1. SPS coating studies 28 February, 2016TE-VSC1 Present situation of the development of e-cloud mitigation methods -MD 2011 results -Carbon coating of dipoles -Diagnostic and quality control tools -StSt conditioning in the lab M.Taborelli, P.Costa Pinto, P.Edwards M.Mensi, H.Neupert, C.Yin-Vallgren M.Holz, F. Caspers

2. 28 February, 2016TE-VSC2 Summary of MD measurements of electron cloud monitors (ECM) in SPS for various SEY (StSt, a-C, DLC..)

3. ECM in 2011 28 February, 2016TE-VSC3 Carbon coated 3y in SPS StSt8 Clearing electrode (StSt) Carbon/StSt Half coated H StSt 9 Carbon coated hollow cathode MD w19 MD w21 MD w33 MD w35 Is it sufficient to coat half of the pipe or to insert a coated “gutter” ? Do clearing electrodes work up to nominal B-fields ? Carbon/StSt Half coated V Carbon coated 3y in SPS

4. Coating half of the beam pipe? 28 February, 2016TE-VSC4 At 25 ns the half coated has only 10 times lower current (500 times lower for 50 ns) than StSt, whereas a full carbon coating has at least 1000 times lower current We need to coat upper and lower faces and a gutter is not a solution Half coated V StSt9 Int e-cloud/int BCT Half coated V

5. Principle test of clearing electrode 28 February, 2016TE-VSC5 2011, StStalumina plasma spray, to be installed in January 2012 in SPS From KEK (Y. Suetsugu) to CERN Al 2 O 3 W

6. Results with 25 ns beam and different B fields 28 February, 2016TE-VSC6 o 1.1A o 2.5A o 5A * 25A * 40A +25A, neg V o 25A + 25A, 450Gev 26GeV 3 batches Int e-cloud/int FBCT 0 20 40 60 80 100 120 140 160 180 200 Voltage on the electrode [V] ◊Effective suppression at all tested B fields with low voltage <100V It can be applied on selected spots of the machine, but we did not solve the issue of the space hindrance

7. a-C coatings of vacuum chambers in 2011 28 February, 2016TE-VSC7 magnetron sputtering of chambers in solenoids (done in 2010 for 7 m MBB)  Difficult in MBA due to stronger outgassing during coating  Solved the issue in MBA with simultaneous deposition of NEG to improve pumping (SEY max =1.0 at second trial)  We still do not have a technique enabling magnetic measurements without scratches on the coating

8. a-C coatings in dipoles in 2011, I 28 February, 2016TE-VSC8 magnetron sputtering with permanent magnets (B-field by permanent magnets instead of solenoids to get the correct direction for magnetron)  Works for 1 m prototype  This set-up is too sensitive to magnet-cathode distance, which changes with thermal expansion during coating: unstable conditions, probably impossible to adjust for 7 m length  This option has been discarded in favour of the hollow cathode system

9. a-C coatings in dipoles in 2011, II 28 February, 2016TE-VSC9 Hollow cathode sputtering (coating without B-field with “smart” configuration of the cathode)  In MBB first trial in 0.5m, then 2m and recently 7m (three prototypes done)  Needs cathode movement during deposition to get uniformity  Cathode for MBA under construction

10. Pumpdown of a-C hollow-cathode coated MBB chamber 28 February, 2016 TE-VSC After 2 hours venting to laboratory air =StSt 3.5 times more ougassing than StSt after 10h

11. Diagnostics of dipoles: RF induced multipactor 28 February, 2016TE-VSC11 RF diagnostics: reflected power Pressure rise diagnostics: RGA signal RF power in reflected pressure Time onset MBB

12. At very low fields the multipacting shows a resonant behaviour corresponding to the cyclotron resonance: Cyclotron resonance: in StSt Same behaviour for refelcted power 131.03 MHz

13. Reflected power: StSt versus carbon coated for various “low“ B 28 February, 2016TE-VSC13 35dB 134.471 MHz (same at 130 MHz) (max ~30W on amplifier) carbonStSt 130.960 MHz (max ~30W on amplifier) No onset of multipacting on the carbon coated chamber for fields up to 400A and no measurable pressure rise = 0.16T

14. 28 February, 2016TE-VSC14 With power supply at SM 18 Reflected power: carbon coated at high B  Reflected power at 3000A, but only once. No reflected power at any field in further runs even at higher B (5000A).  Pressure rise observed for the same event and minor pressure rise in further runs even for low B, but no reflected power.  Possible explanations: -multipacting occurring outside of the coated pipe? -parts displaced by high B field modify the geometry to induce multipacting? -induced remanent magnetization in parts ouside the pipe? We need more tests at SM 18 and a dedicated newtork analyzer 11A 3000A 4000A 5000A =2T

15. Diagnostics in SPS in situ: Pick up 28 February, 2016TE-VSC15 22 mm 42 mm grid for RF screening MBB  Measure the e-cloud electron current in a coated and an uncoated dipole and the pressure rise during e-cloud  Under construction  Simulation (G.Rumolo, G.Iadarola) for different grid configurations are in progress  Ready during 2012

16. Unbaked StSt conditioning I 28 February, 2016TE-VSC16 Conditioning in the lab, irradiated with 500eV electrons: Emax increases slightly δmax≥ 1.3 (for 10 -1 Clb/mm 2 = 10 18 el/mm 2 ) Conditioning in SPS (mobile sample), e-cloud electrons: Emax decreases δmax≥ 1.5, but not necessarily a final limit

17. 28 February, 2016TE-VSC17 Unbaked StSt conditioning II -The same behaviour of Emax was found on copper (N.Hilleret, electrons and also with photons from EPA) -A different behaviour in the lab is found by S.Kato, KEK for irradiation with 5KeV electrons, with δmax< 1.3 and decrease of Emax Conclusions:  -The mechanism of conditioning in the machine and in the lab is not exactly the same  -It is not proved that we can reach δmax< 1.3 on StSt

18. Next 28 February, 2016TE-VSC18 Winter 2011/12 -insert 1 MBB + 1 MBA coated in magnetron (still an important option which can be exploited for magnets replaced during maintenance) -insert alumina clearing electrode -insert movable sample in StSt to verify conditioning behavior Mid 2012 -insert 2 MBB + 2 MBA coated in hollow cathode= half-cell with pressure gauges -insert additional 1 MBB + 1 MBA coated in magnetron= half-cell with pressure gauges -insert uncoated/coated MBB dipoles equipped with pick-up electrode

19. Thank you!

20. At very low fields the multipacting shows a resonant behaviour corresponding to the cyclotron resonance: Cyclotron resonance: in StSt Partial pressure [a.u.] Ramps of RF power Same behaviour for refelcted power B field 131.03 MHz