ASIPP In-time retention evaluation by particle balance analysis on HT-7 Y. YANG*, and HT-7 team Institute of Plasma Physics, Chinese Academy of Sciences 2006

In-time retention evaluation by particle balance analysis on HT-7ASIPP 1.Particle balance method for retention evaluation in HT-7 2.System error of retention 3.D inventory in HT-7 inner vacuum vessel 4.Conclusions Outline

ASIPP Particle balance equation for retention evaluation Wall retention is a critical topic for ITER. The long pulses of HT-7 provide good opportunity for the study. Particle balance equation is utilized for retention evaluation since Working gases: commonly D 2, He for a short period. Conditioning : D 2 and He during the experimental ran. Pumping: 4 cryo-pumps and 4 TMP station. Vacuum Diagnostics: Six ion gauges for vacuum vessel; One diaphragm gauge for fueling tank; One QMS RGA analyzer. [i][i] Y. Yang, 16 th PSI conference, Portland, USA, (2004) In-time retention evaluation by particle balance analysis on HT-7

ASIPP Main error sources of particle balance method For V tank, volume of fueling tank, error could be limited lower than 3% (including that from the Gas Injection System). For P tank, pressure of tank, error could be limited lower than <7%. Error of Q puff could be limited lower than 10%. For P vv, pressure of vacuum vessel, error could be <15% after calibration with pure gases. For S, pumping speed, which is obtained by measuring pumping quantity and pressure evolution, error could be suppressed <20%. Error of Q extract could be limited lower than 35%. In-time retention evaluation by particle balance analysis on HT-7

ASIPP Other potential errors: Pressure distribution (influence P vv, S) Gas type (influence P vv ) Response time* (influence P vv, S, P tank ) Error of absolute retention evaluation: Retention ratio evaluation with particle balance method could be limited lower than 50% value after careful design of Gas Injection System and regular calibration of gauges on HT-7. It’s extremely difficult to suppress error low than 40% value. * for GIS tens of ms, for pumping  1s, for gauge tens to hundreds of ms. In-time retention evaluation by particle balance analysis on HT-7

ASIPP Take shot as an example: Q puff =149*2.3=342Pal. With 3 TMP, pumping speed=843l/s. From QMS, H 2 /D 2 =2:3. Conversion factor of D2 for P vv measurement=2.4. So, Q extract =110Pal  retention =68%±16% In-time retention evaluation by particle balance analysis on HT-7

ASIPP Main error sources of relative evaluation (I) Pressure distribution depends on pumping & puffing position, basically uniform when without plasma pressure (>1e-3Pa) within 300ms. Effect on P vv, previously ~10%, 0 with the new multi-port P monitoring system. Effect on S, same as P vv. Magenta: during discharge; Blue: after discharge. Shot 78800, puff from Loc5, pump from Loc3. In-time retention evaluation by particle balance analysis on HT-7

ASIPP Main error sources of relative evaluation (II) Gas type QMS shows for pure D 2, P2/P4~3% (right upper plot), similar to P1/P2 (~2%) for pure H 2. Thus assume P2,P3,P4 represents H 2,HD,D 2 respectively, and bearing the same partial pressure sensitivity factor. A typical QMS plot is shown (right lower), illustrating that basically H isotopes occupy more than 95% of the residual gas. Effect on P vv, ~10%. Response time GIS puffs gas into vacuum vessel in tens of ms and distributes evenly in <300 ms. For long pulses, Q extract happens mainly within a few to 10 seconds after plasma termination. QMS samples every 1s, while gauge responses every tens to hundreds of ms. In-time retention evaluation by particle balance analysis on HT-7

ASIPP Error of Q puff could be limited lower than 7% (from DAQ). Error of Q extract could be lower than 10% (from QMS) Thus, retention could be compared relatively with the error of <20%. The evaluation is suited for long pulse discharges, which generate big pressure variation and provide long enough time for Residual Gas Analysis. Main error sources of relative evaluation (III) In-time retention evaluation by particle balance analysis on HT-7

ASIPP In HT-7, effective pumping speed is very low during the discharge. Particle balance shows that about 60% of the fuelled gas is retained relatively permanently inside the chamber. Longer pulse tends to cause higher retention quantity. The majority of the dynamic inventory is released and pumped within a couple of seconds after the pulse termination. Nov28Dec04Dec12Dec14Dec17Later 1st Boronization (70s/2.6E20 /80%) 2nd Boronization (300s/5.9E20 /88%) (300s/5.8E20 /76%) 3rd boronization (10s/1.2E20 /46%) In-time retention evaluation by particle balance analysis on HT-7

ASIPP Pumping speed effect on D retention: not distinguishable. Inventory comparison by relative evaluation S.N.S D [l/s]H 2 /D 2 Q puff [Pal/s] Q extract [Pal/s] retentionerror / %16% / %16% Disruption effect on D retention: disruption favors less retention. S.N.S D [l/s]H2/D2retentionerror /289%3% /277%5% /262%8% In-time retention evaluation by particle balance analysis on HT-7

ASIPP D inventory in HT-7 inner vacuum vessel Brown, before discharge; Red, during discharge; Blue, after discharge. All the gauges in the inner vacuum vessel show that pressure drops soon after the plasma is formed, keeps relatively steady in a very low value, and rises quickly to a very high value before decaying gradually. No position inside the chamber is observed to confine large amount of neutral particles during the discharge. In-time retention evaluation by particle balance analysis on HT-7

ASIPP H inventory in HT-7 inner vacuum vessel QMS shows that hydrogen in the released gas could be after discharge as high as 50% (even higher after boronization). QMS (He plasma) By courtesy of M. SU Large amount of H release during the discharges. H/(H+D) ratio evolution By courtesy of J. HUANG In-time retention evaluation by particle balance analysis on HT-7

ASIPP Possible mechanism D is trapped after being puffed into the chamber. When without plasma, it desorbed relatively easier; while with plasma, it’s trapped more firmly. The isotopic exchange leads to the release of H from the bores in graphite tiles. Effective pumping speed is very low during the discharge. Disruption could cause T wall rise in some areas, and suppress retention. In-time retention evaluation by particle balance analysis on HT-7

ASIPP 1.After careful design and calibration, error could be 50% for quantitative evaluation of D retention. For relative evaluation error could be suppressed fewer than 20%, providing a practical tool for retention study. 2.Particle balance shows that about 60% of the fuelled gas is retained relatively permanently. More retention happens in longer pulse. 3.Recycled H ranges from 10% to 80% of the released gas after plasma termination, depending on the wall condition. 4.Pumping speed has negligible effect on D retention. 5.Disruption helps to decrease D retention. Conclusion In-time retention evaluation by particle balance analysis on HT-7