1. Vacuum Problems in the ILC Damping Ring
Dr. Oleg B. Malyshev
ASTeC
CCLRC Daresbury Laboratory
22nd June 2005
ILC European Regional Meeting and ILC-BDIR
Royal Holloway University of London

2. Interaction between the Beam and Residual Gas Molecules
Bremsstrahlung
Ionisation energy loss
Single Coulomb scattering
Multiple Coulomb scattering
Inelastic
Elastic
Interaction
Since the beam particles losses due to collision with residual gas particles is proportional to the gas density
the criteria for ‘good vacuum’ for the dumping rings is the following:
it must be either much less than acceptable level of losses
or much less than the beam particles losses due to different effects Quantum, Touschek, particle lifetime, etc,
This criteria was not scientifically defined yet!!! 10-8 mbar?
ILC European Regional Meeting and ILC-BDIR
Royal Holloway University of London
22nd July 2005

3. Sources of Gas in a Vacuum System: Thermal Desorption
(or thermal outgassing) means:
Molecules diffusing through the bulk material of the vacuum chamber (from subsurface layers to a few mm), entering the surface and desorbing from it
Molecules adsorbed on the surface (initially or after the air venting) and desorbing when vacuum chamber is pumped
Outgassing rate depends on many factors: choice of material, cleaning procedure, pumping time, etc...
Vacuum
Air
ILC European Regional Meeting and ILC-BDIR
Royal Holloway University of London
22nd July 2005

4. Sources of Gas in a Vacuum System: PSD
The same as thermal desorption, PSD depends on:
Choice of material
Cleaning procedure
History of material
Pumping time
Additionally it depends on
Energy of photons
Photon flux
Integral photon dose
Temperature
Photon stimulated desorption (PSD) is one of the most important sources of gas in the presence of SR.
Gas molecules may desorb from a surface when and where photoelectrons leave and arrive at a surface
H2O CO H2 e- e-
CO2   H2
CH4
ILC European Regional Meeting and ILC-BDIR
Royal Holloway University of London
22nd July 2005

5. Sources of Gas in a Vacuum System: PSD
Photodesorption yields,  (molecules/photon), as a function of accumulated photon dose, D, for different materials measured up to certain doses, these results are extrapolated for use in the design of new machines
Photodesorption yield as function of accumulated photon dose can be described as:
PSD yield for CO for prebaked and in-situ baked stainless steel vacuum chambers.
Yields for doses higher then 1023 photons/m (1 to 10 Amphrs for diamond) are extrapolations.
ILC European Regional Meeting and ILC-BDIR
Royal Holloway University of London
22nd July 2005

6. PSD yield as a function of photon critical energy
The photon stimulated desorbtion (PSD) yield :
known desorbtion yield
at 3 keV
proportional to the photon energy  when  < 1 keV and  > 100 keV
weak dependance on the photon energy 
when 1 keV <  < 100 keV
(=30keV)  (=3keV)
Data summary from many publications from CERN, LURE, BINP, BNL and others
ILC European Regional Meeting and ILC-BDIR
Royal Holloway University of London
22nd July 2005

7. Sources of Gas in a Vacuum System: PSD
Photodesorption yields, (molecules/photon), as a function of accumulated photon dose for different materials for vacuum chamber (data from A. Mathewson, CERN):
ILC European Regional Meeting and ILC-BDIR
Royal Holloway University of London
22nd July 2005

8. Sources of Gas in a Vacuum System: ESD
Electron stimulated desorption (ESD) can be a significant gas source in a vacuum system in a number of cases when the electrons bombard the surface.
The same as thermal desorption and PSD, ESD depends on:
Choice of material
Cleaning procedure
History of material
Pumping time
Additionally it depends on:
Energy of electrons impacting the surface
Electron flux to the surface
Integral electron dose
Temperature
F. Billard, N. Hilleret, G. Vorlaufer. Some Results on the Electron Induced Desorption Yield of OFHC Copper. Vacuum Technical Note 00-32, December , CERN, Geneva
ILC European Regional Meeting and ILC-BDIR
Royal Holloway University of London
22nd July 2005

9. Sources of Gas in a Vacuum System: ESD and BIEM
Beam induced electron multipacting (BIEM) is a significant problem in a vacuum chamber with a positive charged beam:
a free electron is accelerated to the first positively charged bunch;
when the bunch passes the accelerated electron moves with accumulated energy up to hundreds of eV towards the opposite wall and strikes it, this causes:
ESD, which results in a pressure rise
Secondary electrons are then accelerated by the next bunch CO e-
H2O e- e- e- e- e- e- e- e- e- e- e- H2 e-
CO2 e-
CH4 e-
ILC European Regional Meeting and ILC-BDIR
Royal Holloway University of London
22nd July 2005

