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 1.Bunches enter off-axis 2.HOM excited 3.Bunch gets kicked 4.Returns to cavity further off-axis 5.Excites larger HOM field 6.Next bunch gets bigger kick 7.… 8.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) Q 1/2 /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 1.Drive a mode in the cavity 2.Monitor BPM position downstream 3.Turn off driving force 4.Monitor decay of beam’s oscillation – Amplitude = R/Q – Decay constant = Q 5.Position dependence in cavity = monopole, dipole, etc.

11. Exciting the HOM Modulate the bunch charge at frequency f mod – sidebands: Time Beam current

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

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

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 –  f min ~ 1 kHz – Q max ~ 10 7 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  m @ 2 mA

20. Machine setting No quads Short bunch length Position feedback Charge0-77 pC Energy4.9 MeV Bunch spacing20 ns (CW) Bunch length2 ps (rms) @ 77 pC Beam width3 mm (rms) @ 77 pC (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),  f, 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 HTCBPM 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 = 3.3x10 7 GHz -1 Q/f = 4.9x10 6 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 f mod = 5.518 MHz, f center = 2.5 GHz Monitor RF probe on 2 nd SA Vary f mod, record peak height on 2 nd SA 1.3GHz + n(50 MHz) ± f mod f mod = 10.03 MHz, f center = 2.3 GHz

31. Summary of data taken Broad scans, 0.5-25 MHz, 10 KHz steps – Horz off-axis, horz BPM – Vert off-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) 2 nd SA frequency scans for each HTC mode

32. Summary of results f mod (Mhz) f (GHz) Q  f (KHz) BPM Peak (mm) 3.359762.3038.3x10 6 0.280.22 5.1121882.4951.1x10 7 0.221.7 5.2699681.2954.3x10 7 0.0300.62 5.517962.4942.0x10 7 0.122.2 8.8600?1.5x10 4 (GHz -1 ) 700.025 10.03062.2901.3x10 7 0.180.82 10.69982.4891.7x10 6 1.50.16 24.30032.4764.7x10 6 0.120.13