Vacuum Pumping of Large Vessels and Modelling of Extended UHV Systems

Transcription

1 Vacuum Pumping of Large Vessels and Modelling of Extended UHV Systems Georgy L. Saksaganski D.V. Efremov Institute, St Petersburg, Russia An overview of the methods for reducing of the desorption flows and technique of UHV production in controlled fusion devices, accelerators, and colliders are presented. Two types of vacuum vessels are discussed: compact great volume vessels and extended low-aperture chambers. The given data summarise experiences gained in Russian scientific laboratories. A technology which was used for conditioning vessel walls and dynamics of pumping in the tokamak T-15 is described. The T-15 vacuum vessel is made of stainless steel 12X18H10T. The vessel is formed with reinforced segments with 10-20mm thick walls. The segments are connected with bellows. The inner surface area is 515 m 2, the volume is about 65m 3. The walls were conditioned using sequentially programmed ohmic and inductive heating up to 260? C, glow discharges in hydrogen, helium, argon, and krypton, finalising with training discharge in hydrogen. The operating conditions and partial proportions of residual gases are considered for different phases of the conditioning. Dynamic estimates of desorption loads are performed. The technique and results of an experimental modelling of the vacuum pumping line for a 3TeV storage accelerator are presented. A UHV system segment which includes the sections 6400mm in length and of oval cross-sections of 48 x 88 mm 2 and 64 x 76 mm 2 are modelled. The segments are formed with thin-walled steel tubes welded together using the argon-shielded arc welding. The tubes are electrochemically polished inside. Wall conditioning is made without heating and includes only glow argon discharge under variable operation conditions. An outline of modelled dynamic behaviour of the UHV system during pumping, partial content of residual gases, and estimates of the system "memory" and desorption loads are presented. Capabilities of desorption flow reduction with the use of an aluminium coating is discussed too. It is as follows: wall internal surfaces are coated with a thin aluminium layer in a pure inert atmosphere. As experiment implies such coating exhibits a very low absorbability in respect to water steam and other active gases. Vacuum vessels with all-coated walls can be pumped out so that to provide extra-low pressure (= 10-9 Pa) without additional thermal vacuum degassing even if it reside in air over prolonged period. This coating is highly reflective at low temperatures, especially in the IR spectrum. This technology seems to be very promising in fabricating of effective UHV system.

2 Vacuum Pumping of Large Vessels and Modeling of Extended UHV Systems Georgy L.Saksagansky D.V.Efremov Scientific Research Institute of Electrophysical Apparatus St.Petersburg, Russia

3 Contents The technology of pumping and walls conditioning of Tokamak T-15; Experimental modeling UHV systems of Accelerating-Storage Complex on 3 TeV: warm stage, cold stage; The coating of metallic film for reduction of wall desorption; Vacuum technology KATRIN proposals. For preparation of this report there were used the information of D.V.Efremov Institute (St.Petersburg); Institute of High Energy Physics (Protvino); I.V.Kurchatov Institute (Moscow); Vacris firm (St.Petersburg) 2

4 Abstract An overview of the methods for reducing of the desorption flows and technique of UHV production in controlled fusion devices, accelerators, and colliders are presented. Two types of vacuum vessels are discussed: compact great volume vessels and extended low-aperture chambers. The given data summarises experiences gained in Russian scientific laboratories. The technology that was used for conditioning vessel walls and dynamics of pumping in the tokamak T-15 is described. The T-15 vacuum vessel is made of stainless steel 12X18H10T. The vessel is formed with reinforced segments with 10-20mm thick walls. The segments are connected by bellows. The inner surface area is 515 m 2, the volume is about 65 m 3. The walls were conditioned using sequentially programmed ohmic and inductive heating up to 260 C, glow discharges in hydrogen, helium, argon, and krypton, finalising with training discharge in hydrogen. The operating conditions and partial proportions of residual gases are considered for different phases of the conditioning. Dynamic estimation of desorption loads is performed. 3

5 The technique and results of the experimental modelling of the vacuum system for the 3 TeV accelerating-storage complex are presented. The UHV system segment which includes the sections 6400 mm in length and of oval cross-sections of 48 x 88 mm 2 and 64 x 76 mm 2 are modelled. The segments are formed with thin-walled steel tubes welded together using the argon-shielded arc welding. The tubes are electrochemically polished inside. Wall conditioning is made without heating and includes only argon glow discharge under variable operation conditions. The outline of modelled dynamic behaviour of the UHV system during pumping, partial content of residual gases, and estimates of the desorption loads are presented. Capabilities of desorption flow reduction with the use of an aluminium coating is discussed too. It is as follows: wall internal surfaces are coated with a thin aluminium layer in a pure inert atmosphere. As experiment implies such coating exhibits a very low absorbability in respect to water steam and other active gases. Vacuum vessels with Al- coated walls can be pumped out so that to provide extra-low pressure (p= 10-9 Pa) without additional thermal vacuum outgassing even if it reside in air over prolonged period. This coating is highly reflective at low temperatures, especially in the IR spectrum. This technology seems to be very promising in fabricating of effective UHV system. 4

