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Surface related properties as an essential ingredient to e-cloud simulations.

Surface related properties as an essential ingredient to e-cloud simulations. The e- cloud problem: a SS-oriented review. Surface science techniques to provide input parameters. Some selected results Future work and implications. R. Cimino LNF-INFN Frascati (Roma) Italy. The “e-cloud”.

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Surface related properties as an essential ingredient to e-cloud simulations.

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  1. Surface related properties as an essential ingredient to e-cloud simulations. • The e- cloud problem: a SS-oriented review. • Surface science techniques to provide input parameters. • Some selected results • Future work and implications • R. Cimino LNF-INFN Frascati (Roma) Italy. ECLOUD04, Napa, April 19-23, 04.

  2. The “e-cloud” e--cloud and Surface Science Let us see what Surface Science properties are (or may) be important in determining the eventual occurrence and size of an e-cloud build-up, (hence beam, and/or pressure instabilities) by describing the case of the: L.H.C. arcs. ECLOUD04, Napa, April 19-23, 04.

  3. Static Cold Bore @ 1.9 K Beam Screen @ 5K< T <20K Extreme High Static Vacuum (<< 10-13 Torr) Radial Distance T=0, without beam ECLOUD04, Napa, April 19-23, 04.

  4. calculation Synchrotron Radiation: Ec = 44 e V @ LHC CB BS Radial Distance + + + + + + + + + + + + + + + + + + p + Time = 0 ECLOUD04, Napa, April 19-23, 04.

  5. Surface Science 1 Photon reflectivity: CB BS Radial Distance + + + + + + + + + + + + + + + + + + Saw tooth Flat Cu p + N. Mahne et al. App. Surf. Sci. 04, to be published Time = 2 nsec ECLOUD04, Napa, April 19-23, 04.

  6. Surface Science 2 Photoemission:(vs. hn, Q, E,T, B) CB BS Radial Distance + + + + + + + + + + + + + + + + + + p + Time = 5 nsec ECLOUD04, Napa, April 19-23, 04.

  7. observation Even in absence of SR: e- from ionization of residual gas… etc SPS (M.Jimenez) e- Radial Distance + + + + + + + + + + + + + + + + + + p + Beam induced multipacting is observed in SPS where no e- are photoemitted. Time = 5 nsec ECLOUD04, Napa, April 19-23, 04.

  8. simulation Beam induced el. acceleration CB BS (F.Zimmermann) Radial Distance + + + + + + + + + + + + + + + + + + p + Time = 10 nsec ECLOUD04, Napa, April 19-23, 04.

  9. e- induced e- emission Surface Science 3 CB BS SEY Radial Distance + + + + + + + + + + + + + + + + + + N. Hilleret Time = 15 nsec ECLOUD04, Napa, April 19-23, 04.

  10. e- induced e- emission vs. E Surface Science 4 CB BS Ep=300 eV Radial Distance + + + + + + + + + + + + + + + + + + (R.E. Kirby) Time = 20 ns ECLOUD04, Napa, April 19-23, 04.

  11. simulation e- cloud Build-up CB BS Radial Distance + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + P + P + (F. Zimmermann) Time = 25 ns Time structure vs Simulations. ECLOUD04, Napa, April 19-23, 04.

  12. e- induced heat load Surface Science 5 CB BS 100 e- @1 eV Do they give the same heat load as 1 e-@100 eV ??? Radial Distance + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + p + p + Time = 25 ns ECLOUD04, Napa, April 19-23, 04.

  13. ph. or e- induced desorption Surface Science 6 and simulation CB BS Dynamic pressure increase !!! Radial Distance + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + p + p + Desorbed gas Time = 25 ns ECLOUD04, Napa, April 19-23, 04.

  14. Beam scrubbing effect Surface Science 7 CB BS Radial Distance + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + P + P + Time = 25 ns The nominal LHC operation relies on SCRUBBING ECLOUD04, Napa, April 19-23, 04.

  15. Surface science inputs : 1 (Photon - reflectivity) 2 Photoemission Yield and Photoemission induced el. energy distribution 3 Electron induced el. emission yield (SEY) 4 Electron induced energy distribution curves 5 (Heat load) 6 (Photon and electron induced desorption) 7 Surface properties changes during conditioning. 8 Chemical modifications vs conditioning. ECLOUD04, Napa, April 19-23, 04.

  16. Experiment with Synchrotron radiation ECLOUD04, Napa, April 19-23, 04.

  17. EDC vs. photon Dose See: R. Cimino et al PRST 2 063201 (1999) ECLOUD04, Napa, April 19-23, 04.

  18. Photoemission with Monochromatic light ECLOUD04, Napa, April 19-23, 04.

  19. A surface science lab. • µ-metal chamber; • En. & angle res. analyser; • Low T manipulator; • LEED - Auger RFA; • Faraday cup. • Low energy electron gun • Mass spectrometer • Sample preparation ECLOUD04, Napa, April 19-23, 04.

  20. Characterization of the e- beam • Stable between 30 - 400 eV • Currents from few nA to µA 20µC/h/mm2 - 20mC/h/mm2 • Intense spot (less than 0.5 mm) with low background • Position and intensity stability vs Time, Energy and Filament current. 1mm slot ECLOUD04, Napa, April 19-23, 04.

  21. Characterization of the RFA as an electron energy analyser • On Ausample Vs Electron energy and Sample Bias: • Need of a Bias voltage to measure Secondaries from sample and not a superposition of them and those produced internally by the grids of the analyser. ECLOUD04, Napa, April 19-23, 04.

  22. Characterization of the electron source and sample Bias • The actual Primary energy impinging on the sample is: Ep = Egun + E bias • It is possible to vary Egun and E bias to measure at low Ep keeping the Egun in a region where it is stable (>30 eV) and recording EDC. ECLOUD04, Napa, April 19-23, 04.

