1. S. Jachmich (slide 1) Vessel Conditioning SL-Training, Nov 2010 Vessel conditioning Stefan Jachmich SL-Training 2010

2. S. Jachmich (slide 2) Vessel Conditioning SL-Training, Nov 2010Outline  Vacuum condition  Torus pumping  Vessel baking  Glow discharge cleaning  Beryllium evaporation  Residual gas analysis

3. S. Jachmich (slide 3) Vessel Conditioning SL-Training, Nov 2010 Vessel conditioning Good quality of vaccuum and surface condition are essential for successful repeatable plasma operation Sequence to recondition vessel after shutdown:  Pump-down of vessel  Vessel baking  Glow Discharge Cleaning  Be-evaporation  Plasma conditioning

4. S. Jachmich (slide 4) Vessel Conditioning SL-Training, Nov 2010 Pumping system  Turbo pumps compress gas molecules into fore-vaccuum chamber  Four turbomolecular pumps (~2000 l s -1 ) connected to the torus via two pumping chambers (Octant 1&5) (  Xmimic:  “vc/tps/tt01-2, “vc/tps/tt01-2 )  Sufficient to get below 10 -6 mbar and to operate  All pumped gases go through the active gas handling system (AGHS)

5. S. Jachmich (slide 5) Vessel Conditioning SL-Training, Nov 2010Cryopumps  Cryopumps reduce chamber pressure by condensing gas at low temperature  Process: Cryocondensation, Cryosorption, Cryotrapping  Six large cryopumps: Pumped Divertor (PD) 2x, NIB4, NIB8 and LH  All cryo-pumps except PDs can be sealed off from torus  Achievable vaccuum depends on temperature of trapping panels  Three temperature states: (1) Warm, (2) LN 2 (~77K), (3) LHe (~4.7K)  LN 2 : absorbs water vapour and some CO  LHe (supercritical): pumps D 2 and Hydrocarbons  Ar-frosting: pumps He by cryotrapping  Cryopumps have a limited capacity and must be regenerated regularly (risk of spontaneous regeneration for experiments with large gas loads!)

6. S. Jachmich (slide 6) Vessel Conditioning SL-Training, Nov 2010PD-cryopump  If regeneration is required by your programme: check machine configuration table, check with EIC/SL of previous and next session  Operation without LHe is possible, however:  density control more difficult  higher LH-threshold  landing of pulse has to be more careful  Symptoms of possible problems with PD:  slow pump down after pulse  impurity spikes during pump down  oscillations of torus base pressure between pulses  Status of PDs: (  Xmimic:  “vc/crs/oct15”,  Xpad:  cgrt/VC/slow/.../.../VC/C-PD1-HEO<TMP {He-temperature of PD1})  Inventory of PDs (JOI7.5): (  Xmimic:  “vc/inv/inventory)

7. S. Jachmich (slide 7) Vessel Conditioning SL-Training, Nov 2010Baking  Increases outgasing rate of impurities  Increase GDC-effectiveness  Faster recovery from discharges  Improves density control and pulse termination  At JET: thermal expansion of vessel necessary to free from MVP packing blocks  Operation temperature typically 200 o C  Baking temperature: 320 o C, dT/dt ~ +/- 10 o C  High baking temperature increases outgassing and diffusion

8. S. Jachmich (slide 8) Vessel Conditioning SL-Training, Nov 2010 Glow discharge cleaning (GDC)  Helps to release impurities from wall materials  Four electrodes in Octants 2, 4, 6, and 8  Working gases: D 2, He at 10 -2 mbar  PD has to be warmed up to LN 2  Ions accelerated to the walls of the vessel  Two cleaning processes: (1) direct chemical reactions, (2) ion induced desoprtion  Removed products are pumped out of the vessel  Fraction of the working gas will implanted into the wall => gas will be released into vessels  Allow for outgassing after GDC

9. S. Jachmich (slide 9) Vessel Conditioning SL-Training, Nov 2010 Deuterium or Helium Glow?  Hydrogen (H2, D2) GDC is primarily reactive: Released impurities: H 2 O, CO, CH x, CD x (e.g. Methane)  Large quantities of hydrogen can get stored in the wall and released during pulses => difficult density control  Helium GDC works mainly by ion induced desoprtion: Released impurities: H 2 O, CO, CO 2, H 2, D 2  Possible plasma contamination following a He-glow  Deuterium GDC is often followed by a Helium GDC  Needed before or after your experiment: obtain JPEC/Coord-approval + raise paperwork  If required after unplanned events (disruption): check with CoordCM, Vacuum, Cryo

10. S. Jachmich (slide 10) Vessel Conditioning SL-Training, Nov 2010Be-evaporation  Berylium is an oxygen getter, forms a stable oxide => reduction of Oxygen in plasma  Does not form stable compounds with deuterium => reduction of Deuterium wall loading  Four evaporator heads in Oct. 1,3,5,7  Typically 2 heads for 2 hrs (incl. heat up to 900 o C)  Good vacuum conditions for Be-evaporation required (low H 2 O and N 2 part. press.) (JOI 7.1  Xmimic:  “vc/codas/sys )  Needed before or after your experiment: obtain JPEC/Coord-approval + raise paperwork

11. S. Jachmich (slide 11) Vessel Conditioning SL-Training, Nov 2010 Residual gas analysis (RGA)  Quadrupole mass spectrometers installed in pumping chamber  Primarily to identify air or water leaks and to assess oxygen removal rates of D 2 -GDC  Complicated cracking pattern: List of masses for molecules  POG  Handbook  RGA-list   Xpad:  local_vc/spectra/qs1/... (in [A]); To calibrate: p torus / ∑(largest peaks) (usually masses 2-4)  Time trace for mass YY: cgrt/VC/slow/.../VC/MS1-TREND<MPX:YY  If peak mass 14 (N x ) and mass 16 (O x ) are similar then probably air leak

12. S. Jachmich (slide 12) Vessel Conditioning SL-Training, Nov 2010 Vessel condition for operation In the morning at start of operational day:  Assess torus condition  Torus pressure <3*10 -6 mbar (  Xpad:  open “EIC/cgrt-pennings-today”) (  Xmimic:  “vc/codas/sys”)  Vessel condition is categorized by partial pressure of water, Carbonoxides, Nitrogen  Residual gas analyser, RGA:  Xpad:  local_vc/spectra/qs1/...  Refer to JOI 7.2 for details

13. S. Jachmich (slide 13) Vessel Conditioning SL-Training, Nov 2010 Vessel deconditioning  Some experiments implicate deconditioning of the machine (e.g. disruption studies, impurity seeding, runaways etc.)  Check JOI 1.3 and agree on re-conditioning procedure using form in appendix  Use recovery pulse to get back in operation if struggeling with breakdown  Cleaning pulses:  in principle plasma conditioning mostly effective using long pulses with high ion flux and energy  sweep over relevant limiter and divertor areas  Guidance note on conditioning procedure:  POG  News&Notes