1. Recent Studies on Electron Cloud at KEKB 1.EC studies at KEKB 2.Recent results –Clearing Electrode –Groove surface –EC measurement in Q and solenoid field 3.Summary Y. Suetsugu for KEKB Group 2016/9/302008/12/10 INFN 1

2. EC studies at KEKB 2016/9/302008/12/10 INFN 2 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)

3. Recent Results 2016/9/302008/12/10 INFN 3 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

4. EC studies at KEKB 2016/9/302008/12/10 INFN 4 KEKB positron ring Basic parameters Energy 3.5 GeV Circumference3016.26 m Nominal bunch current1~1.3 mA Nominal bunch charge10~13 nC Nominal bunch spacing6~8 ns Harmonic number5120 RMS beam size (x/y)0.42/0.06 mm Betatron tune45.51/43.57 RF voltage8 MV Synchrotron tune0.024 Radiation damping time40 ms

5. Clearing Electrode 2016/9/302008/12/10 INFN 5 R-pipe=38mm bunch intensity=9.36e10 3.5 Bunch Spacing B = 0.75 T  max = 1.4 Electron Density in magnetic field Two peaks Electrode (+) L. Wang et al, EPAC2006, p.1489 Electrons decrease drastically! Very effective in simulations

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, Al 2 O 3, by thermal spray. –Low beam impedance, high thermal conductivity (reported in ILCDR08 and ILCWS08) 2016/9/302008/12/10 INFN 6 Stainless steel Tungsten Al 2 O 3 40 mm 440 mm

7. Clearing Electrode Properties 2016/9/302008/12/10 INFN 7 0.2 mm electrode 0.2 mm Al 2 O 3 Z // ~ a few Ohm, R/Q ~ 0.1 Z // reduced to ~1/5 compared to the case of 1 mm thick. k ~1.5x10 10 V/C Dissipated power is ~ 120 W (@1.6 A,1585 bunches) Impedance (Z // ) [  ] Frequency [Hz] 31.2  C (  T = 6  C) Stainless steel Tungsten Al 2 O 3 28  C (  T = 3  C) Cooling water 0.5 mm electrode 1.0 mm Al 2 O 3 Longitudinal impedanceTemperature of electrode Input power = 100 W DT ~ 6  C owing to good thermal conductivity between the electrode and chamber.

8. Clearing Electrode Electron number was measured by a monitor with RFA. –7 strips to measure spatial distribution 2016/9/302008/12/10 INFN 8 Collectors Repeller (Retarding grid) Monitor Holes Shield Applied voltage Collectors: +100V Retarding Grid: 0 ~ -1 kV Measurement: DC mode Monitor holes (  2 mm, 3mm pitch)

9. Clearing Electrode The electrode and the monitor were set face to face in a test chamber. 2016/9/302008/12/10 INFN 9 R47 Beam 5 [Electrode] [Monitor] Magnetic field Monitor Electrode Al-alloy chamber (Not coated) Cooling water Inside view

10. Clearing Electrode Test chamber was installed into a wiggler magnet –Beam current (I b ) ~1600 mA –Bunch spacing (B s ) 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: 2x10 17 photons/s/m 2016/9/302008/12/10 INFN 10 Test chamber Beam Gate Valve

11. Clearing Electrode 2016/9/302008/12/10 INFN 11 1.6x10 -5 1.2x10 -5 8.0x10 -6 4.0x10 -7 0.0 1000 1600 1500 1400 1300 1200 1100 #1#2 #3 #4 #5#6#7 I e [A] I b [mA] Collectors V r = 0 V, V elec = 0 V [Linear scale] Electron distribution splits to two peaks at high current. Spatial growth and energy distribution of EC center 4 x 10 -6 3 x 10 -6 2 x 10 -6 1 x 10 -6 0 Collectors V r [kV] High energy electrons are at the beam position. Center 1585 bunches (B s ~ 6 ns)

12. V r =  1.0 kV B = 0.77 T [Logarithmic scale] Clearing Electrode Effect of electrode potential –Drastic decrease in electron density was demonstrated by applying positive voltage. 2016/9/302008/12/10 INFN 12 V elec [V] I e [A] Collector s 1585 bunches (B s ~ 6 ns) ~1600 mA V elec = 0 V -500 V +500 V V r = -1 kV ~1x10 12 e - /m 3 Electron density decreased to 1/10 at V elec = + 100 ~ 200 V 1/100 at V elec = + 300 ~ 400 V ?

