1. Quick Review of Secondary Electron Yield Studies to Mitigate Electron Cloud Effect in Accelerator F. Le Pimpec (PSI) R. Kirby, F. King, M. Pivi (SLAC) MULCOPIM 08 Valencia, Spain 2008

2. 25.09.2008 F. Le Pimpec 2 Outline EC mechanism → impact on circulating beam Luminosity & non Luminosity affecting solution Need for theoretical understanding Machine & Lab solution : → Technical remedies → Surface approach (Lab studies) → In-situ machine studies Summary

3. 25.09.2008 F. Le Pimpec 3 U. Irizo Ariz RHIC (up) & LHC (down) multipacting schema F. Ruggiero Electron cloud mechanism Beam Damage And it could be even worse… G. Rumolo e-cloud can also arise without multipacting : Primary e - produced by Ions and Photons

4. 25.09.2008 F. Le Pimpec 4 Possible Remedies Reduce the number of charges by bunch → lower the luminosity – Sorry not acceptable Increase the gap between bunches → lower the luminosity – Sorry not acceptable Remove some bunches → lower the luminosity – Sorry not acceptable Get a surface which does not emit much electrons upon photon, ion or electron bombardment → Killing emitted electrons at the emission point : ex Winding solenoid (NA everywhere), clearing electron electrodes… → Play with the surface material : Nature, Geometry, Chemistry

5. 25.09.2008 F. Le Pimpec 5 Understanding the problem Need for theoretical understanding of the ECE ● many codes have been developed for accelerators : POSINT, ECLOUD, EPI, PEI … Code repository URL which includes ESA ESTEC ® ( http://oraweb.cern.ch/pls/hhh/code_website.startup ) ● General EC webpage at CERN : ( http://ab-abp-rlc.web.cern.ch/ab%2Dabp%2Drlc%2Decloud/ ) Understanding of : ● Build-up of the cloud (model : SE- Yield- Energy spectrum, UHV chber  properties …) ● Instability of the running beam due to EC → Need experimental input on SEY – also near 0 eV incident electron energy EPAC 2004 : Comparison of some codes ( THPLT017 ) “ Build-up and instability simulation codes can produce results that vary by factors 3– 100. The differences reflect a strong sensitivity to modeling details. ”

6. 25.09.2008 F. Le Pimpec 6 SEY at 0 eV !!! Buildup of electron cloud depending of the parameterization taken at 0 eV incident electron energy; simulation for LHC (SLAC-TN-04-046) Can Low-Energy Electrons Affect High-Energy Physics Accelerators? R. Cimino et al Phys. Rev. Lett. 93, 014801 (2004) R. Cimino et al, See Ecloud’04 proceedings

7. 25.09.2008 F. Le Pimpec 7 Technical Solutions Clearing Electrodes → Potent device, as long as their curing effects on EC does not create new problems (wakefield, gas desorption, ion instabilities …) → “ At ECL2, the optimum configuration of clearing electrodes was investigated, their impedance for different layouts estimated by two independent programs, GdfidL and HFSS, respectively, and the clearing efficiency explored in simulations. “ (CERN-2007) Winding Solenoids : → Very effective techniques in field free region, but not easy to wind everywhere (usually Bz <50 gauss). → Since year 2000 more than 95 % of the drift region of KEKB are covered. “Blow up is now almost suppressed up to 1.7 A (3.06 RF bucket spacing)” Y. Suetsugu (ILCDR 2008 – Cornell University) solenoid windings PEP 2

8. 25.09.2008 F. Le Pimpec 8 Technical Solutions Photon Killer : → In LHC the sawtooth is used to avoid reflection of photons far away from the emission point. EC is confined where emitted photons strike the wall → The other approach is to use an Antechamber with photon absorber where photons are trapped. Technique commonly used in most e + /e - machine. Photons Pumping with NEG or with distributed TiSp pumps Y. Tanimoto, KEK EPAC08 CERN Be window CERN – LHC

9. 25.09.2008 F. Le Pimpec 9 Surface studies TiZrV/Al Nb NH et al Appl. Phys. A 76, (2003) As Received = air exposed As received, TiZrV getters and TiN samples have a  max > 1.6 Thermal treatment is effective in reducing the SEY, but you need to be able to heat up the device. So far “we” haven’t found a material with an as received  max below 1.5 We must play tricks : heating, particles bombardment, roughness change … NIM A (551),187, 2005

10. 25.09.2008 F. Le Pimpec 10 Electron conditioning CERN (NH) - 2000 However, as the e-cloud disappears the conditioning dies off ! But we get steady help from the SR (CERN and KEK results) and some extra help from residual gas ions kicked to the wall Electrons from the e-cloud can condition surfaces. Hopefully in a reasonable time scale ! e-e- NIM A (551),187, 2005

11. 25.09.2008 F. Le Pimpec 11 NIM A (564),44-50, 2006 Ion conditioning What about CO ion exposure ? (chemistry) 250 eV ion exposure (P~10 -8 Torr) CERN (NH) - 2000 e-e- SEY after PGD + Air exposure PGDs (~500 V - 0.1 Torr) are extremely efficient in lowering the SEY to pure metal values in short time (~30 minutes) Downside : Not so trivial to make a PGD in-situ Using the beam itself to condition the wall is of course an option

12. 25.09.2008 F. Le Pimpec 12 Artificial roughness Simulation Cu M. Pivi et al to be published in JAP  = 60°

13. 25.09.2008 F. Le Pimpec 13 Artificial roughness & coatings 1” h x w x = 5 x 1 x 1 [mm] TiZrV/Al SEY experiment SR, ions and electrons do produce electrons & increase the pressure (PSD, ESD, ISD). In the other hand they cleanup the surface and lower the SEY over time. Thence, surfaces in an accelerator environment might not behave or condition as in the lab

14. 25.09.2008 F. Le Pimpec 14 In-situ machine studies Most of the major laboratory (ANL, BNL, CERN, KEK, LLNL, SLAC) are measuring the EC and the distribution in energy of the electron – usually use of a special RFA device (See ILCDR08 – Cornell University) e - detectors : measure e - Hrztl distribution & energy M. Pivi et al Now samples and even full length UHV chamber are exposed to accelerator operation. NEG coating was found to not only suppress EC but SEY results were in agreement with what was measured in lab (see A. Rossi ecloud’04) Groove chamber Flat chamber Electron detectors Connecting Flange PEP-II LER straight section e+  Arc bend B1 magnet upstream M. Pivi et al

15. 25.09.2008 F. Le Pimpec 15 Laboratory After e- Irradiation LER After Exposure LER Before Exposure S. Kato, KEK Review 2007 After Exposure to beam: Drastic decease of  max for all samples. Results are almost consistent with those results obtained at Lab. Accelerator SEY studies Y. Suetsugu in ILCDR08

16. 25.09.2008 F. Le Pimpec 16 Summary ECE still is very stimulating theoretically and experimentally : Number of workshop and special meetings: Ecloud’07, ECL2, ILCDR08... Material research mainly done in laboratory, in depth SEY studies at CERN - KEK - SLAC (use of AES - XPS first with air exposure yet without) especially on coatings. Test in accelerator confirms laboratory research - Grooves and coatings are effective in mitigating EC. Beam-surface interaction helps conditioning the UHV chambers, as expected. This workshop is a good bridge between Space and Accelerator community - common problems - A ccelerator physicist were/are unaware of your work

17. F. Le Pimpec Acknowledgement Colleagues from the accelerator community B. Henrist, G. Rumolo, M. Furman, H. Hseuh, F. Zimmermann …