1. Recent Electron-Cloud Mitigation Studies at KEK E-cloud mitigation mini-workshop on 20-21 November at CERN Kyo Shibata (for KEKB Group)

2. Introduction E-cloud mitigation mini-workshop on 20-21 November at CERN KEKB : double ring collider with one collision point High Energy Ring (HER) : Electron ring Energy : 8 GeV, Current :  970 mA, bunch space : 3  6 ns, bunch charge :  0.9  10 -8 C Low Energy Ring (LER) : Positron ring Energy : 3.5 GeV, Current :  1600 mA, bunch space : 3  6 ns, bunch charge :  1.2  10 -8 C E-cloud study items at LER TiN coating Clearing electrode Graphitization RFA type electron detectors e-e- e+e+ clearing electrode RFA in Q-mag RFA in solenoid TiN, Graphitization TiN (antechamber) Belle Detector Wiggler Beam duct with antechambers and so on… 1 Circumference : 3016 m Groove surface groove surface

3. Clearing Electrode 1 E-cloud mitigation mini-workshop on 20-21 November at CERN Mitigation of E-cloud in Magnets Clearing electrode and electron detector was installed in Wiggler magnet. (placed at the center of pole) To ascertain the effect of electrode, the electron density was measured from just under electrode. Electron detector Clearing electrode Test chamber Y. Suetsugu, ILCDR2008 2 Wiggler magnet Magnetic filed : 0.77 T Effective length : 346 mm Aperture (height) : 110 mm Tolerance for high beam current was also tested.

4. Clearing Electrode 2 E-cloud mitigation mini-workshop on 20-21 November at CERN Cross-section Drawing Electrode [W] (~ +1 kV) Collector (7 strips, w 5 mm) Cooling channel Insulator [Al 2 O 3 ] ( t ~0.2 mm) Grid (~ 1 kV) Vacuum seal by metal O-ring Cooling Water paths Shield Monitor Block Electrode Block  2 mm holes R47 Limited by magnet aperture ( w 40, l 440, t ~0.1) Beam 7 strips measure the horizontal spatial distribution of the e-cloud. Y. Suetsugu, ILCDR2008 Very thin electrode (0.1 mm, Tungsten) and insulator (0.2 mm, Al 2 O 3 ) were developed. 3

5. Clearing Electrode 3 E-cloud mitigation mini-workshop on 20-21 November at CERN Top view & side view 5 Electron Monitor Block Electrode Block Detachable Block Feed-through Magnet Top view Side view Y. Suetsugu, ILCDR2008 4

6. Clearing Electrode 4 E-cloud mitigation mini-workshop on 20-21 November at CERN Results Drastic decrease in electron density was demonstrated by applying positive voltage. Y. Suetsugu, ILCDR2008 5 V r =  1.0 kV B = 0.77 T [Logarithmic scale] I e [A] Collectors V elec [V] I e [A] Collector s V r = 0 V B = 0.77 T [Logarithmic scale] 1 x 10 -5 1 x 10 -6 1 x 10 -7 1 x 10 -8 1 x 10 -9 1585 bunches (B s ~ 6 ns) ~1600 mA

7. Groove surface (preliminary) E-cloud mitigation mini-workshop on 20-21 November at CERN Effect of groove surface will be ascertained this autumn. (collaboration with SLAC) Electrode will be replaced by groove surface. 6 Same setup for clearing electrode is utilized. Groove structure was designed and manufactured in SLAC. TiN~50 nm Flat surface with TiN coating is now tested for reference. R47 Beam [Groove] [Monitor] Magnetic field Y. Suetsugu, ILCDR2008 Groove surface [SUS + TiN] Flat surface [SUS + TiN] TiN~200 nm

8. TiN coating 1 E-cloud mitigation mini-workshop on 20-21 November at CERN Reduction of SEY of beam duct by coating TiN coating system for long beam ducts was built at KEK. Several beam ducts have been coated with TiN, and installed in KEKB LER. ~4 m Gas inlet Duct Pumping system Solenoid Coating was done by DC magnetron sputtering of titanium in Ar and N 2. Thickness : 200 nm Maximum SEY of TiN film on sample piece was 0.84 (electron dose : 0.001 C/mm 2 ) Uncoated TiN coated K. Shibata, EPAC2008 7

9. TiN coating 2 E-cloud mitigation mini-workshop on 20-21 November at CERN Measurement of electron density in KEKB Electron density in circular beam duct (  94 mm) Copper TiN (KEK) TiN (BNL) TiN (KEK) TiN (BNL) Electron Microscope Image K. Kanazawa H. Hisamatsu and M. Nishiwaki f 94 mm 8

