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Recent Studies on Electron Cloud Mitigation at KEKB |

Explore various electron cloud studies at KEKB positron ring, including clearing electrode use, groove structure experiments, and mitigation techniques in magnets. Stay updated with the latest results and properties like electrode impedance and temperature. |

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Recent Studies on Electron Cloud Mitigation at KEKB |

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  1. Recent Studies on Electron Cloud at KEKB Y. Suetsugu for KEKBGroup EC studies at KEKB Recent results Clearing Electrode Groove surface EC measurement in Q and solenoid field Summary 2008/12/10 INFN

  2. EC studies at KEKB • Various kind of EC studies have been made using KEKB positron ring. • Diagnostics of instabilities • Beam size, synchro-betatron sidebands • EC measurement • Electron monitor with RFA • SEY estimation and measurement • In ring, at laboratory • Mitigation techniques • Solenoid (drift space) • Beam duct with antechamber • Surface to reduce SEY (TiN or NEG coating, etc) 2008/12/10 INFN

  3. Recent Results • Several new studies have started this year. • Mitigation: • Clearing Electrode • Mitigation in magnets • Groove structure • Mitigation in magnets • TiN coating (continued) • Coating in KEK, and combination with antechamber • Measurement of EC • Measurement in solenoid and Q-magnet • Newly developed electron monitor 2008/12/10 INFN

  4. EC studies at KEKB • KEKB positron ring • Basic parameters • Energy 3.5 GeV • Circumference 3016.26 m • Nominal bunch current 1~1.3 mA • Nominal bunch charge 10~13 nC • Nominal bunch spacing 6~8 ns • Harmonic number 5120 • RMS beam size (x/y) 0.42/0.06 mm • Betatron tune 45.51/43.57 • RF voltage 8 MV • Synchrotron tune 0.024 • Radiation damping time 40 ms 2008/12/10 INFN

  5. Clearing Electrode • Very effective in simulations R-pipe=38mm bunch intensity=9.36e10 3.5 Bunch Spacing B = 0.75 T dmax = 1.4 Electrode (+) Electron Density in magnetic field Two peaks Electrons decrease drastically! L. Wang et al, EPAC2006, p.1489 2008/12/10 INFN

  6. Clearing Electrode • New strip-line type electrode was developed. • Very thin electrode and insulator; • Electrode: ~0.1 mm, Tungsten, by thermal spray. • Insulator: ~0.2 mm, Al2O3, by thermal spray. • Low beam impedance, high thermal conductivity (reported in ILCDR08 and ILCWS08) Tungsten Al2O3 40 mm 440 mm Stainless steel 2008/12/10 INFN

  7. Clearing Electrode • Properties 0.5 mm electrode 1.0 mm Al2O3 Longitudinal impedance Temperature of electrode 0.2 mm electrode 0.2 mm Al2O3 Tungsten 31.2 C(DT = 6 C) Al2O3 Impedance (Z//) [W] Stainless steel Cooling water 28 C (DT = 3 C) Frequency [Hz] • Z// ~ a few Ohm, R/Q ~ 0.1 • Z// reduced to ~1/5 compared to the case of 1 mm thick. • k ~1.5x1010 V/C • Dissipated power is ~ 120 W (@1.6 A,1585 bunches) • Input power = 100 W • DT ~ 6 C owing to good thermal conductivity between the electrode and chamber. 2008/12/10 INFN

  8. Clearing Electrode • Electron number was measured by a monitor with RFA. • 7 strips to measure spatial distribution Repeller (Retarding grid) Collectors Shield Monitor Holes Monitor holes (f2 mm, 3mm pitch) • Applied voltage • Collectors: +100V • Retarding Grid: 0 ~ -1 kV • Measurement: DC mode 2008/12/10 INFN

  9. Clearing Electrode • The electrode and the monitor were set face to face in a test chamber. Electrode Cooling water [Electrode] R47 Magnetic field Beam [Monitor] Al-alloy chamber (Not coated) Inside view Monitor 5 2008/12/10 INFN

