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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-7 ASIPPASIPP

1. Particle balance method for retention evaluation in HT-7

2. System error of retention

3. Retention evaluation on HT-7

4. H/D inventory in HT-7

5. Conclusions

Outline

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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.

SdtPQ

VPQ

QQQ

vvextract

tanktankpuff

retentionextractpuffl

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.

In-time retention evaluation by particle balance analysis on HT-7

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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%.

tanktankpuff VPQ

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%.

SdtPQ vvextract

In-time retention evaluation by particle balance analysis on HT-7

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•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.

In-time retention evaluation by particle balance analysis on HT-7

Other error sources of particle balance method (I)

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•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.

In-time retention evaluation by particle balance analysis on HT-7

Other error sources of particle balance method (II)

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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.

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.

Brief summary on error sources

In-time retention evaluation by particle balance analysis on HT-7

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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%

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.

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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

1stBoronization

81338(70s/2.6E20/80%)

2ndBoronization

83000(300s/5.9E20/88%)83026(300s/5.8E20/76%)

3rdboronization

84247(10s/1.2E20/46%)

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.

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In HT-7, effective pumping speed is very low during the discharge.

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 2/3 342 103 68% 16%

In-time retention evaluation by particle balance analysis on HT-7

Retention evaluation on HT-7 (III) Pumping speed

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•Disruption effect on D retention: disruption favors less retention.

S.N. SD[l/s] H2/D2 Retent’ Disrupt’

79152 843 1/2 89% - 3%

79158 843 1/2 77% + 5%

79164 843 1/2 62% + 8%

In-time retention evaluation by particle balance analysis on HT-7

Retention evaluation on HT-7 (III) Disruption effect

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D inventory in HT-7 inner vacuum vessel

1. 0E-04

1. 0E-03

1. 0E-02

1. 0E-011

2

34

5

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

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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

In-time retention evaluation by particle balance analysis on HT-7

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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.

In-time retention evaluation by particle balance analysis on HT-7

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1. Particle balance shows that about 60% of the fuelled gas is retained relatively permanently.

2. For relative evaluation error could be at 20%, providing a practical tool for retention study.

3. More retention happens in longer pulse.

4. Pumping speed has negligible effect on D retention.

5. Disruption helps to decrease D retention.

6. Recycled H ranges from 10% to 80% of the released gas after plasma termination, depending on the wall condition.

Conclusion

In-time retention evaluation by particle balance analysis on HT-7