Updates of EC Studies at KEKB 1.EC studies at KEKB 2.Recent results 1.Clearing Electrode 2.Groove surface 3.TiN coating 4.Measurement of EC in solenoid and Q-magnet 3.Summary Y. Suetsugu for KEKB Group 2016/3/20ILCWS Chicago Univ. 1

EC studies at KEKB 2016/3/20ILCWS Chicago Univ. 2 Various kind of EC studies have been performed using KEKB positron ring. Basic parameters of KEKB positron ring Energy 3.5 GeV Circumference m Nominal bunch current1~1.3 mA Nominal bunch spacing3~4 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 –Diagnostics of instabilities, EC measurement, SEY estimation and mitigation techniques

EC studies at KEKB 2016/3/20ILCWS Chicago Univ. 3 Diagnostics of instability –Beam size measurement by interferometer –Measurement of synchro-betatron sidebands Direct evidence of Head-Tail instability due to EC –Coupled bunch instability Transverse coupled-bunch instability. J. Flanagan, ECLOUD07 Unstable modes w/o solenoid field M. Tobiyama, ECLOUD07

EC studies at KEKB 2016/3/20ILCWS Chicago Univ. 4 Measurement of EC –Electron Monitor with RFA –Estimation of electron density near to beam In drift space so far K. Kanazawa, ECLOUD07

EC studies at KEKB 2016/3/20ILCWS Chicago Univ. 5 Estimation of SEY –SEY measurement at laboratory –Measurement using samples: exposed to beam –Estimation from measured electron current, utilizing a simulation. S. Kato, KEK Review 2007

EC studies at KEKB 2016/3/20ILCWS Chicago Univ. 6 Mitigation technique –Solenoid (drift space) –Beam duct with antechamber –Surface to reduce SEY (TiN or NEG coating, etc)

Recent Results 2016/3/20ILCWS Chicago Univ. 7 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

Clearing Electrode New strip 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 –(already reported in ILC08, Cornel, June, 2008) 2016/3/20ILCWS Chicago Univ. 8 Stainless steel Tungsten Al 2 O 3 40 mm 440 mm

Clearing Electrode Electron number was measured by a monitor with RFA –7 strips to measure spatial distribution 2016/3/20ILCWS Chicago Univ. 9 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)

Clearing Electrode The electrode and monitor are set face to face in a test chamber 2016/3/20ILCWS Chicago Univ. 10 R47 Beam 5 [Electrode] [Monitor] Magnetic field Monitor Electrode Al-alloy chamber (Not coated) Cooling water Inside view

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 2016/3/20ILCWS Chicago Univ. 11 Test chamber Beam Gate Valve

Clearing Electrode 2016/3/20ILCWS Chicago Univ mV/div. 5ns/div. Collectors 1.6 x x x x #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 bunches (B s ~ 6 ns) Spatial growth of EC during beam injection –Check of monitor center

Clearing Electrode Energy distribution of electron –High energy electrons are around the center (beam orbit) 2016/3/20ILCWS Chicago Univ x x x x I e [A] Collectors V r [kV] 1 x V elec = 0 V [Logarithmic scale] 4 x x x x V elec = 0 V [Linear scale] 0 Collectors V r [kV] 1585 bunches (B s ~ 6 ns) ~1600 mA center

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. 2016/3/20ILCWS Chicago Univ. 14 I e [A] Collectors V elec [V] I e [A] Collector s 1585 bunches (B s ~ 6 ns) ~1600 mA V r = 0 V B = 0.77 T [Logarithmic scale] 1 x x x x x V elec = 0 V -500 V +500 V

Clearing Electrode Effect of electrode potential 2016/3/20ILCWS Chicago Univ. 15 V r = 0 V V r = -1 kV Electron density decreased to 1/10 at V elec = ~ 200 V 1/100 at V elec = ~ 400 V 1585 bunches (B s ~ 6 ns) ~1600 mA ~1x10 12 e - /m 3 Complicated behavior at negative V elec.

Clearing Electrode Issues to be solved (1) –Simulation of electron behaviors including RFA structure is required to fully understand the observation. 2016/3/20ILCWS Chicago Univ. 16 Measurement Simulation (  max = 1.2) Model

Clearing Electrode Issues to be solved (2) –Improvement in the structure of connection is undergoing. –Next electrode will be tested next spring. 2016/3/20ILCWS Chicago Univ. 17

Groove structure 2016/3/20ILCWS Chicago Univ. 18 Experiment has just begun in collaboration with SLAC (M. Pivi) Utilize the same set up for clearing electrode. –The same electron monitor Wiggler magnets B = 0.77 T R47 Beam [Groove] [Monitor] Magnetic field