6 Tokamak?-15 Background pressure Pa Chamber volume 65 m 3 Surface area 515 m 2 Chamber material stainless steel 12?18? 10? Effective pumping speed 9600 l/s Modes of vacuum condition of the chamber: ohm and induction warming up prior to warming up q = W/m 2 in the process of chamber warming up q = W/m 2 after warming up q = W/m 2 induction discharge in hydrogen or helium; glow discharge in argon or krypton; training discharge in hydrogen. 5

7 Partial pressure of residual gases partial pressure, Pa 10-2 prior to warming up (T=300K) mass number partial pressure, Pa 10-2 in the process of chamber 18 warming up (T=420K) mass number partial pressure, Pa 10-3 after warming up (T=300K) mass number 6

8 Accelerating-Storage Complex (ASC) Background pressure < ?? Operating pressure Chamber length (closed on a circle) Chamber average diameter < Pa m 70 mm Chamber material stainless steel 09?18?10? Modes of vacuum-technological condition of the chamber: electrochemical polishing of internal surface; ultrasonic cleaning of internal surface; surface treatment by glow discharge plasma in argon: average doze of bombarding particles - ~(1 3 ) ion/cm 2 ; incident ion energy 350 ev; maximal intensity of the discharge at Ar pressure ( ) Pa. 7

9 8 Ring (I) and disk (2) electrodes for glow discharge initiation The diagram of the model of ASC warm stage 1. sputter-ion pump 2. electrode for glow discharge initiation 3. gauge 4. omegatron-type massspectrometer 5. Electrode for indication of Sputter-ion pump 6. viewing window 7. connection to pumping unit 8. gas dosing valve

10 Partial pressure of residual gases partial pressure, Pa H 2 CH 4 H 2 O Stay on air 10 days Without argon treatment Pumping 185 hours CO mass number Ar partial pressure, Pa hours in N 2 medium H 2 CH 4 H 2 O Argon treatment 15 min Pumping 40 hours CO mass number Ar specific outgassing rate, W/m H CH 4 H 2 O CO Ar mass number 9

11 The diagram of the model of ASC cold stage 1. viewing window 2. gauge 3. omegatron-type mass-spectrometer 4. valve 5. gas dosing valve 6. sputter-ion pump 7. warm chamber 8. cool chamber 9. electrode for glow discharge initiation 10. zeolite 10

12 Outgassing in different modes of wall conditioning for cold stage chamber specific outgassing rate, W/m Evacuation Evacuation and argon treatment Evacuation, argon treatment and cooling cryosurface by liquid nitrogen 11

13 Thin-film coatings of a surface for reduction of its adsorption ability Thermal sputtering Al, Cu, Ag, Cr, V, etc. in pure He passing at pressure 2-10 Pa (partial pressure of residual gases <10-3 Pa) Coating technology effects: Reduction of a roughness degree in times Creation barrier to migration of gases from thickness of a material Very low emissivity factor e (see table) Surface Surface temperatures,? Emissivity factor, e 1 Al film deposited in He passing flow under???=8 Pa Al film deposited in vacuum at p= Pa. Cu film deposited in He passing flow under???=8 Pa

14 1 cap 7 fore-vacuum pump 13 tube 2 plate 8 gas dosing valve 14 rotary motion feedthroudh 3 vacuum gate valve 9 tube 15 workpiece 4 LN 2 baffle 10 LHe bottle 16 quartz halogen lamps 5 turbomolecular pump 11 valve 17 evaporators 6 valve 12 valve 13

15 Design principle and KATRIN vacuum technology (proposals) conception «vacuum in vacuum» (hermetic thin-walled liner with K controlled temperature inside the ultra high vacuum chamber). using of construction materials double vacuum melted orgering and cleaning of inner liner surfaces (special vibro-mechanical treatment, electro-chemical holishing AL coating ) multi-stage combined wall conditioning of liner, using autonomy technological pumping system with continuous leakage and pressure and mass-spector messurement: programmable heating and termical outgasing on 420K; glow discharge cleaning in dynamical mode in dry gas medium: - hydrogen (active renewal medium) - argon (distraction of oxide films and hydrogen replace) - helium (argon replace) termical outgassing on K programmable cooling down to cryogenic temherature 14

16 transition in to own ultra high vacuum pumps routine wall-conditioning technology of ultra high vacuum chamber AL coating of the VV interiors EHV pumps: - liners: distribution pumps with microporous cryogetter (absorbent carbon) - VV: cryogetter pumps; - pumping system: turbo-molecular pumps An experimental modeling of the conditioning/pumping processes is required to choose optimum operating conditions. 15

17 Functional diagram of KATRIN vacuum system Cryopump Liner T=80K Cryopump p ~ 10-7 Pa Cooling pipe LN Warm Chamber Pumping 16