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

2. e--cloud and Surface Science
The “e-cloud”
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. Extreme High Static Vacuum (<< 10-13 Torr)
Cold 1.9 K
Static
Beam 5K< T <20K
Extreme High Static Vacuum (<< Torr)
Radial Distance
T=0, without beam
ECLOUD04, Napa, April 19-23, 04.

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

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

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

7. e- from ionization of residual gas… etc
Even in absence of SR:
observation
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. Beam induced el. acceleration
simulation CB BS
(F.Zimmermann)
Radial Distance
+ +
p +
Time = 10 nsec
ECLOUD04, Napa, April 19-23, 04.

9. e- induced e- emission Radial Distance Time = 15 nsec
Surface Science 3 CB BS
Radial Distance
SEY
+ +
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. Time structure vs Simulations.
e- cloud Build-up
simulation CB BS
Radial Distance
+ +
+ +
P +
P +
(F. Zimmermann)
Time = 25 ns
Time structure vs Simulations.
ECLOUD04, Napa, April 19-23, 04.

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

13. Surface Science 6 and simulation
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 Radial Distance Time = 25 ns Surface Science 7CB 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 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 Au sample 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. All measured data have an Resolution of =1.2 eV
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, ) 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 energy in 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. Secondaries and Reflected Electron VERSUS Primary Energy
What has been measured on f.s. Cu
Secondaries and Reflected Electron VERSUS Primary Energy
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. Special thanks to I.R. Collins.
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.