10. TiN coating 3 E-cloud mitigation mini-workshop on 20-21 November at CERN Measurement of electron density in KEKB Electron density in beam duct with antechambers Combination of beam duct with antechambers and TiN coating is a promising candidate for future high current machines. f 90 mm Y. Suetsugu, ILCDR2008 9

11. Graphitization 1 E-cloud mitigation mini-workshop on 20-21 November at CERN Electron beam induced graphitization is also studied. Graphitized surfaces have shown low SEY in laboratory experiments. Setup for graphitization of copper beam duct was newly developed. Maximum SEY decreased to 1.0-1.1 (electron irradiated : 0.0016 C/mm 2 ) 500 eV electrons irradiate to duct surface. Electron Gun Filament 1.35m Copper Duct Sample Piece M. Nishiwaki and S. Kato, Vasscaa4 2008 10 Emission Current Density : 170  A/cm 2

12. Graphitization 2 E-cloud mitigation mini-workshop on 20-21 November at CERN Measurement of electron density in KEKB Graphitization is effective to reduce electron cloud density. Effect is less than TiN and NEG. Graphite layer was too thin (FWHM  10 nm). Thicker carbon coating on copper duct is in preparation. M. Nishiwaki and S. Kato, Vasscaa4 2008 11 [4.200.3]

13. Monitor used without solenoid field Monitor used with solenoid field Groove SR Detector 1 Detector 2 Near Beam Electron Cloud Density Measurement in Magnetic Filed with RFA 1 E-cloud mitigation mini-workshop on 20-21 November at CERN Measurement of electron density in solenoid coil Only high energy electrons produced near the bunch can enter the groove and reach the detector behind it. With the help of simulation detector current is converted into the density near the beam. y[mm] x[mm] +15 -15 +15 0 0 Starting points of electrons that enter the detector Only these electrons reach the detector. Beam Detector Bunch size:  x = 0.434mm,  y = 0.061mm,  z = 6mm, B = 50 G, N B =   K. Kanazawa and H. Fukuma 12

14. E-cloud mitigation mini-workshop on 20-21 November at CERN Groove Inside of the chamber 13 Near Beam Electron Cloud Density Measurement in Magnetic Filed with RFA 2

15. E-cloud mitigation mini-workshop on 20-21 November at CERN Electron cloud density with solenoid field (B = 50 G) 1) COD The difference in two detectors may be due to ; The measured current in a solenoid field might have included electrons drifting along the wall. K. Kanazawa and H. Fukuma 14 (0.005 T) B=0 B=50 G, Detector 1 B=50 G, Detector 2 Near Beam Electron Cloud Density Measurement in Magnetic Filed with RFA 3 Preliminary result 2) Relative position to the primary synchrotron radiation 3) Output offset of amplifier in measurement system.

16. E-cloud mitigation mini-workshop on 20-21 November at CERN Measurement of electron density in Q-magnet Electrons accelerated by a bunch along X-axis reach the detector. With the help of simulation detector current is converted into the density near the beam. Electrons accelerated with small angle to X-axis moves spirally around X-axis losing their energy along X-axis to the spiral motion. Electrons with sufficient energy and direction close to X-axis reach the detector. X-axis Detector Detector 2 Detector 1 SR +20 0 -20 x [mm] -200 20 Detector y [mm] The big island is an expected source. Other small islands are unexpected. Detector Bias = -1 keV  x = 0.35 mm,  y = 0.094 mm,  z = 6.0 mm, B’ = 3.32 T/m, N B = 7.5  10 10 K. Kanazawa and H. Fukuma 15 Near Beam Electron Cloud Density Measurement in Magnetic Filed with RFA 4

17. Detector 16 E-cloud mitigation mini-workshop on 20-21 November at CERN Near Beam Electron Cloud Density Measurement in Magnetic Filed with RFA 5

18. E-cloud mitigation mini-workshop on 20-21 November at CERN Electron cloud density with quadrupole field (B’ = 3.32 T/m) The difference in two detectors may be due to ; The observed value in Q-Magnet is close to the estimation by simulation K. Kanazawa and H. Fukuma 17 Near Beam Electron Cloud Density Measurement in Magnetic Filed with RFA 6 Preliminary result 1) COD 2) Relative position to the primary synchrotron radiation 3) Output offset of amplifier in measurement system.

19. Summary E-cloud mitigation mini-workshop on 20-21 November at CERN Various studies on the electron cloud mitigation have been done at KEKB positron ring. TiN coating Clearing electrode Graphitization Beam duct with antechambers Mitigation methods, such as clearing electrode and coating gave reasonable effect. New RFA type electron detectors was developed and installed in KEKB LER to measure the electron cloud density in solenoid coil and quadrupole magnet. 18 Groove surface For SuperKEKB : Drift space : antechamber + solenoid + TiN coating Magnet space : antechamber + TiN coating + clearing electrode? groove surface?