10. Sources of Gas in a Vacuum System: ESD and BIEM
BIEM is also a potential problem in a vacuum chamber with a negatively charged beam:
a free electron is repelled from the first negatively charged bunch
the repelled electron with an accumulated energy up to hundreds of eV strikes the wall, this causes:
ESD, which result in a pressure rise
Secondary electrons, depending upon their position may be repelled by the next bunch with energies sufficient to create more secondary electrons. CO
H2O e- e- e- e- e- e- e- e- e- e- e- e- e- H2 e-
CO2
CH4 e-
ILC European Regional Meeting and ILC-BDIR
Royal Holloway University of London
22nd July 2005

11. Sources of Gas in a Vacuum System: ISD
Ion stimulated desorption (ISD) can be a significant gas source in a vacuum system where the ion beam bombards the surface. There is very little data, most work has been done at CERN.
The same as thermal desorption, PSD and ESD, the ISD depends on: choice of material, cleaning procedure, history of material and pumping time.
It is also depends on:
Mass, charge and energy of ions impacting the surface
Ion flux to the surface
Integral ion dose
Temperature
ILC European Regional Meeting and ILC-BDIR
Royal Holloway University of London
22nd July 2005

12. Ion Induced Pressure Instability
When the positive charged beam particles colliding with residual gas molecules ionise them, these ions are accelerated towards the vacuum chamber wall. This causes ion induced gas desorption, the pressure rises and more molecules will be ionised, accelerated and bombard the wall…
where
Q = gas desorption,
Seff = effective pumping speed,
 = ion induced desorption yield
 = ionisation cross section,
I = beam current.
When I  Ic (or )
then gas density (pressure) increases dramatically!
ILC European Regional Meeting and ILC-BDIR
Royal Holloway University of London
22nd July 2005

13. Dust Particles in a Vacuum Chamber
The dust micro-particle in the beam vacuum chamber might be ionised by photons or photoelectrons and
then be trapped by the beam electric field.
This may cause the significant loss of the beam.
Potential sources of the dust micro-particles:
Dust from the atmosphere during storage, installation or venting
Dust from moving parts: manipulators,bellows, valves, etc
Micro-particles from getters, cryosorbers
Micro-particles from working Ion Pumps.
How to avoid:
Proper cleaning and storing of components
Positioning of potential dust sources in regard to the beam
Clean environment when vacuum chamber is open
Clean gas for venting (for example, boil-off nitrogen)
ILC European Regional Meeting and ILC-BDIR
Royal Holloway University of London
22nd July 2005

14. NEG coated vacuum chamber under SR
Dynamic pressure rise for the Stainless Steel (baked at 300C for 24 hrs) and TiZrV coated vacuum chambers (activated at 190C for 24 hrs)
ILC European Regional Meeting and ILC-BDIR
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22nd July 2005

15. NEG Coated Vacuum Chamber: Main Parameters
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16. NEG Coated Vacuum Chamber: SR Induced Pumping
NEG TiZrV coated surface saturated with CO (i.e. no pumping speed) exposed to SR
ILC European Regional Meeting and ILC-BDIR
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22nd July 2005

17. Photon stimulated desorption in Tesla Damping Ring
There are two main sources of SR in the damping ring:
Dipoles in the arcs and
Wigglers in the straights.
The photon stimulated desorption is calculated as:
where
 is the photon stimulated desorption yield
 is the critical energy of photon
D is the photon dose
 is the photon flux
E = 5 GeV is the beam energy
B is the magnet field
I = A is the beam current
ILC European Regional Meeting and ILC-BDIR
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22nd July 2005

18. SR from Dipoles and Wigglers in TESLA damping ring
B (T)
0.2
1.8 
3.3 keV
~30 keV
m (photons/(ms))
71017
1.11019
tot (photons/s)
2  5.51020
2  61021
Pm (kW/m)
0.067 8
Ptot (kW)
2  88
2  4500
ILC European Regional Meeting and ILC-BDIR
Royal Holloway University of London
22nd July 2005

19. Initial desorption yields for Al and SS (after about 1021 photons/m)  = 3 keV
Material
Aluminium
Stainless steel Cu
NEG coated SS H2
0.05
610-4
810-4
1.510-5 CO
0.025
710-4
210-4
110-5
CO2
0.012
410-4
310-4
210-6
CH4
310-3
410-5
210-7
ILC European Regional Meeting and ILC-BDIR
Royal Holloway University of London
22nd July 2005

20. Initial gas density (pressure)
Photon stimulated gas desorption:
This would be the highest value for vacuum chamber at room temperature, the desorption reduces with an accumulated photon dose.
Thermal stimulated desorption must be considered in either case and is the lowers limit. After baking it can be in the range of mbarlt/(scm2)
A tube with 50-mm inner diameter was studied at this stage in stead the real vacuum chamber cross section.
The vacuum system will be conductance limited, the average gas density with lumped pumps only is approximately proportional to 1/d3
ILC European Regional Meeting and ILC-BDIR
Royal Holloway University of London
22nd July 2005

