1. In-time retention evaluation by particle balance analysis on HT-7
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. Outline Particle balance method for retention evaluation in HT-7
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
D inventory in HT-7 inner vacuum vessel
Conclusions

3. ASIPP Particle balance equation for retention evaluation
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.
[i] Y. Yang, 16th PSI conference, Portland, USA, (2004)

4. ASIPP Main error sources of particle balance method
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 Other potential errors: Pressure distribution (influence Pvv, S)
In-time retention evaluation by particle balance analysis on HT-7
Other potential errors:
Pressure distribution (influence Pvv, S)
Gas type (influence Pvv)
Response time* (influence Pvv, S, Ptank)
* for GIS tens of ms, for pumping 1s, for gauge tens to hundreds of ms.
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.

6. ASIPP Take shot 78467 as an example: Qpuff=149*2.3=342Pal.
In-time retention evaluation by particle balance analysis on HT-7
Take shot as an example:
Qpuff=149*2.3=342Pal.
With 3 TMP, pumping speed=843l/s.
From QMS, H2/D2=2:3.
Conversion factor of D2 for Pvv measurement=2.4.
So, Qextract=110Pal
retention=68%±16%

7. ASIPP Main error sources of relative evaluation (I)
In-time retention evaluation by particle balance analysis on HT-7
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 Pvv, previously ~10%, 0 with the new multi-port P monitoring system.
Effect on S, same as Pvv.
Magenta: during discharge;
Blue: after discharge.
Shot 78800, puff from Loc5, pump from Loc3.

8. ASIPP Main error sources of relative evaluation (II) Gas type
In-time retention evaluation by particle balance analysis on HT-7
Main error sources of relative evaluation (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.
Effect on Pvv, ~10%.
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.

9. ASIPP Main error sources of relative evaluation (III)
In-time retention evaluation by particle balance analysis on HT-7
Main error sources of relative evaluation (III)
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.

10. ASIPP
In-time retention evaluation by particle balance analysis on HT-7
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.
Nov28
Dec04
Dec12
Dec14
Dec17
Later
1st
Boronization
81338
(70s/2.6E20
/80%)
2nd
83000
(300s/5.9E20
/88%)
83026
(300s/5.8E20
/76%)
3rd
boronization
84247
(10s/1.2E20
/46%)

11. ASIPP Inventory comparison by relative evaluation
In-time retention evaluation by particle balance analysis on HT-7
Inventory comparison by relative evaluation
Pumping speed effect on D retention: not distinguishable.
S.N.
SD[l/s]
H2/D2
Qpuff [Pal/s]
Qextract [Pal/s]
retention
error
78466
369
2/3
321
110
68%
16%
78467
843
342
103
Disruption effect on D retention: disruption favors less retention.
S.N.
SD[l/s]
H2/D2
retention
error
79152
843
1/2
89% 3%
79158
77% 5%
79164
62% 8%

12. ASIPP D inventory in HT-7 inner vacuum vessel
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 H inventory in HT-7 inner vacuum vessel
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
(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 Possible mechanism
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
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.
Particle balance shows that about 60% of the fuelled gas is retained relatively permanently. More retention happens in longer pulse.
Recycled H ranges from 10% to 80% of the released gas after plasma termination, depending on the wall condition.
Pumping speed has negligible effect on D retention.
Disruption helps to decrease D retention.