Groove structure 2016/3/20ILCWS Chicago Univ. 19 Test of a triangular-type groove structure –In a magnetic field of 0.77 T TiN~50 nm SS + TiN coating Designed and manufactured in SLAC M. Pivi

Groove structure 2016/3/20ILCWS Chicago Univ. 20 At present, a flat surface with TiN coating is installed and being tested. –Reference value The surface will be changed to grooved structure this week. TiN~200 nm

Groove structure 2016/3/20ILCWS Chicago Univ. 21 Electron density is lower than W surface (electrode) by factors ~ one order. –Aging is proceeding. V r = -1 kV V r = 0 V Preliminary result 1585 bunches (B s ~ 6 ns) ~1600 mA

Groove structure 2016/3/20ILCWS Chicago Univ. 22 But, electron number is still higher than the case with clearing electrode of V elec > V. Twin peaks are clearly observed. Preliminary result I e [A] Collector s V r = 0 V B = 0.77 T [Linear scale] Beam Current [mA] bunches (B s ~ 6 ns) ~1600 mA Vr = -1 kV V elec = -1 kV

TiN Coating Combination of beam pipe with antechambers – Coating system available for ~4 m pipe was set up in KEK. –Thickness is ~200 nm, which is determined from adhesiveness of film and  max (~0.84). 2016/3/20ILCWS Chicago Univ. 23 ~4 m Gas inlet Duct Pumping system Solenoid ~3.6 m  90 mm K. Shibata, EPAC2008

TiN Coating Coating was successful –Only beam channel region Beam pipe with antechambers with TiN coating was installed into the ring. –Reduction in electron number at high current region was confirmed. 2016/3/20ILCWS Chicago Univ. 24 Coated at only beam channel part f 90 mm Drift region

Measurement of EC in solenoid Installed at a drift region, in a controllable solenoid. Monitor: without repeller (grid). Energy is selected geometrically. 2016/3/20ILCWS Chicago Univ. 25 Monitor used without solenoid field Monitor used with solenoid field Groove SR Detector 1 Detector 2 y[mm] x[mm] Only these electrons reach the detector. Beam Detector K. Kanazawa

Measurement of EC in solenoid Installed this summer Setup in the ring 2016/3/20ILCWS Chicago Univ. 26 Detector Placed at the center of a coil.

Measurement of EC in solenoid Result –Electron density decreased by 2 orders. 2016/3/20ILCWS Chicago Univ. 27 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.

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. 2016/3/20ILCWS Chicago Univ. 28 X-axis Detector Detector 2 Detector 1 SR x [mm] y [mm] K. Kanazawa Detector Beam

Measurement of EC Q-magnet Installed this summer Setup in the ring 2016/3/20ILCWS Chicago Univ. 29 Detector Placed at the end of a yoke.

Measurement of EC Q-magnet Result –Electron density near to that expected by a simulation was obtained. 2016/3/20ILCWS Chicago Univ. 30 K. Kanazawa Preliminary result –The difference in two detectors may be due to; 1) COD 2) Relative position to the primary SR.

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. –TiN coating in a long beam pipe with antechambers reduced the electron density at high current region –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 –Drift space : Antechamber + Solenoid + TiN coating –In magnets: Antechamber + TiN coating (+  ) 2016/3/20ILCWS Chicago Univ. 31

Backup slide 2016/3/20ILCWS Chicago Univ. 32

Simulation [Preliminary] Trajectory of electrons –In magnetic field, but cyclotron motion was neglected for simplicity. –1/1585/3 (Bs ~ 6 ns) 2016/3/20ILCWS Chicago Univ. 33 V elec = 0VV elec = +500 V

Simulation [Preliminary] Spatial distribution of Measured Electron Current (Ie) –Vr = 0 kV, 4/200/3 (Bs = 6 ns) –B=0.75 T 2016/3/20ILCWS Chicago Univ. 34 Measurement Simulation (  max = 1.2)

Simulation [Preliminary] Spatial distribution of Measured Electron Current (Ie) –Vr = -0.2 kV, 4/200/3 (Bs = 6 ns) –B=0.75 T 2016/3/20ILCWS Chicago Univ. 35 Measurement Simulation (  max = 1.2)

Simulation [Preliminary] Measured Electron Current (Ie) for different fill patterns –Vr = 0 kV –B=0.75 T 2016/3/20ILCWS Chicago Univ. 36 Measurement Simulation (  max = 1.2)

Note model 2016/3/20ILCWS Chicago Univ. 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)

Note Cal (preliminary) 2016/3/20ILCWS Chicago Univ. 38

Note W SEY (preliminary) 2016/3/20ILCWS Chicago Univ. 39