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Vessel conditioning

Vessel conditioning. Stefan Jachmich SL-Training 2010. Outline. Glow discharge cleaning Beryllium evaporation Residual gas analysis. Vacuum condition Torus pumping Vessel baking. Vessel conditioning.

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Vessel conditioning

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  1. Vessel conditioning Stefan Jachmich SL-Training 2010

  2. Outline • Glow discharge cleaning • Beryllium evaporation • Residual gas analysis • Vacuum condition • Torus pumping • Vessel baking

  3. 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. 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. Cryopumps • 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) LN2 (~77K), (3) LHe (~4.7K) • LN2 : absorbs water vapour and some CO • LHe (supercritical): pumps D2 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. PD-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. Baking • 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 200oC • Baking temperature: 320oC, dT/dt ~ +/- 10oC • High baking temperature increases outgassing and diffusion

  8. Glow discharge cleaning (GDC) • Helps to release impurities from wall materials • Four electrodes in Octants 2, 4, 6, and 8 • Working gases: D2, He at 10-2 mbar • PD has to be warmed up to LN2 • 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. Deuterium or Helium Glow? • Hydrogen (H2, D2) GDC is primarily reactive: Released impurities: H2O, CO, CHx, CDx (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: H2O, CO, CO2, H2, D2 • 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. Be-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 900oC) • Good vacuum conditions for Be-evaporation required (low H2O and N2 part. press.) (JOI 7.1 Xmimic: “vc/codas/sys ) • Needed before or after your experiment: obtain JPEC/Coord-approval + raise paperwork

  11. 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 D2-GDC • Complicated cracking pattern: List of masses for molecules POG  Handbook RGA-list • Xpad:  local_vc/spectra/qs1/... (in [A]); To calibrate: ptorus / ∑(largest peaks) (usually masses 2-4) • Time trace for mass YY: cgrt/VC/slow/.../VC/MS1-TREND<MPX:YY • If peak mass 14 (Nx) and mass 16 (Ox) are similar then probably air leak

  12. 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. 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

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