  10. Clearing Electrode • Test chamber was installed into a wiggler magnet • Beam current (Ib) ~1600 mA • Bunch spacing (Bs) 4 ~16 ns • Wiggler magnet. • Magnetic field: 0.77 T • Effective length: 346 mm • Aperture (height): 110 mm • Placed at the center of pole • SR: 2x1017 photons/s/m Test chamber Gate Valve Gate Valve Beam 2008/12/10 INFN

  11. Clearing Electrode • Spatial growth and energy distribution of EC 1585 bunches (Bs ~ 6 ns) Vr = 0 V, Velec = 0 V [Linear scale] 1000 4 x 10-6 1100 3 x 10-6 Center 1.6x10-5 1200 2 x 10-6 1.2x10-5 1300 1 x 10-6 Ie [A] Ib [mA] 8.0x10-6 Vr [kV] 1400 0 4.0x10-7 1500 Collectors center 1600 0.0 Electron distribution splits to two peaks at high current. High energy electrons are at the beam position. #1 #2 #4 #3 #5 #6 #7 Collectors 2008/12/10 INFN

  12. Clearing Electrode • Effect of electrode potential • Drastic decrease in electron density was demonstrated by applying positive voltage. 1585 bunches (Bs ~ 6 ns) ~1600 mA ? Vr = 1.0 kV B = 0.77 T [Logarithmic scale] Vr = -1 kV -500 V ~1x1012 e-/m3 Velec = 0 V Velec [V] Ie [A] Electron density decreased to 1/10 at Velec = + 100 ~ 200 V 1/100 at Velec = + 300 ~ 400 V +500 V Collectors 2008/12/10 INFN

  13. Groove structure • Groove structure: reduce the effective SEY geometrically. • Effective even in magnet. Grooved structure in magnet (simulation) Lanfa Wang, SLAC 2008/12/10 INFN

  14. Groove structure • Experiment has just begun under collaboration with SLAC (M. Pivi) • Utilize the same set up for clearing electrode. • The same experimental setup as that of electrode Wiggler magnets B = 0.77 T [Groove] R47 Magnetic field Beam [Monitor] 2008/12/10 INFN

  15. Groove structure • Triangular-type groove structure, with TiN • In a magnetic field of 0.77 T • Compared with the data for a flat surface (TiN) and electrode (W, Velec = 0 V) Designed and manufactured in SLAC SS + TiN coating TiN~50 nm M. Pivi 2008/12/10 INFN

  16. Groove structure • Electron density for groove surface is lower than W surface (electrode) by ~2 orders, and than flat surface by ~ one order. • Aging is proceeding. Vr = -1 kV Preliminary result 1585 bunches (Bs ~ 6 ns) ~1600 mA 2008/12/10 INFN

  17. Groove structure • But, density is still higher than the case with clearing electrode of Velec > + 300 V by ~ one order. • Now accumulating data. Vr = -1 kV Preliminary result 1585 bunches (Bs ~ 6 ns) ~1600 mA Collectors 2008/12/10 INFN

  18. Measurement of EC in solenoid • Installed at a drift region, in a controllable solenoid. • Monitor: Without repeller (grid). • Energy is selected by groove geometrically. +15 Monitor used without solenoid field Only these electrons reach the detector. Detector 1 y[mm] B 0 e- Beam SR Detector Detector 2 -15 -15 0 +15 Monitor used with solenoid field Groove K. Kanazawa x[mm] 2008/12/10 INFN

  19. Measurement of EC in solenoid • The first attempt so far. • Installed into the KEKB positron ring this summer. Whole view Groove Placed at the center of a coil. 2008/12/10 INFN

  20. Measurement of EC in solenoid • Result • Electron density decreased by 2 orders. • The difference in two detectors may be due to; • 1) COD • 2) Relative position to • the primary SR • The measured current in a solenoid field might have included electrons drifting along the wall. Preliminary result K. Kanazawa 2008/12/10 INFN

  21. Measurement of EC Q-magnet • Installed into a Q-magnet with wide-aperture • Electrons that coming along the magnetic fields are counted by a monitor. +20 X-axis Detector Detector Detector 2 SR x [mm] 0 Beam Detector 1 -20 -20 0 +20 y [mm] K. Kanazawa 2008/12/10 INFN

