Achieving Ultra-High Vacuum without (in-situ) Bakeout

Transcription

1 Achieving Ultra-High Vacuum without (in-situ) Bakeout Matthew Cox Diamond Light Source, UK WS 63 Ávila September

2 What is Diamond Light Source? The UK national synchrotron facility 3 rd generation light source Generates brilliant beams of light, from infra red to hard X rays, for a range of science applications Construction began in early 2003 User operations began on schedule in January 2007 with 7 beamlines operational 19 beamlines operational + 3 being installed (September 2010) Can ultimately host up to 40 beamlines WS 63 Ávila September

3 Aerial view WS 63 Ávila September

4 Diamond layout Booster synchrotron 3 GeV electron storage ring 562m circumference <10-9 mbar 100 MeV Linac Front end Beamline WS 63 Ávila September

5 Why ultra high vacuum? Storage ring p< 10 9 mbar to reduce interactions of circulating electrons with residual gas molecules If the pressure is too high Stored beam lifetime is reduced Unwanted Gas Bremsstrahlung radiation is produced Effects scale as Z 2 so high Z gases must be minimised H 2 ΣZ 2 = 2 H 2 O ΣZ 2 = 66 CO ΣZ 2 = 100 Ar ΣZ 2 = 324 KrΣZ 2 = 1296 Beamlines Hydrocarbons cause contamination of x ray optics (mirrors, gratings, crystals) Halogen containing compounds (particularly fluorine) can poison Non Evaporable Getter (NEG) pumps and coatings Some sample environments require UHV X ray beam quality reduction due to gas phase absorption and scattering Residual gas Hydrogen often predominant but not a problem low Z, non contaminating Biggest problems usually water and hydrocarbons WS 63 Ávila September

6 Storage ring vacuum engineering High radiation environment in shielding tunnel Materials Stainless steel 316LN Copper OFHC Ceramic Electronics Personnel access High heat loads and power densities RF compatibility Long narrow beam channels with little space for vacuum pumps: conductance limited In situ bakeout only in a few sections (mainly cost, in a few cases for technical reasons) Reliability 24/7 operation, 5 shutdowns a year Remote control and monitoring WS 63 Ávila September

7 Basic Pumping Equation At steady state p = Q/S p= local pressure Q = local gas sources S = local pumping speed Important practically Local number density ( pressure ) integrated along the beam path Residual gas composition Time to reach acceptable operating UHV pressure Following new component installation or intervention Elapsed time and beam dose (Photon Stimulated Desorption) WS 63 Ávila September

8 Leaks Virtual Leaks Permeation Sources of gas Q Insignificant with proper design, material choices, manufacture, cleaning, assembly and conditioning Gas flows to / from different pressure regions Differential pumping, flow restrictions + windows where possible Thermal desorption Reduce by materials, processing, procedures Water adsorption during interventions followed by desorption can be a problem Photon stimulated desorption (PSD) in accelerators and beamlines Dominates following new vessel introduction on machine startup Reduces by orders of magnitude after a period of beam conditioning Radiation degradation of materials Avoid fluorocarbons!!! WS 63 Ávila September

9 Local pumping speed S Conventional lumped pumps Differential (noble) diode Sputter Ion Pumps over 500 in the storage ring NEG cartridges where space limited Titanium Sublimation Pumps high gas load areas, boost SIP locally Distributed pumps NEG coatings long narrow insertion device vessels only where conventional pumps cannot be fitted in. Activation following interventions is inconvenient. 6 straights with NEG vessels. Cryo surfaces No dedicated cryo pumps Useful by product of cryo devices with another function which cannot be baked and often have high thermal outgassing rate at room temperature Sometimes a problem as the gas condenses where we don t want it Release of adsorbed gas on warm up can be awkward Superconducting RF cavities 4.2K Cold bore superconducting insertion devices (wigglers) 4.2K/20K Cryogenic Permanent Magnet Undulator (CPMU) 150K LN2 cooled monochromators in beamlines 80K WS 63 Ávila September

10 Diamond bakeout policy Preference is not to bake out in situ Limited intervention time Labour intensive and prone to human error Expensive Jackets, heaters and controllers Magnet apertures Thermal expansion Ex situ prebake and installation under dry nitrogen purge Aqueous cleaning (steam/alkaline detergent/di water/ultrasonic for stainless steel Citronox for copper) Ex situ bakeout typically 250 C vessel at supplier, 200 C assemblies at Diamond No significant performance difference between in situ baked and ex situ baked invacuum undulators. Max temp 120 C Dry nitrogen flow helps to prevent water vapour from entering the system Air Products BIP nitrogen (H 2 O<20ppb, THC<100ppb), baked supply lines etc In situ bake out NEG coating activation Where significant exposure to atmospheric moisture is unavoidable Where ultimatre pressue or gas composition is particularly critical WS 63 Ávila September

11 Diamond bakeout experience Generally works well Problems occur Items which cannot be baked Often saved by below room temperature operation Superconducting RF cavities 4.2K LN2 cooled monochromator 100K Cold bore superconducting insertion devices (wigglers, undulators) 4.2K/20K Cryogenic Permanent Magnet Undulator (CPMU) 150K Sometimes we just have to pump for a long time (months before installation) Sometimes we have to accept a higher local pressure, e.g. injection straight started operations at around 10 8 mbar now around 10 9 mbar Complex items which sometimes can t be cleaned or baked Water cooled beamline x ray monos (encoders, motors, wiring, bearings ) Interventions with long atmospheric exposure and no bakeout possibility or not enough time for bakeout Beamlines! WS 63 Ávila September

