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Advanced Beam Bunch Length Monitoring Techniques in Accelerator Physics

Detailed overview of bunch length measurement techniques, monitor requirements, and coherent radiation detectors for beam diagnostics in accelerator physics. Discusses calibration, precision measurement, and spectral power spectrum analysis.

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Advanced Beam Bunch Length Monitoring Techniques in Accelerator Physics

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  1. Steve Smith for J. Frisch, T. Borden, H. Loos, T. Montagne, M. Ross, D. Schultz, J. Wu, et al April 20, 2006

  2. Applications of Bunch Length • Beam longitudinal profile for “accelerator physics” • Calibrated profile needed to understand machine • Measurement can be low rate, invasive • Bunch length signal for feedback • Non invasive • Signal at full repetition rate of beam • Only need an output which is monotonic and stable with respect to bunch length tuning phases

  3. Bunch Length Monitor Requirements • After BC1: • 80 to 360 microns at 1nC • 130 – 600 GHz Gaussian width • 25 to 120 microns at 0.2nC • 400GHz – 2THz Gaussian width • After BC2: • 8 to 40 microns at 1nC • 1.2THz to 6 THz • 4 to 20 microns at 0.2nC • 2.4THz to 12 THz • Bunches not Gaussian  frequency distribution will be somewhat different. • Goal for commissioning run: • Instrument and commission BC1 • Gain operational experience • Discussion here almost entirely for BC1

  4. Measurement Options • Temporal • Works like a high speed oscilloscope. • Transverse deflection Cavity (LOLA) • Electro-optical measurement • Spectral • Measure power spectrum radiated by beam • Coherent radiation • Any kind: • Synchrotron • Edge • Diffraction • Gap • Spectral measurement does not include phase information is lost.

  5. Precision Measurement • Transverse RF deflection structure (LOLA) • High resolution • directly calibrated • using known phase shifts. • Measurement from LOLA: • TTF at DESY • 4 micron resolution (13 fs) demonstrated  • Intercepting 13 femtosecond FWHM spike! 1 picosecond • LOLA deflection cavity installed in LCLS will be used as the “Gold Standard” bunch length measurement • Beam physics experiments • Calibration of “spectral” detectors • Run LOLA at some slow rate (as needed) • to maintain calibration of non-intercepting bunch length monitors

  6. Coherent Radiation Detectors • BC1 range is 100GHz to 1THz • (BC2 to 10THz) • Corresponds to the 100um to few mm wavelength range for BC1 • Two approaches: • Waveguides • Optics • Standard microwave waveguide techniques difficult above 10 GHz • near impossible at THz • Free space quasi-optical techniques difficult at longer wavelengths (mm) due to diffraction. • Materials absorption not well known in this frequency range. • Calibrated measurement difficult • Saved by LOLA • Use both free-space and waveguide technology at BC1

  7. Spectral Measurements • Detect coherent radiation two ways: • CSR or Edge radiation in a bend • Coherent radiation from ceramic gap • Both provide order of a microJoule of energy. • CSR/edge radiation provides somewhat more power and lower divergence • easier to collect on the detector. • Radiation from last bend of BC1 available • BL11 is CSR/edge radiation detector • Easy to add ceramic gap downstream. • BL12 is gap radiation detector • Calibration of bunch length vs. spectral power: • difficult to do from first principles • but we have transverse cavity (LOLA) • As long as signal is monotonic and reproducible, we can do periodic calibrations • Eliminates the most serious problems with spectral detection.

  8. Detectors • High performance mm-wave detectors are cryogenic. • Used for astronomy, etc. • Avoid cryogenics if possible • Room temperature detectors in principle have an energy sensitivity of Ethermal ~ kBT ~10-20J. • Real detectors much worse • Two common technologies: • Pyroelectric • Diodes

  9. Diodes vs. Pyroelectrics • Diodes limited to ~750GHz • Diodes have better sensitivity • Diodes have worse dynamic range, ~10,000:1, but this is probably not a limit • Diodes more expensive ($5K at high frequencies), • rather than $500 (including preamplifier for pyro • Diodes are more damage sensitive.

