s. jachmich (slide 1) vessel conditioning sl-training, nov 2010 vessel conditioning stefan jachmich...

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

Vessel conditioning

Stefan Jachmich

SL-Training 2010


OutlineOutlineVacuum condition

Torus pumping

Vessel baking

Glow discharge cleaning

Beryllium evaporation

Residual gas analysis


Vessel conditioningVessel conditioning

Good quality of vaccuum and surface condition are essential for successful repeatable plasma operation

Sequence to recondition vessel after shutdown:Pump-down of vesselVessel bakingGlow Discharge CleaningBe-evaporationPlasma conditioning


Pumping systemPumping 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)


CryopumpsCryopumps 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!)


PD-cryopumpPD-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)


BakingBaking

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


Glow discharge cleaning (GDC)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


Deuterium or Helium Glow?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


Be-evaporationBe-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


0.0E+00

2.0E-08

4.0E-08

6.0E-08

0 5 10 15 20 25 30 35 40 45 50Mass [AMU]

pa

rtia

l p

res

su

re [

mb

ar]

before Be-evap

after Be-evap, before ops

Residual gas analysis (RGA)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

0.0E+00

1.0E-09

2.0E-09

14 19 24 29 34 39 44 49

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:YYIf peak mass 14 (Nx) and mass 16 (Ox) are similar then probably air leak


Vessel condition for operationVessel 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


Vessel deconditioningVessel 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

s. jachmich (slide 1) vessel conditioning sl-training, nov 2010 vessel conditioning stefan jachmich...

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