) • Y’ = Y/Ref • Calibrate: • move BPM • Observe normalized amplitude vs. BPM position • Can use other BPMs (uncalibrated) to remove beam jitter • Extract phase & scale of position signal in normalized amplitude • Measurement • Rotate normalized amplitude by phase angle from calibration • Project real component • Scale and remove position offset • Position signal is projected from complex amplitudes • DO NOT detect power then try to get sign from phase • Ignoring phase results in poor resolution near origin

  16. Waveform / Spectrum • Cavity IF waveform sampled at 119 MHz • 16 bit digitizer • Extract amplitude, phase of • X, Y, Reference

  17. Move BPM Measure complex amplitudes (X, Y, Ref) Normalized amplitude = Position/Reference (Complex) Remove beam jitter using adjacent upstream BPMs Fit complex normalized amplitude to mover position Repeat for off-axis component Calibration

  18. Resolution Measurement • Measure resolution via correlation between BPMs • Coherent acquisition over • Many pulses e.g. 120 pulses • Many BPMs, e.g all 36 cavity BPMs • Least-squares fit of each BPM (X,Y) • to linear combination of neighboring BPMs • Model-independent • Slightly biased estimate • underestimates resolution very slightly due to fit • Insignificant bias for Npulses >> Nbpms • Overestimates resolution slightly, assumes other BPMs noise-free • Real resolution should be better by roughly 10% • Correction would depend on beta functions

  19. Resolution Measurement Fit the BPM on the 13th undulator girder to a linear combination of Y measured in the 3 previous BPMs and next 3 for 120 beam pulses.

  20. Position Resolution • Resolution measured at 145 pC/pulse • 120 beam pulses 10 Jan 2012 • Typical (median) resolutions: • sx ~ 220 nm • sy ~ 190 nm (2 outliers s > 1 micron) • Distribution of measured resolution:

  21. Resolution at Low Charge • LCLS designed to operate from 200 pC to 1 nC • Spec creep during design: • What about running at 100 pC? • Spec creep during commissioning: • How about 10 pC? • Or 1 pC? • Typical (median) resolution at 20 pC: sx,y ~ 1.2 mm Overdesign when you can; you may need it!

  22. Undulator Quadrupole Alignment after Beam-Based Alignment Earth’s field effect (0.4 G) 8 mm rms undulators installed (with m-metal) • Vary each quadrupole magnet gradient by 30% sequentially • Record kick angle using both upstream & downstream BPMs, adjusting for incoming jitter • Calculate quadrupole magnet transverse offsets Z (m)

  23. Measuring Stability in Presence of Beam Jitter • Accumulate data parasitic to beam operations • Take data periodically 120 shots every 20 min over 3½ days • Total >15,000 beam pulses • Ignore first 10 girders • undulator feedback moves these to maintain launch into undulator • Ignore downstream girders • periodic mover calibration running • Beam jitter ~10 microns at this time • Beam steering sometimes > 100 microns • Must remove real beam motions: • Take one run (120 pulses) from middle of weekend to learn correlation between each BPM and its neighbors • Fit linear coefficients to predict: • Xn from Xn-1 and Xn+1 • Yn from Yn-1 and Yn+1 • Use these coefficients to predict Xn, Yn • Compare measurement to prediction BPMs pulse-by-pulse

  24. BPM Stability X resolution ~700 nm Stable to ~ 1 micron /day Y resolution ~180nm Stable to < 200 nm over 3 days

  25. Long Term Stability Issue • In early commissioning we found substantial gain changes in some X-band BPM receivers • Gain drifts 0.1 dB/week in some cases • Up to 10 dB worst case • Frightening possibilities: • Radiation • Overvoltage • Found: • Probable hydrogen poisoning from chip packaging • Chip vendor app note says “Don’t use in hermetic packaging without hydrogen getter” • Slowly replacing LNAs as we can interrupt operation

  26. Improvements • Lower noise figure possible • Noise figure dominated by input attenuator • Can absorb out of band power without attenuating in-band signal • Will improve resolution by up to 14 dB in LCLS-II • Faster digitizer multibunch operation • 30 ns bunch spacing • In-line calibration • Can introduce calibration signal from opposite ports • Presently terminated • Subliminal calibration • Can calibrate with beam motion << beam jitter • Could perform continuous calibration while lasing • using with few-micron amplitude motion

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