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Differential Residual Gas Analysis in the HT-7 Tokamak

J.S Hu and HT-7 vacuum groupJ.S Hu and HT-7 vacuum groupInstitute of Plasma Physics, Chinese Academic of Science, Hefei, Anhui 2300Institute of Plasma Physics, Chinese Academic of Science, Hefei, Anhui 2300

31, P.R.China31, P.R.China E-mailE-mail:: HuJS@ipp.ac.cn

44thth General Scientific Assembly General Scientific Assembly of Asia Plasma & Fusion Association On of Asia Plasma & Fusion Association On

New Development of Plasma Physics and Fusion TechnologyNew Development of Plasma Physics and Fusion TechnologyHangzhou, China Oct 13 –16, 2003 Hangzhou, China Oct 13 –16, 2003

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Abstract(1)

This paper mainly discussed residual gas analysis during plasma discharge in HT-7 2003 spring campaign. The differential RGA mainly measured hydrogen (isotopes) and H2O, D2O, CO, CO2, CH3, O2, which were main impurities in carbon limiter HT-7 device during plasma discharge.

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Abstract (2)

1) The ratio of hydrogen to deuterium in deuterium plasma discharge were calculated with RGA data before and after boronization respectively.

2) The partial pressure of m/q=28 were tracked in the differential chamber in shots close to the discharges with disruption.

3) During long pulse plasma discharges, the impurities and hydrogen (isotopes) partial pressure were scanned in real time in the differential chamber.

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Index • Introduction • 1.1 Introduce HT-7 vacuum system• 1.2 Actively cooling Graphite limiter• 1.3 Introduce HT-7 differential RGA system

• RGA analysis• 2.1 The ratio of H/(H+D) during plasma discharge• 2.2 Neutral D2 behavior during long plasma discharge• 2.3 Neutral impurities and hydrogen behavior during long plasma discharge • 2.4 Impurities caused by plasma with disruption &Plasma discharge cleanin

g • Conclusions

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1.1 Introduce HT-7 vacuum system1.1 Introduce HT-7 vacuum system

1.1. Two turbo molecular pumping stations;

2. Two cryo-pumps (pumping speed 104l/s);

3. The main chamber volume is about 4800L.

4. The ultimate pressure of the device is lower than 1.0×10-5 Pa.

5. The vacuum pressure was kept at 1.0~2×10-5Pa between plasma shots.

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1.2 Actively cooling Graphite limiter

GBST1308 (1%B, 2.5%Si, 7.5%Ti) doped graphite with thermal conductivity up to 180 W/m.K

• 6MW/m2 heat loads for 100s with active cooling• 200m SiC gradient coatings

Water cooled poloidal and toroidal belt graphite limiters

Water cooled poloidal graphite limiterWater cooling system of the toroidal belt limiter

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1.3 Introduce HT-7 differential RGA system 1.3 Introduce HT-7 differential RGA system

1. Composition: One turbo pump system, one gauge and one Quadruple Mass Spectrometer (RGA200). Nominal pumping speed 600l/s.

2. Connected to the main HT-7 chamber: by a 1m long bellows tube of 40mm diameter.

3. It was baked to 100oC during whole HT-7 campaign to prevent the wall from absorbing the transition particles.

4. The ultimate pressure of the differential chamber was about 2.0×10-7Pa after baking.

5. After connection with main chamber, the ultimate pressure was 1.0×10-6Pa as the pressure in main chamber was 2.0×10-5Pa and bellows tube was baked to 100oC.

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2.1 The ratio of H/(H+D) during plasma discharge

1. After plasma discharge termination the pressure of deuterium and hydrogen increased very fast because the ionized D or H was recombined to molecules.

2. The ratio of H/(H+D) in filling deuterium plasma discharge was measured according to the their increased pressure after the discharge termination.

3. Gas accumulation:

4. Average value. PSdtQ

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The ratio of H/(H+D) during plasma discharge

1. In first day in 2003 HT-7 spring campaign, at the first 30 shots the ratio of H/(H+D) is near 0.5.

2. Then with increase plasma density, the ratio became unstable.

3. As the campaign advanced, the ratio H/(H+D) reached stable with almost same plasma parameters.

4. During plasma discharge with main parameters as IP=140~160KA, ne≈1×1019/m3 , t=1~2s, during shot 56168 to 56194 and from 56621 to 56647, before the first boronization in this campaign the average value of the ratio is about 0.15.

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The ratio of H/(H+D) during plasma discharge

1. After boronization with C2

B10H14.2.At early, the ratio was nea

r 1. 3.Then the ratio decreased sl

owly and reaches 0.5 around 40 shots

4.The ratio decreased approx linear.

5. The ratio change reflects that boron film characteristics and edge recycling changed.

The ratio of H/(H+D) curves with discharges after 1# and 4# boronization.

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2.2 Neutral D2 behavior during long plasma discharge

1. At initial fueling, PD2 was high, which existed as molecules.

2. Goes down very fast at plasma startup. 3. Almost stable and only little higher than the backgroun

d during whole plasma discharge. Nearly steady at the edge plasma by PSI, which is beneficial for long pulse discharge.

4. Increased very much by recombination of D+ ions with electrons after termination of plasma.

5. The increased partial pressure reached a maximum two or three seconds later, and then decreased in relation to the pumping speed .

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Typical time evolution of the deuterium partial pressure in the differential chamber during plasma discharge

1. The line averaged electrons density was 5×1018/m3 in shot 56766.

2. 1.5×10-5Pa at initial fueling. Then it went down quickly to about 1.1×10-7 Pa at startup plasma, little higher than its background 5×10-8 Pa.

3. stable during whole plasma discharge.

