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Electron Cloud Studies at KEKB

Electron Cloud Studies at KEKB. J.W. Flanagan 2007.1.30 @ Cornell LEPP. Also: K. Ohmi, E. Benedetto, H. Fukuma, Y. Funakoshi, S. Hiramatsu, T. Ieiri, H. Ikeda, K. Kanazawa, Y.Ohnishi, K. Oide, E. Perevedentsev, M. Tobiyama, S. Uehara, S. Uno, S.S. Win, Others…. Overview.

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Electron Cloud Studies at KEKB

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  1. Electron Cloud Studies at KEKB J.W. Flanagan 2007.1.30 @ Cornell LEPP Also: K. Ohmi, E. Benedetto, H. Fukuma, Y. Funakoshi, S. Hiramatsu, T. Ieiri, H. Ikeda, K. Kanazawa, Y.Ohnishi, K. Oide, E. Perevedentsev, M. Tobiyama, S. Uehara, S. Uno, S.S. Win, Others…

  2. Overview • Brief summary of beam measurements made at KEKB, in particular as they may relate to ones which can be made at CESR • Size (Fukuma) • Tune shift (Ieiri) • Coupled-bunch mode spectra (Tobiyama) • Single-bunch head-tail sideband signal • Focus on this – would like to try to see at CESR • Luminosity-related measurements

  3. Electron cloud-induced beam blow-up • Vertical beam blow-up has been observed at KEKB LER (positron ring) at bunch currents above a threshold of ~0.35 mA/bunch, at a spacing of 4 rf buckets (~8 ns). • Blow-up due to photo-electrons kicked from wall by synchrotron radiation, which form clouds that interact with the positron beam. • Bunch-current blowup threshold can be raised by the use of solenoids around the beam pipe. • Currently about 95% of drift space in LER is covered.

  4. LER Electron-cloud suppression solenoids Fukuma et al., “Study of Vertical Beam Blow-up in KEKB LER,” HEAC01 proceedings

  5. Blow-up measured by SR interferometer Fukuma et al., “Study of Vertical Beam Blow-up in KEKB LER,” HEAC01 proceedings

  6. Bunch-by-bunch beam size along train as measured by gated camera Fukuma et al., “Study of Vertical Beam Blow-up in KEKB LER,” HEAC01 proceedings

  7. Gated Camera Observations of Trailing Witness Bunch J.W. Flanagan et al., Proc. EPAC00 (2000) 1119

  8. Detection of photoelectrons at wall Ohnishi et al., “DETECTION OF PHOTOELECTRON CLOUD IN POSITRON RING AT KEKB,” HEAC01

  9. Tune Shift MeasurementsT.Ieiri, et al.,PRST-AB 5, 094402 (2002) Train Bunches Witness Bunches

  10. Tune Shift after Train w/o Solenoids T.Ieiri

  11. Width of Tune Spectrum  Damping Rate is proportional to Frequency Width  Spectrum Width depends on various effects Wb: Radiation damping + Head-tail damping (Chromaticity, Impedance) + Beam-Beam + Electron Cloud Wc: Feedback damping + Current ripple + Nonlinear magnetic field W0: RBW (Resolution Band Width) T.Ieiri

  12. Width vs Bunch Current w/o Solenoids I_train=380 mA ξx=1.7 ξy=4.8  電流バンチとCloud密度 増加でwidth減少 T.Ieiri

  13. Beam spectrum measurements • Bunch Oscillation Recorder • Digitizer synched to RF clock, plus 20-MByte memory. • Can record 4096 turns x 5120 buckets worth of data. • Calculate Fourier power spectrum of each bunch separately. • Inputs: • Feedback BPMs • 6 mm diameter button electrodes • 2 GHz （4xfrf) detection frequency, 750 MHz bandpass • Fast PMT

  14. Coupled-Bunch Measurements M. Tobiyama et al., PRST-AB 9, 012801 (2006) • Abstract : • A coupled bunch instability caused by an electron cloud has been observed in the KEKB LER. The time evolution of the instabilities just after the turning-off the transverse bunch feedback was recorded with several weak solenoid-field conditions, which are used to suppress the vertical blowup of the beam size due to the electron cloud. • The mode spectra and their growth rates of the coupled-bunch instabilities were compared with simulations of electrons moving in drift space, a weak solenoid field and a strong bending field. • Mode spectra without a solenoid field support the model where the instability is dominated by the electron clouds in the drift space with a lower secondary yield of photoelectron 2,max = 1.0 rather than 1.5. With the solenoid field, the behavior of unstable modes and the growth rate with the strength of solenoid field also support the simulation with lower secondary emission yield.