  23. Characterization of EDC line-shape as a function of analyser resolution • The actual EDC line shape is an important parameter of the BIEM simulation codes. • All measured data have an Resolution of =1.2 eV • The measured line-shape depends on the an. Res. ECLOUD04, Napa, April 19-23, 04.

  24. Characterization of EDC line-shape as a function of analyser resolution Photoemission results(R. Cimino, I.R. Collins, V. Baglin Phys. Rev. Acc. & Beam 2, 63201 99) do show, on some as- received samples, very sharp secondaries. • Data should be collected with good an. resolution ECLOUD04, Napa, April 19-23, 04.

  25. The set-up + the bias give us access to the very low energy electrons: • By applying a bias to the sample: Ep = Egun + E bias Energy Distribution of el. incident on LHC chamber of r=40mm. (A. Rossi) • It is possible to measure from ZERO primary energyin a region where the gun is stable (Egun >30 eV). 250 Wall chamber half dimensions: hxy=22, 18 mm (F.Zimmermann) 150 ECLOUD04, Napa, April 19-23, 04.

  26. Measure of Secondary e- YIELD • At each Primary energy we can measure Igun (with the Faraday cup)and Isample. Igun - Isample  Igun R. Cimino et al Phys. Rev. Lett. in print ECLOUD04, Napa, April 19-23, 04.

  27. Measure of Secondary e- YIELD • Each single point in a DELTA plot gives the total number of electrons emitted at a give primary energy. The energy distribution of such emitted electrons is important for the simulations. R. Cimino et al Phys. Rev. Lett. in print ECLOUD04, Napa, April 19-23, 04.

  28. What has been measured on f.s. Cu • Energy Distribution Curves as function of Ep R. Cimino et al Phys. Rev. Lett. in print ECLOUD04, Napa, April 19-23, 04.

  29. What has been measured on f.s. Cu • Energy Distribution Curves as function of Ep R. Cimino et al Phys. Rev. Lett. in print ECLOUD04, Napa, April 19-23, 04.

  30. What has been measured on f.s. Cu • Energy Distribution Curves as function of Ep R. Cimino et al Phys. Rev. Lett. in print ECLOUD04, Napa, April 19-23, 04.

  31. What has been measured on f.s. Cu • Integrating the curves gives the Percentage of Secondaries and Reflected electrons • To separate “true secondaries” from “rediffused electrons” is arbitrary and has not been considered in this analysis. ECLOUD04, Napa, April 19-23, 04.

  32. Secondariesand Reflected Electron VERSUS Primary Energy What has been measured on f.s. Cu R. Cimino et al Phys. Rev. Lett. in print ECLOUD04, Napa, April 19-23, 04.

  33. What has been measured on f.s. Cu • we can single out the contribution to of the secondaries and the reflected electrons versus primary energy. Igun - Isample  Igun ECLOUD04, Napa, April 19-23, 04.

  34. What has been measured on f.s. Cu The value for the minimum, its energy and width vary with sample, sample spot, scrubbing and T. A systematic study is necessary to give values. It is clear that the reflected component plays a major role in  at low energy. ECLOUD04, Napa, April 19-23, 04.

  35. Implication Implication Implication • Low energy electrons have a long survival time. Explains observations at KEK, SPS, PSR, LANL…. ECLOUD04, Napa, April 19-23, 04.

  36. Implication • Low energy electrons have a long survival time (in agreement with observation on PSR at LANL…). • In FELs a low repetition rate is supposed to ensure no e- cloud problems. BIEM has to be considered. • BIEM simulations need to be updated for the LHC and other machines. • Reflected el. are NOT absorbed and do not directly contribute to heat load !!! • However they will be accelerated by the following bunches, gaining energy to be deposited on the BS. ECLOUD04, Napa, April 19-23, 04.

  37. Simulated average heat load in an LHC DM as a function of Nb, considering the elastic reflection (dashed line) or ignoring it (full line) Calculation done extrapolating measured SEY to : dM=1.7 EM=240 eV See: R. Cimino et al: Phys. Rev. Lett. in print ECLOUD04, Napa, April 19-23, 04.

  38. Simulated EC density for the GSI-SIS18 with (dashed line) and without (full line) elastic reflection. See:R. Cimino et al.: Phys. Rev. Lett. in print ECLOUD04, Napa, April 19-23, 04.

  39. Back to Scrubbing or conditioning: • from LHC PR 472 (Aug. 2001): • “…Although the phenomenon of conditioning has been obtained reproducibly on many samples, the exact mechanism leading to this effect is not properly understood. This is of course not a comfortable situation as the LHC operation at nominal intensities relies on this effect…” Surface science Can give a deeper understanding of the chemical processes occurring at surfaces. ECLOUD04, Napa, April 19-23, 04.

  40. Surface science vs. Scrubbing • Indeed beam induced conditioning (or scrubbing) acts by modifying surface properties and, therefore, can and should be studied by SS techniques!!! Some preliminary results… ECLOUD04, Napa, April 19-23, 04.

  41. CONCLUSION: SURFACE SCIENCE can produce essential inputs to tackle the e-cloud problem. High quality study and use of frontiers techniques like photoemission with Synchrotron radiation seems to be important to understand material properties as required to correctly predict accelerators performances in presence of an e-cloud. ECLOUD04, Napa, April 19-23, 04.

  42. Acknowledgments: • This work was only possible tanks to: - A. G. Mattewson and O. Gröbner - V. Baglin and the CERN vacuum Group. - F. Zimmermann, F. Ruggero, M. Pivi, M. Furman, G. Bellodi, G. Rumolo, etc.. Special thanks to I.R. Collins. ECLOUD04, Napa, April 19-23, 04.

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