12 Diamond Storage Ring Vacuum Performance (1) 3 years into operation, storage ring 2342 A.h of accumulated beam dose at 3 GeV Static pressure (no beam) 2.6 x mbar Dynamic pressure (250 ma) 5.6 x mbar Diamond wide 10,000 vacuum PVs, 1TB vacuum data accumulated! Specified dynamic pressure < 10 9 mbar Gas composition according to RGA is mainly H 2, the balance mainly CO Beam current 2h Thermal vs PSD contributions? Storage ring pressure WS 63 Ávila September

13 Diamond Storage Ring Vacuum Performance (2) 10% 1% Uncalibrated RGA scan with beam off 98% H 2, 2% CO, also CH 4, CO 2, H 2 O + some low level contaminants at the 0.05% level 0.1% WS 63 Ávila September

14 1.0E-08 Storage ring beam conditioning Dynamic pressure (mbar/ma) 1.0E E E-11 Pressure Lifetime Current x lifetime (ma.h) 1.0E Beam dose (A.h) Beam / vacuum effects (PSD, lifetime) get smaller with time and WS-63 Ávila September 2010 dose and more difficult to separate 14 different contributions Comparison of modelling and results Work in Progress Malyshev/Cox

15 Calculated pressure in a standard Diamond cell during beam conditioning 1.E-07 DIAMOND Storage Ring Cell with 5m ID make-up Girder 1 Girder 2 Girder 3 ID Make-up 1.E-08 No Beam 10mA 0Ah 100mA 0Ah 100mA 0.1 Ah 300 ma 1000 Ah Pressure (mbar) 1.E-09 1.E-10 NEG 300 l/s 300 l/s 300 l/s 500 l/s 300 l/s 300 l/s 500 l/s 300 l/s 300 l/s 500 l/s 300 l/s 500 l/s 300 l/s NEG 300 l/s 300 l/s 300 l/s 1.E Distance (mm) Plot shown using Diamond in house PressureProfile program Q th = mbar l/s/sqcm η0 = 10 4 molecules/photon m=28 Comparison of modelling and results Work in Progress Malyshev/Cox WS-63 Ávila September 2010

16 Dipole and crotch vessel Material 316LN stainless steel walls 316LN flanges OFHC copper absorbers CuBe spring contacts Processes TIG welding e-beam welding Explosion bonding Vacuum brazing Quantities Full prototype Production run 52 off (26 pairs) over 12 months EVC 11 Salamanca Sept

17 Photon absorbers Discrete crotch and finger absorbers, OFHC copper vacuum brazed Explosion bonded OFHC copper distributed absorber

18 SR build (1) Vessel pre-assembly at the supplier + clean bake Vessel dimensional inspection at the supplier Vessel delivery to Diamond 6 m long vacuum string assembly on trolley Vacuum string lifting into the bakeout oven Vacuum bakeout 200 EVC 11 Salamanca Sept

19 SR build (2) Girder integration Girder installation Vacuum interconnections Girder installation complete Installing the 5 m long NEGcoated Al vessel for beamline I06 Storage ring complete EVC 11 Salamanca Sept

20 In vacuum undulator 9 installed Two moveable arrays 2 m long of permanent magnets in vacuum Bakeout 120 C 3 weeks (SmCo magnets) 1300l/s ion pumps +GP500 NEG + 2 TSPs p ~ 5x10-10 mbar (no beam) WS-63 Ávila September 2010

21 Cryogenic Permanent Magnet Undulator (CPMU) Similar to in-vac ID magnet except arrays LN2 cooled ~150K 1 installed Vessel pre-baked Assembly non-bakeable (FeNdB magnets) p ~ 1x10-8 mbar RT (no beam) p ~ 5x10-10 mbar cold (no beam) WS-63 Ávila September 2010

22 Superconducting multipole wiggler 2 similar devices installed I15 and I12 I15 parameters: 3.5 T 49 pole magnet Cold bore 4.2 K /20K vacuum vessel Cleanliness not ideal for UHV! Pressure at ends ~ 2x10-9 mbar cold Peaks around 10-5 mbar when warmed No pressure measurement inside the beam tube WS-63 Ávila September

23 Beamline I13 external (planned) Twin 150m long external x ray beam pipes 100 to 200 mm diameter. Windowless. Need to achieve UHV <5x10 9 mbar at both ends Exposed to ambient temperature variation = outgassing rate variation Installation in ambient conditions (long tent) UK October/November >100% humidity! Critical to control water adsorption/release. In situ bake not possible Pressure along the pipes not critical, only at the ends Planned solution is to have no pumps along the pipes with differential pumping at the end. Calculations show could be order of mbar in the middle with specific outgas rate 10 9 mbar l/s/sqcm! Pressure gauges included + provision to fit pumps if needed. 2/3 gate valves. Considered pre air baking, lab tests in progress but will probably use vacuum pre baked (250 stainless steel 304 WS-63 Ávila September 2010

24 Possible points for discussions Control of water adsorption / desorption Wall material Coatings Pre treatments, e.g. air baking? Intervention procedures humidity control How to accelerate removal of adsorbed water without bakeout, e.g. N 2 backfill and pump mechanism? Most effective cleaning methods for UHV components to reduce Thermal outgassing PSD yield and accelerate beam conditioning What is beam conditioning process? WS 63 Ávila September

25 Thank you WS 63 Ávila September