21. Layout for vacuum calculations
A tube with 50-mm inner diameter was studied at this stage in stead the real vacuum chamber cross section.
The vacuum system will be conductance limited, the average gas density with lumped pumps only is approximately proportional to 1/d3 L
Pumping ports
ILC European Regional Meeting and ILC-BDIR
Royal Holloway University of London
22nd July 2005

22. Gas density along the arc vacuum chamber
Stainless steel baked in situ at 300C for 24 hrs
Photon stimulated desorbtion for 4 gases and thermal desorbtion with t=10-12 mbarl/(scm2)
ILC European Regional Meeting and ILC-BDIR
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22nd July 2005

23. Gas density along the NEG coated arc vacuum chamber
NEG coating does not pumpCH4
ILC European Regional Meeting and ILC-BDIR
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22nd July 2005

24. Gas density along the arcs
Stainless steel vacuum chamber
The photon stimulated desorption is much larger than thermal stimulated desorption.
The average gas density along the stainless steel vacuum chamber with lumped pumps only is too high: 10-8 mbar can be reached installing 1000-l/s pump every meter
NEG coating
NEG coating of the vacuum chamber reduces desorption and introduces distributed pumping speed: 10-8 mbar (mainly CH4) can be reached by installing 10-l/s pump every 20 meter (to pump hydro-carbons and noble gases)
ILC European Regional Meeting and ILC-BDIR
Royal Holloway University of London
22nd July 2005

25. Gas density along the wiggler vacuum chamber
Cu baked in situ at 200C for 24 hrs
Photon stimulated desorbtion for 4 gases
ILC European Regional Meeting and ILC-BDIR
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22nd July 2005

26. Gas density along the NEG coated wiggler vacuum chamber
NEG coating does not pumpCH4
ILC European Regional Meeting and ILC-BDIR
Royal Holloway University of London
22nd July 2005

27. Gas density along the wiggler sections
Copper vacuum chamber
The photon stimulated desorption is much larger than thermal stimulated desorption.
The average gas density along the stainless steel vacuum chamber with lumped pumps only is too high: 10-8 mbar can be reached installing 1000-l/s pump every 10 cm – almost distributed pumping.
NEG coating
NEG coating of the vacuum chamber reduces desorption and introduces distributed pumping speed: 210-8 mbar (H2 and CH4) can be reached by installing 30-l/s pump every 5 meters (to pump mainly hydro-carbons and noble gases)
ILC European Regional Meeting and ILC-BDIR
Royal Holloway University of London
22nd July 2005

28. Photon flux along the straights downstream the arcs
Gas density is proportional to the photon flux therefore
the distance between pumps may be larger than in the arcs or
NEG coating is no linger the only vacuum solution from m downstream the arc
ILC European Regional Meeting and ILC-BDIR
Royal Holloway University of London
22nd July 2005

29. Section where photon flux is negligible
In the section where the photon flux is less than 1015 photons/m the thermal desorption is the main source of gas. These straights can be made of stainless steel tube with 30 l/s pump every m.
ILC European Regional Meeting and ILC-BDIR
Royal Holloway University of London
22nd July 2005

30. What is needed from the accelerator science
What column density is required along the dumping ring (10-6 or 10-8 mbar)
How long the beam will be used for conditioning
Whether the impedance of the NEG coated wall is acceptable (~250 Ohmcm)
Required vacuum chamber apertures along DR or at least the gap in dipoles and wigglers
ILC European Regional Meeting and ILC-BDIR
Royal Holloway University of London
22nd July 2005

31. Wiggler vacuum chamber will absorb significant SR power:
Other considerations
BIEM may cause much higher outgassing – awaiting results of modelling (power, impact electron flux, energy distribution)
Wiggler vacuum chamber will absorb significant SR power:
Vacuum chamber design will depends on solution of how to deal with that power
SR power absorber coated by NEG has never been studied (thermal expansion coefficients, film adhesion, vacuum properties…)
Arcs vacuum chamber will require cooling for mechanical stability
ILC European Regional Meeting and ILC-BDIR
Royal Holloway University of London
22nd July 2005

32. Including BIEM in to vacuum model will be the following step
Conclusions
NEG coating of vacuum chamber along both the arcs and the wigglers as well as a few tens meters downstream of both looks to be the only possible solution to fulfil vacuum requirement for the dumping ring
Considering thermal and photon stimulation desorption only the most of the straights can be made of stainless steel tube with 30 l/s pumps located every m
Including BIEM in to vacuum model will be the following step
Power dissipation from SR and BIEM have to be considered
NEG coated power absorber needs to be studied
ILC European Regional Meeting and ILC-BDIR
Royal Holloway University of London
22nd July 2005