13. Groove structure 2016/9/302008/12/10 INFN 13 Groove structure: reduce the effective SEY geometrically. Effective even in magnet. Grooved structure in magnet (simulation) Lanfa Wang, SLAC

14. Groove structure 2016/9/302008/12/10 INFN 14 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 R47 Beam [Groove] [Monitor] Magnetic field

15. Groove structure 2016/9/302008/12/10 INFN 15 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, V elec = 0 V) TiN~50 nm SS + TiN coating Designed and manufactured in SLAC M. Pivi

16. Groove structure 2016/9/302008/12/10 INFN 16 Electron density for groove surface is lower than W surface (electrode) by ~2 orders, and than flat surface by ~ one order. –Aging is proceeding. V r = -1 kV 1585 bunches (B s ~ 6 ns) ~1600 mA Preliminary result

17. Groove structure 2016/9/302008/12/10 INFN 17 But, density is still higher than the case with clearing electrode of V elec > + 300 V by ~ one order. –Now accumulating data. Collector s 1585 bunches (B s ~ 6 ns) ~1600 mA Vr = -1 kV Preliminary result

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. 2016/9/302008/12/10 INFN 18 Monitor used without solenoid field Monitor used with solenoid field Groove SR Detector 1 Detector 2 y[mm] x[mm] +15 -15 0 Only these electrons reach the detector. Beam Detector +150-15 K. Kanazawa B e-

19. Measurement of EC in solenoid The first attempt so far. Installed into the KEKB positron ring this summer. 2016/9/302008/12/10 INFN 19 Whole view Placed at the center of a coil. Groove

20. Measurement of EC in solenoid Result –Electron density decreased by 2 orders. 2016/9/302008/12/10 INFN 20 Preliminary result K. Kanazawa –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.

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. 2016/9/302008/12/10 INFN 21 X-axis Detector Detector 2 Detector 1 SR x [mm] y [mm] K. Kanazawa Detector Beam +20 -20 0 +200-20

22. Measurement of EC Q-magnet Installed this summer 2016/9/302008/12/10 INFN 22 Detector Placed at the end of a yoke. Detector

23. Measurement of EC Q-magnet Result –Electron density near to that expected by a simulation was obtained. 2016/9/302008/12/10 INFN 23 K. Kanazawa 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.

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 (+  ) 2016/9/302008/12/10 INFN 24

25. Backup slide 2016/9/302008/12/10 INFN 25

26. Backup slide Growth of electrons 2016/9/302008/12/10 INFN 26 Flat Groove 1585 bunches (B s ~ 6 ns) Vr = 0V I [mA] Collector No. Ie [A] I [mA] 1300 1600 1300 1600

27. Backup slide Energy distributions of electrons 2016/9/302008/12/10 INFN 27 Flat Groove 1585 bunches (B s ~ 6 ns) ~1600 mA Vr [V] Collector No. Ie [A]

28. Backup slide 2016/9/302008/12/10 INFN 28

29. Clearing Electrode 2016/9/302008/12/10 INFN 29 RF properties (calculation by MAFIA) –Thin electrode and insulator  Low beam impedance 0.2 mm electrode 0.2 mm Al 2 O 3 Z // ~ a few Ohm Z // reduced to ~1/5 compared to the case of 1 mm thick. R/Q ~ 0.1 k ~1.5x10 10 V/C including the connection part (2 electrodes). Dissipated power is ~ 120 W for 1 electrode. (@1.6 A,1585 bunches) Electrode only (2 electrodes). Impedance (Z // ) [  ] Frequency [Hz] Loss Factor [V C -1 ] Thickness of Insulator [mm]

30. Clearing Electrode 2016/9/302008/12/10 INFN 30 Electric potential by the electrode Potential distribution +500 V +120 V 0 V 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.

31. Clearing Electrode Issues to be solved (1) –Simulation of electron behaviors including RFA structure is required to fully understand the observation. 2016/9/302008/12/10 INFN 31 Measurement Simulation (  max = 1.2) Model

32. Clearing Electrode Issues to be solved (2) –Improvement in the structure of connection is undergoing. –Next electrode will be tested next spring. 2016/9/302008/12/10 INFN 32

33. Simulation [Preliminary] Trajectory of electrons –In magnetic field, but cyclotron motion was neglected for simplicity. –1/1585/3 (Bs ~ 6 ns) 2016/9/302008/12/10 INFN 33 V elec = 0VV elec = +500 V

34. Simulation [Preliminary] Spatial distribution of Measured Electron Current (Ie) –Vr = 0 kV, 4/200/3 (Bs = 6 ns) –B=0.75 T 2016/9/302008/12/10 INFN 34 Measurement Simulation (  max = 1.2)

35. Simulation [Preliminary] Spatial distribution of Measured Electron Current (Ie) –Vr = -0.2 kV, 4/200/3 (Bs = 6 ns) –B=0.75 T 2016/9/302008/12/10 INFN 35 Measurement Simulation (  max = 1.2)

36. Simulation [Preliminary] Measured Electron Current (Ie) for different fill patterns –Vr = 0 kV –B=0.75 T 2016/9/302008/12/10 INFN 36 Measurement Simulation (  max = 1.2)

37. Note model 2016/9/302008/12/10 INFN 37 Monitoring Holes (0V) Shielding Grid (0V) Retarding Grid (-1kV) Collector (+100V) High energy e - Acceleration e-e- (-> Interaction with bunches) High energy e - (-> Interaction with bunches)

38. Note Cal (preliminary) 2016/9/302008/12/10 INFN 38