  15. Analysis Procedure

  16. Horizontal Mode Spectra Bsol=0% Bsol=10% Bsol=20% Bsol=100%

  17. Horizontal: Time evolution Bsol=0% Bsol=10% Bsol=20% Bsol=100%

  18. Horizontal: Effect of Chromaticity xx=-0.3 xx=-0.7

  19. Vertical Mode Spectra Bsol=0% Bsol=10% Bsol=20% Bsol=100%

  20. Vertical: Time Evolution Bsol=0% Bsol=10% Bsol=20% Bsol=100%

  21. Vertical: Effect of Chromaticity xy=5.2 xy=4.2

  22. Single-bunch measurements: synchro-betatron sidebands • Vertical betatron sidebands found at KEKB which appear to be signatures of fast head-tail instability due to electron clouds. • J.W. Flanagan, K. Ohmi, H. Fukuma, S. Hiramatsu, M. Tobiyama and E. Perevedentsev, PRL 94, 054801 (2005) • Presence of sidebands also associated with loss of luminosity during collision. • J.W. Flanagan, K. Ohmi, H. Fukuma, S. Hiramatsu, H. Ikeda, M. Tobiyama, S. Uehara, S. Uno, and E. Perevedentsev, Proc. PAC05, p. 680 (2005) • Further studies have been performed: • Single beam studies: • Varying RF voltage • Varying chromaticity • Varying initial beam size below blow-up threshold (emittance) • In-collision studies: • Looking at specific luminosity below sideband appearance threshold • Looking at specific luminosity closer to head and tail of LER bunch

  23. Fourier power spectrum of BPM data V. Tune Sideband Peak • LER single beam, 4 trains, 100 bunches per train, 4 rf bucket spacing • Solenoids off: beam size increased from 60 mm ->283 mm at 400 mA • Vertical feedback gain lowered • This brings out the vertical tune without external excitation

  24. Spectra of Bunches 1-10 Bunch 1 Bunch 6 Bunch 2 Bunch 7 Bunch 3 Bunch 8 Bunch 4 Bunch 9 Bunch 5 Bunch 10

  25. Synchro-betatron sideband characteristics • Sideband appears at beam-size blow-up threshold, initially at ~nb+ns, with separation distance from nb increasing as cloud density increases. • Sideband peak moves with betatron peak when betatron tune is changed. • Sideband separation from nb changes with change in ns. (Flanagan et al., EPAC06)

  26. Model focusing wake a=wR/2Q Mode spectrum using model wake and airbag charge distribution. Sideband-betatron peak separation dependence on synchrotron tune reproduced.

  27. Simulations of electron cloud induced head-tail instabilityE. Benedetto, K. Ohmi Simulation (PEHTS) (HEADTAIL gives similar results) Betatron sideband Tail of train Head of train Head-tail regime Incoherent regime

  28. Feedback does not suppress the sideband • Bunch by bunch feedback suppresses only betatron amplitude. Betatron sideband Simulation (PEHTS) Sideband signal is Integrated over the train

  29. PMT setup Partially block beam image with black cardboard, and measure light intensity of the visible part with a PMT. The PMT signal is buffered and then recorded using a feedback BOR digitizer/memory board. y: 100 mV/division x: 10 ns/division

  30. Beam Blow-up Measurement 4-bucket spacing, 600 bunches 4 trains, 150 bunches/train, 4 rf bucket spacing Vertical beam size at IP (mm) Blow-up threshold 250 mA, 0.42 mA/bunch LER Beam Current (mA)

  31. PMT Spectra FB BPM Spectra 100 mA, 2.3 mm 200 mA, 2.7 mm 250 mA, 2.9 mm Blow-up Threshold 300 mA, 3.6 mm 450 mA, 4.9 mm 1.0 Tune 0.5 1.0 Tune 0.5

  32. Summary of BPM + PMT measurements • Sideband peak appears in both BPM and PMT measurements. • Two different types of detector • Sideband peak appears in both instruments only at and above the beam-size blow-up threshold of beam current • Other measurements show that the amplitude of the sideband peak at constant beam current is affected by the strength of the solenoid field. Stronger solenoid field  smaller sideband peak.

  33. Time series data BPM Data (Position) PMT Data (Size)

  34. Time Development of Instability: 512-turn slices BPM data

  35. Blow-up Pattern Analysis: PMT data Find a bunch with characteristic blow-up pattern, and take spectra of 3 stages separately. Then average the 3 spectra over all bunches that have this pattern.