  10. Waveguide Attenuation • Waveguides available as small as WR-0.51 • Internal Dimensions: 130um X 65um • Frequency 1.4-2.2 THz • Attenuation can be very high for small waveguide • 3dB/M at 100GHz • 17dB/M at 300GHz • 100dB/M at 1THz • (attenuations from empirical fit to data) • Limits use of waveguide at high frequency

  11. Waveguide vs. Free space Compare Rayleigh length for free space with sigma = .5cm relative to length for 10dB attenuation in Waveguide Approximate cross-over At 400GHz

  12. Coherent Synchrotron Radiation • Narrow opening angle, large transverse size at end of magnet suggest use of free space optics to image onto detector. • Expect order of 1uJ collected on detector. • >1000X Pyroelectric sensor sensitivity. • No advantage to diodes here • Since free space optics works well at high frequencies, this seems a good solution for frequencies >~250GHz

  13. Conceptual Design Diagnostics Focusing 200mm DR 10mm Bend ER Mirror Focusing f = 200mm SR 38mm 200mm

  14. BL11 Bunch length monitor • Use CS/edge radiation • free space • pyroelectric detector. • Systems like this already in use • M. Hogan at SPPS • Retractable mirror in vacuum. • Use flat mirror • Off axis parabola would collect slightly more signal • but has difficult alignment issues • Slight modification of existing vacuum chamber and insertion design • Hole for beam passage • Small optical table for detector components • Insertable wavelength filters. • Alignment diode • has phase space similar to mm-wave radiation

  15. BL 11 Quasi-Optical / Pyroelectric Monitor • Image coherent synchrotron & edge radiation on pyroelectric detector

  16. BC1 Radiation Distribution • Wavelength 1mm • 200mm downstream of BC1 • Near field integration of “acceleration field” • Edge length « λγ² • Mainly ER from both bend edges, 4x larger than SR • Radiation from Entrance edge hits vacuum chamber Horizontal Pol. Vertical Pol.

  17. Propagate Gauss-Laguerre Modes • Use Gauss-Laguerre modes with radial mode number 1 for field of each polarization • Needs γ/2 transverse modes to get correct far field distribution Horizontal polarization at magnet edge λ = 1cm γ = 500

  18. CER Transmission Through Optics For one polarization, normalized to total 2π emission 3 cm-1 at detector 15 cm-1 3 cm-1 15 cm-1

  19. Transverse Profile Through Optics 3 cm-1 15 cm-1

  20. Is Interference of CER & CDR a Problem? • Get field at detector for CER and CDR • CDR is not focused on detector • Wave front curvature differs from CER • Intensity at detector shows narrow fringe pattern • Fringes much faster than changes in form factor • Conclusion: CDR can be ignored

  21. Predicted Detector Signal vs Bunch Length

  22. Pyroelectric Detectors • Crystal which converts thermal directly to electrical output • LiTaO3 • “physics” is fast – nanosecond • coatings can slow down the detectors. • Integrate all input energy (DC-gamma rays) • Very good linearity up to damage threshold. • Act as current sources, approximately 1uC/J • Noise limited by preamplifier.

  23. Pyroelectric Detector Sensitivity • ELTEC420m3 • 5mm diameter detector (20mm2). • 0.3 uC/Joule sensitivity • Detector capacitance Cd ~100 pF • A good charge preamplifier (Amptek A250F) should see 300 electrons RMS noise • based on 100pf capacitance • Corresponds to 0.15nJ detector noise. • Parts cost: • Detector $75 • Pre-amplfier about $500. • Threshold sensitivity ~ 7.5pJ/mm2

  24. BL11 Bunch Length Monitor:Development Plan • Use of flat mirror in vacuum and existing chamber / mover design minimizes engineering before installation • Optics and detectors on table can be modified as needed • only humidity proof cover required • Serves as a model for the BC2 bunch length monitor • Short bunch length / high frequencies requires pyro detectors • allows for easy use of free space optics .