  22. Measurement of EC Q-magnet • Installed this summer Detector Placed at the end of a yoke. Detector 2008/12/10 INFN

  23. Measurement of EC Q-magnet • Result • Electron density near to that expected by a simulation was obtained. Preliminary result • The difference in two detectors may be due to; • 1) COD • 2) Relative position to the primary SR. The results will be presented in PAC05 in detail. K. Kanazawa 2008/12/10 INFN

  24. Summary • Various EC studies are undergoing at KEKB • Updates: • Clearing electrode in bending magnetic field was found to be very effective in reducing electron density • Measurement for grooved structure in bending magnetic field has just started. The reduction by one order was observed. • Measurement of electron density in a solenoid field and Q-magnet has just started, and the preliminary values were obtained for the first time. • Strategy for KEKB upgrade (at present) • Drift space : Antechamber + Solenoid + TiN coating • In magnets: Antechamber + TiN coating (+a) 2008/12/10 INFN

  25. Backup slide 2008/12/10 INFN

  26. Backup slide • Growth of electrons 1585 bunches (Bs ~ 6 ns) Vr = 0V Groove Flat 1300 1300 Ie [A] Ie [A] I [mA] I [mA] 1600 1600 Collector No. Collector No. 2008/12/10 INFN

  27. Backup slide • Energy distributions of electrons 1585 bunches (Bs ~ 6 ns) ~1600 mA Groove Flat Ie [A] Ie [A] Vr [V] Vr [V] Collector No. Collector No. 2008/12/10 INFN

  28. Backup slide 2008/12/10 INFN

  29. Clearing Electrode • RF properties (calculation by MAFIA) • Thin electrode and insulator Low beam impedance 0.2 mm electrode 0.2 mm Al2O3 Impedance (Z//) [W] Loss Factor [V C-1] Electrode only (2 electrodes). Frequency [Hz] Thickness of Insulator [mm] • Z// ~ a few Ohm • Z// reduced to ~1/5 compared to the case of 1 mm thick. • R/Q ~ 0.1 • k ~1.5x1010 V/C including the connection part (2 electrodes). • Dissipated power is ~ 120 W for 1 electrode. (@1.6 A,1585 bunches) 2008/12/10 INFN

  30. Clearing Electrode • Electric potential by the electrode • Potential distribution • Similar structure to “Invisible Electrode” by F. Caspers (PAC07). • Difference: Electrode is made of pure metal (W). • We used pure metal: (1) To avoid Joule loss of the electrode due to high current (2) To reduce voltage drop along the long electrode. 0 V +120 V +500 V 2008/12/10 INFN

  31. Clearing Electrode • Issues to be solved (1) • Simulation of electron behaviors including RFA structure is required to fully understand the observation. Model Measurement Simulation(dmax = 1.2) 2008/12/10 INFN

  32. Clearing Electrode • Issues to be solved (2) • Improvement in the structure of connection is undergoing. • Next electrode will be tested next spring. 2008/12/10 INFN

  33. Simulation [Preliminary] • Trajectory of electrons • In magnetic field, but cyclotron motion was neglected for simplicity. • 1/1585/3 (Bs ~ 6 ns) Velec = 0V Velec = +500 V 2008/12/10 INFN

  34. Simulation [Preliminary] • Spatial distribution of Measured Electron Current (Ie) • Vr = 0 kV, 4/200/3 (Bs = 6 ns) • B=0.75 T Simulation(dmax = 1.2) Measurement 2008/12/10 INFN

  35. Simulation [Preliminary] • Spatial distribution of Measured Electron Current (Ie) • Vr = -0.2 kV, 4/200/3 (Bs = 6 ns) • B=0.75 T Simulation(dmax = 1.2) Measurement 2008/12/10 INFN

  36. Simulation [Preliminary] • Measured Electron Current (Ie) for different fill patterns • Vr = 0 kV • B=0.75 T Measurement Simulation(dmax = 1.2) 2008/12/10 INFN

  37. Note • model High energy e- (-> Interaction with bunches) High energy e- (-> Interaction with bunches) Acceleration Monitoring Holes (0V) e- Shielding Grid (0V) Retarding Grid (-1kV) Collector (+100V) 2008/12/10 INFN

  38. Note • Cal (preliminary) 2008/12/10 INFN

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