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

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

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

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

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

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

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

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 2000, CERN, Geneva ILC European Regional Meeting and ILC-BDIR Royal Holloway University of London 22nd July 2005

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

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

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

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

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

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 Royal Holloway University of London 22nd July 2005

NEG Coated Vacuum Chamber: Main Parameters ILC European Regional Meeting and ILC-BDIR Royal Holloway University of London 22nd July 2005

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 Royal Holloway University of London 22nd July 2005

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 = 0.159 A is the beam current ILC European Regional Meeting and ILC-BDIR Royal Holloway University of London 22nd July 2005

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

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

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 10-12 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

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

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 Royal Holloway University of London 22nd July 2005

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

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

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 Royal Holloway University of London 22nd July 2005

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

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

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 40-80 m downstream the arc ILC European Regional Meeting and ILC-BDIR Royal Holloway University of London 22nd July 2005

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 20-30 m. ILC European Regional Meeting and ILC-BDIR Royal Holloway University of London 22nd July 2005

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

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

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 20-30 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