  36. Summary of time domain data • Sideband oscillation has a burst-like structure. • Sideband peak is present at a low level, then grows and damps in a burst lasting ~500 turns (5 ms). • During this burst, beam size grows ~5% from its already blown-up state • Immediately after burst is complete, sideband peak is absent, until beam size damps back down.

  37. Threshold dependence on synchrotron tune dns Data: 23 Dec 2003 Vc = 8 MV Vc = 6 MV 25 Bunch 1 0.5 Tune 0.75 0.5 Tune 0.75 • Data taken at 8 MV and 6 MV. • Sequence: 8 MV×2, 6 MV×2, 8 MV×2, 6 MV×1 • Synch. Tune = 0.0237 @ 8 MeV, 0.0203 @ 6 MeV (measured from spectral peak). • Peak frequency bin in sideband region found, peak height averaged for 8 MV and 6 MV subsets separately.

  38. Sideband Peak Height Near Threshold at Different ns ns=0.0203 ns=0.0237

  39. Effect of changing RF voltage200 bunches/train, 4-bucket spacing, 0.6 mA/bunch, xy = 4.27 νy Sideband • Vc = 8 MV • FB gain = -14.9 dB Tail----------Head Tail----------Head • Vc = 6 MV • FB gain = -18.3 dB 0.5-------------------------------------------------Tune-----------------------------------------0.7

  40. Effect of varying synchrotron tune (RF voltage) Sideband-tune separation does not change Or does it? Hard to tell.

  41. Model Spectrum Dns

  42. Effect of changing RF voltage Separation is found to be close to Dns towards the head of the train, and it decreases going towards the back of the train, where the cloud density is higher. Dns Dns

  43. Effect of changing RF voltage • Sideband onset is delayed along train (~3 bunches). • Confirms previous results. • Betatron peak growth is not delayed. • Note that it peaks just before sideband appears

  44. Effect of Changing ns (RF Voltage) • Conclusion: • Threshold and separation between betatron peak and sideband peak are found to depend on ns, in agreement with model.

  45. Effect of Changing Chromaticity • An effect predicted by head-tail theory is that the e-cloud density threshold for the onset of the instability should go up if the vertical chromaticity is raised. • Feedback gain was changed at each beam current to make ny visible. To make sure this did not affect results, also took data at same chromaticity and two different feedback gains.

  46. Sidebands at Different xy ny Noise Sideband xy = 1.27 xy = 4.27 FB Gain = -13.1 dB FB Gain = -14.9 dB Joining/Splitting? xy = 6.27 xy = 6.27 FB Gain = -14.9 dB FB Gain = -18.3 dB

  47. Sideband Peak Heights xy=1.27 xy=4.27 xy=6.27, FB gain = -14.9dB xy=6.27, FB gain = -18.3 dB • Note: For KEKB, Dxy ~ 3 should correspond to a change in cloud-density threshold of ~10%

  48. Simulated E-cloud build-up at KEKBWang et al., PRSTAB 5 124402 (2002) Bunch: 10 20 30

  49. Effect of Changing Chromaticity • The lower the chromaticity, the earlier in the train the sideband appears. (No change is seen for two different feedback gains at xy=6.3, as expected.) Raising xy from 1.3 to 4.3 pushes the onset of the instability back ~10 bunches along the train, as does further increasing xy from 4.3 to 6.3. From simulations of electron cloud build-up (L.F. Wang, et al., PRSTAB 5 124402 (2002)), these would correspond do changes in the electron cloud density of ~20-40%. • In numerical simulations (K. Ohmi, Proc. 2001 PAC, Chicago, p. 1895 (2001)), changing xy from 0 to 12 raises the threshold by a factor of 2, from 5x1011 electrons/m3 to 1x1012 electrons/m3. Scaling from this, each change in xy used in the machine study would be expected to change the threshold by ~20. • Basic agreement between simulations and experimental results.

  50. nx ny Side-band Effect of Forced Excitation • Experiment performed on a non-colliding bunch during physics operation, with the cloud-suppression solenoids on. A test bunch was inserted between two bunches (high cloud region). • Test bunch was excited near the vertical tune. Bunch-by-bunch feedback was turned off for this bunch. • As the excitation amplitude increased, in addition to the betatron peak amplitude increasing, the sideband amplitude decreased, dropping below detectable levels at the highest excitation amplitude. • Mechanism under investigation

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