ASIPP

1. 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

2. ASIPP In-time retention evaluation by particle balance analysis on HT-7 Outline • Particle balance method for retention evaluation in HT-7 • System error of retention • Retention evaluation on HT-7 • H/D inventory in HT-7 • Conclusions

3. ASIPP In-time retention evaluation by particle balance analysis on HT-7 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 2004. Working gases: commonly D2, He for a short period. Conditioning : D2 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.

4. ASIPP In-time retention evaluation by particle balance analysis on HT-7 Main error sources of particle balance method For Vtank, volume of fueling tank, error could be limited lower than 3% (including that from the Gas Injection System). For Ptank, pressure of tank, error could be limited lower than <7%. Error of Qpuff could be limited lower than 10%. For Pvv, 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 Qextract could be limited lower than 35%.

5. ASIPP In-time retention evaluation by particle balance analysis on HT-7 Other error sources of particle balance method (I) • Pressure distribution depends on pumping & puffing position, basically uniform when without plasma & @higher pressure (>1e-3Pa) within 300ms. Magenta: during discharge; Blue: after discharge. Shot 78800, puff from Loc5, pump from Loc3.

6. ASIPP In-time retention evaluation by particle balance analysis on HT-7 Other error sources of particle balance method (II) • Gas type QMS shows for pure D2, P2/P4~3% (right upper plot), similar to P1/P2 (~2%) for pure H2. Thus assume P2,P3,P4 represents H2,HD,D2 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. • Response time GIS puffs gas into vacuum vessel in tens of ms and distributes evenly in <300 ms. For long pulses, Qextract happens mainly within a few to 10 seconds after plasma termination. QMS samples every 1s, while gauge responses every tens to hundreds of ms.

7. ASIPP In-time retention evaluation by particle balance analysis on HT-7 Brief summary on error sources Retention ratio evaluation with particle balance method could be limited lower than 50% valueafter 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. Error of Qpuff could be limited lower than 7% (from DAQ). Error of Qextract 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.

8. ASIPP In-time retention evaluation by particle balance analysis on HT-7 Retention evaluation on HT-7 (I) General Particle balance shows that about 60% of the fuelled gas is retained relatively permanently inside the chamber. With 3 TMP, pumping speed=843l/s. From QMS, H2/D2=2:3. Conversion factor of D2 for Pvv=2.4. Qpuff=342Pal ~9.2E19 molecules. Qextract=110Pal ~3.0E19 molecules. retention=68%±16%

9. ASIPP In-time retention evaluation by particle balance analysis on HT-7 Retention evaluation on HT-7 (II) Pulse duration 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.

10. ASIPP In-time retention evaluation by particle balance analysis on HT-7 Retention evaluation on HT-7 (III) Pumping speed In HT-7, effective pumping speed is very low during the discharge. Pumping speed effect on D retention: not distinguishable.

11. ASIPP In-time retention evaluation by particle balance analysis on HT-7 Retention evaluation on HT-7 (III) Disruption effect Disruption effect on D retention: disruption favors less retention.

12. ASIPP In-time retention evaluation by particle balance analysis on HT-7 D inventory in HT-7 inner vacuum vessel 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. Brown, before discharge; Red, during discharge; Blue, after discharge.

13. ASIPP In-time retention evaluation by particle balance analysis on HT-7 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 78178-78218 (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

14. ASIPP In-time retention evaluation by particle balance analysis on HT-7 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 Twall rise in some areas, and suppress retention.

15. ASIPP In-time retention evaluation by particle balance analysis on HT-7 Conclusion • Particle balance shows that about 60% of the fuelled gas is retained relatively permanently. • For relative evaluation error could be at 20%, providing a practical tool for retention study. • More retention happens in longer pulse. • Pumping speed has negligible effect on D retention. • Disruption helps to decrease D retention. • Recycled H ranges from 10% to 80% of the released gas after plasma termination,depending on the wall condition.