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Beam-based Measurements of HOMs in the HTC

Beam-based Measurements of HOMs in the HTC. Adam Bartnik for ERL Team, Daniel Hall , John Dobbins, Mike Billing, Matthias Liepe , Ivan Bazarov. Summary. What I will talk about Introduction Our experiment Raw data What I won’t talk about

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Beam-based Measurements of HOMs in the HTC

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  1. Beam-based Measurements of HOMs in the HTC Adam Bartnik for ERL Team, Daniel Hall, John Dobbins, Mike Billing, Matthias Liepe, Ivan Bazarov

  2. Summary • What I will talk about • Introduction • Our experiment • Raw data • What I won’t talk about • Detailed analysis of the data (stay tuned…)

  3. New Injector Layout: HTC From ICM To Dump Beam goes this way

  4. Higher order modes

  5. HOMs excited by wake fields First bunch enters cavity Bunch excites fields Future bunches receive kick

  6. Beam breakup in an ERL • Bunches enter off-axis • HOM excited • Bunch gets kicked • Returns to cavity further off-axis • Excites larger HOM field • Next bunch gets bigger kick • … • BOOM! • Instability occurs above a threshold current

  7. Beam breakup in an ERL • Beam breakup limits currents in an ERL • J-Lab ERL limited to <30 mA by beam breakup (simulation) • Cornell needs > 100 mA • HTC designed carefully with this in mind • Question: how can we estimate the threshold current without building an ERL?

  8. Simulations / Merit function • Simulation • Cavities with given mode ( R/Q , Q, f, …) • Realistic lattice • Add slight randomness to HOM properties • Find some merit function that correlates linearly with threshold current: • (R/Q)Q/f … (R/Q) Q1/2/f… (??) … • Measure merit function in real cavity

  9. Goal: Characterize HOMs • Questions to answer • f • Q • R/Q • Dipole, quadrupole, etc.? • Measuring R/Q requires a beam-based method

  10. Beam-based method • Drive a mode in the cavity • Monitor BPM position downstream • Turn off driving force • Monitor decay of beam’s oscillation • Amplitude = R/Q • Decay constant = Q • Position dependence in cavity = monopole, dipole, etc.

  11. Exciting the HOM • Modulate the bunch charge at frequency fmod • sidebands: Beam current Time

  12. Exciting the HOM • Charge modulation via laser modulation • 1.3 GHz laser • Good: Up to 75 mA current • Good: Easy to search sidebands • Bad: Need to search 0-650 MHz • Prohibitive: Cannot modulate high power laser that fast • 50 MHz laser • Bad: Only 2 mA • Bad: Laborious to find the sideband exciting the mode • Good: Only search from 0-25 MHz • Good: Can directly modulate the (final) laser beam

  13. Monitor BPM Position Last easily accessible BPM 3.4 meters drift

  14. Monitor BPM Position • Spectrum analyzer in zero span mode • Baseband (0-25 MHz) has poor BPM response and background noise • Use sideband around higher harmonic of 50 Mhz • 1.3 GHz is convenient, but also has larger background • 1.3 GHz – 50 Mhz = 1.25 GHz was used instead

  15. Monitor BPM Position f 1.25 GHz 1.3 GHz Laser Switch 50 MHz BPM signal Spectrum Analyzer (1.25 GHz – f) Cathode trigger Pulse Generator BPM 180o hybrid

  16. Expected signal Bunch charge: Position and amplitude modulation • Only position modulation, • Decay gives Q • Peak amplitude gives R/Q BPM Signal BPM Signal On resonance: Otherwise:

  17. Scanning details • Is this no mode, or just a really big/small Q? • Scan with multiple scan lengths / SA bandwidths • What frequencies do we choose? • Scan takes ~20 seconds • 25 MHz / (40 hours / 20 s) = 3.5 kHz • Eventually settled on 10 kHz steps to speed things up BPM Signal

  18. What can we find? • Small width modes will be missed • 10 KHz steps • Dfmin ~ 1 kHz • Qmax ~ 107 • SA bandwidth • Smaller = better noise floor • Larger = faster response (can see smaller Q) • Choose target Q, set bandwidth accordingly

  19. What can we find? • SA noise floor: P ~ -100 dBm • Noise floor ~ 5 mm @ 2 mA

  20. Machine setting • No quads • Short bunch length • Position feedback (from simulation)

  21. 25 MHz modulator performance • Laser pulse measurements • Only 50% modulation depth at 25 MHz

  22. Example data • Fit to exponential • Get (Q/f) , Df, BPM deflection

  23. Broad scan • 10 kHz steps • SA bandwidth: 100 kHz (red), 1 MHz (blue) • Found lots of peaks!

  24. Two cavities! • The beam also passed through the ICM • Repeat with BPM before the HTC Last BPM before HTC

  25. Almost all peaks from ICM BPM after the HTC BPM before the HTC

  26. Modes actually in HTC One of the peaks in this group These peaks

  27. Fine scans • Find peak frequency • Double check expected peak width Q/f = 4.9x106 GHz-1 Q/f = 3.3x107 GHz-1

  28. Position dependence • On resonance • Displace beam vertically or horizontally • Use vertical or horizontal BPM downstream

  29. Example position dependence

  30. Finding the true frequency • Monitor RF probe on 2nd SA • Vary fmod, record peak height on 2nd SA • 1.3GHz + n(50 MHz) ± fmod fmod = 5.518 MHz, fcenter = 2.5 GHz fmod = 10.03 MHz, fcenter = 2.3 GHz

  31. Summary of data taken • Broad scans, 0.5-25 MHz, 10 KHz steps • Horz off-axis, horz BPM • Vertoff-axis, vert BPM • Traces before HTC at each peak (ICM) • Fine scans around each HTC mode • Position dependence • All combinations (vert/horz off-axis, vert/horz BPM) • 2nd SA frequency scans for each HTC mode

  32. Summary of results

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