  25. Radiation from Gap • 1 nC, 200micron bunch, 1cm gap gives about 2 uJ total energy • Calculations fro Juhao Wu • Radiation is distributed over a wide area – difficult to collect. • Corresponds to about 1.6nJ/mm2 for a 2cm radius gap • Pyroelectric detectors (7.5pJ/mm2) marginal (especially for a 0.2nC bunch). • Diode detectors (.03 to 0.4 pJ/mm2 depending on wavelength) look OK. • RF horn will gain 10-20dB in diode detectors • but probably lose 10dB in waveguide at high frequencies • Looks reasonable, limited by waveguide, and diode frequency response to frequencies below about 500GHz.

  26. BL12 Bunch Length Monitor • Located just after the BL11 monitor • Uses gap and diode detectors • Only vacuum component is conventional ceramic gap • Initially instrument with 100GHz diodes • Add higher frequency diodes as needed • Diodes used in pairs to reduce effect of beam motion • 20cm waveguide used to disperse pulse (~1ns), keep peak power reasonable on diodes. • 20dB gain horns on diodes

  27. BL12 Waveguide / Diode Monitor

  28. RF Diode Detectors • Very fast diode to rectify the input signal • Vout Pin for input voltages < diode drop • Typically modest output impedance (~ few KOhm). • Linear output range limited to ~100mV. • Use waveguide dispersion to stretch mm-wave pulse to keep diode in linear range. • Very high sensitivity ~1V/W, or 1mC/J . • Typically connected to waveguide • Many vendors for F<~130GHz • Few (only Virginia diodes found so far) for higher frequencies up to ~800GHz.

  29. RF Diode : 100 GHz • Millitech DXP-10 • WR10 input waveguide • Active area 3mm2. • 20dB gain horn available. • Approx 2KOhm output impedance • Output charge 0.15mC/J. • Capacitance small ~1pf. • Assume A250F charge amplifier • expect ~100 electrons noise • corresponds to 0.1pJ detector noise • Maximum linear signal ~500pJ • Cost ~$1K, preamplifier $500 • Threshold sensitivity 0.03 pJ/mm2 (energy density)

  30. Microwave Diode at 300GHz • Virginia Diodes WR-2.2ZBD • WR-2.2 input waveguide • Active area 0.16mm2 • 20dB gain horn avaialble • Output impedance ~3KOhm • Output charge ~ 1mC/J • Capacitance small ~1pf. • With A250F charge amplifier, expect ~100 electrons noise, corresponds to 0.016 pJ detector noise. • Max linear signal 0.1nJ • Cost $5K, preamplifier $500 • Sensitivity 0.1pJ/mm2 Note, for 750GHz diode get threshold sensitivity ~0.4pJ / mm2

  31. BL12 Development Plan • Similar diodes operating at 100GHz tested in End Station A. • Additional test in end station A in April 2006 • using same electronics as LCLS • Will use pair of diodes to check measurement noise. • Initial test in LCLS will be done with a pair of 100GHz detectors. • As shorter bunch length measurements are required, additional diodes and waveguide can be added • Use of optical breadboard makes installation of new diodes (on optical clamp mount) straightforward. • Will try using a pyroelectric detector mounted next to the gap. • Should be able to measure total mm-wave power • compare with toroid current measurement to get bunch length signal • Very simple and inexpensive system if it works. • In principal extends to very short bunch lengths • Must be calibrated with LOLA

  32. Controls Interface • Pyroelectric detectors, and diodes will use very similar “nuclear physics” type charge sensitive preamplifiers • Signals can be read with a conventional GADC (gated ADC). • Initially will use SLC CAMAC ADC • Existing software for control and histories • Can provide slow feedback to main LCLS EPICS control system for feedback tests. • For high bandwidth feedback convert to EPICS GADC in VME. • Other controls interface is straightforward • pneumatic actuators • temperature monitoring

  33. Summary • Two coherent radiation bunch-length monitors for BC1 • Measure bunch length every pulse • non-intercepting • BL11 • Quasi-optical • Pyroelectric readout • BL12 • Waveguide • Diode readout • Both are calibrated by transverse deflecting cavity

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