4. After the discharge termination, the pressure increased quickly to maximum value 1×10-5Pa three seconds later.

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In some special shots, the pressure increased during whole plasma discharge!APFA 2003

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2.3 Neutral impurities and hydrogen behavior during plasma discharge

1. During long pulse plasma discharges, the impurities were monitored in real time by RGA in the leak-detected mode(16.5ms/point).

2. Due to the boron film on the first surface wall, hydrogen partial pressure in the differential chamber was about 10 times larger than other impurities.

3. The impurities have similar behaviors as deuterium during the plasma discharges.

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As an example, the partial pressure of m/q=28 impurities, mainly CO was chosen in a 47.43s low plasma density (ne=1×1019/m3) discharge (56781)

• Increased from 3×10-8Pa to 5×10-8Pa at the plasma startup,

• Stable at 4.5×10-8Pa for about 20s, • Later grows up slowly to 7×10-8Pa before p

lasma termination, due to the thermal chemical erosion according to graphite limiter temperature.

• After plasma discharge termination, to 4.6×10-7Pa.

• LHCD power :150KW; Plasma current: 61KA•The intensities of CIII and OII emission were also increased slowly after 20s, which was similar to RGA data. •The intensity of CIII impurity emission increased to high level before the discharge termination, and the discharge terminated quickly.

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Partial pressures of several impurities during the plasma discharge of shot 56775 for duration of 41.80s with low plasma density (ne=3×1018/m3). The power of LHCD was 166KW.

1. The hydrogen behavior was different to deuterium. During plasma pulse,

2. Its partial pressure increased from 4×10-7Pa to a maximum value of 1.0×10-

6Pa in the first 25s. 3. Then it went down to a

stabilized value of 8×10-7Pa.

After deuterium breakdown, D+ impacts with graphite limiter and wall surface. Abundant H accumulated at surface or combined in boron film was sputtered. Hydrogen trapped in carbon gives also contribution. By those processes, its partial pressure increased more than other impurities.

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partial pressures of several impurities during the plasma discharge of shot 56775 for duration of 41.80s with low plasma density (ne=3×1018/m3). The power of LHCD was 166KW.

Special behavior of m/q=44 Increased higher than other particles at startup plasma, from 3×10-8Pa to 2.5×10-7Pa, and sustained a few seconds. And it stabilized in large party of plasma pulse. About 10 seconds before plasma termination, it became unstable and had a little increasing.

•The deuterium and m/q=28 behavior in fig5 is similar to the fig2 and fig3 during plasma discharge. •Also the m/q=15,32 is almost same to m/q=28.

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The partial pressure of impurities is corresponding to the thermal chemical erosion according graphite limiter temperature, which will be discussed in other paper.

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After the plasma terminationThe partial pressure of m/q=2 is higher than that of m/q=4, which means that the boron film has great contribution for hydrogen outgassing. Abundant CO and CO2 were formed. The partial pressure of CH3 increased little. But the partial pressure of O2 went down to background very fast.

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Recombined of H, O and C is a complex process. At plasma termination a lots of high energy particles impact with high temperature graphite limiter and abundant C was erosion and ionized. C+ combined with oxygen to form CO and CO2, so carbon-oxygen impurities partial pressure increased large at same time the oxygen pressure go down after plasma termination.

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2.4 Impurities caused by plasma with disruption &Plasma discharge cleaning

After plasma discharge with disruption, PFCs are rapidly heated during plasma instabilities by direct impact of energetic plasma particles and radiation. The deposited energy flux may be high enough to melt and vaporize the surface materials. Abundant impurities were formed by combining each other and were released from wall surface.

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1. the partial pressure in a few shots near the discharges with disruption without LHCD heating.

2. The plasma currents were stable at 150KA.

3. The durations were about 750ms.

4. Densities were little different. 5. Scanned from shot 58060 to

shot 58083. 6. Shot 58067 and 58068 plasma

discharge happened disruption

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Before the discharges with disruption, background at very low, 1×10-8Pa.

After shot 58067, the maximum partial pressure after discharge termination increased to 1×10-7Pa and fast went down. The background increased to 3×10-8Pa. After the second shot discharge with disruption, 58068, the maximum partial pressure increased to 2×10-6Pa. During the plasma discharge, the pressure increased from 3×10-8Pa to 1×10-7Pa.

The impurities produced in discharges with disruption would be weakly adhered to wall and influenced the later a few discharges. As shown in fig6, after shot 58069 and 58070, the maximum pressure increased to about 2×10-6Pa. But later in shot 56072, the pressure after termination was not change and after about 10 shots later the background come back to 1×10-8, which means that good cleaning were made

by a few plasma discharges.

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1. In 2003 HT-7 spring campaign, the ratio of H/(H+D) during plasma discharges was calculated with RGA data.

2. And the ratio changing after boronizations was compared, which is useful to know wall condition and boron film characteristics changing.

3. Neutral impurities and hydrogen behavior at edge plasma were detected by QMS. Together with other plasma diagnostics, it might give useful information for understanding the interaction of plasma and materials.

4. Impurities produced in plasma discharges with disruption were stronger than normal discharges, and influenced the impurity level in later discharges. It induced the background increasing. But those impurities were weakly adhered to wall and easy to remove by a few plasma discharges.

5. Therefore the differential system needs to be updated in next campaign. Some topics should be studied deeply.

3 conclusions ASIPP

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[1] Poschenrieder, W., Venus, G., ASDEX Team, J.Nucl. Mater.111&112(1982) 29[2] G.Federici, J.Nucl.Fusion,VOL.41.(2001)1968[3] ,H.f.Dalla,J.Nucl.mater,93-94,61(1980)[4] W.Poschenrieders, G.Venus, and ASDEX team, J.Nucl.mater.111-112,29(1982) [5] Y.Nakashima, et al, J.Vac.Sci.Technol. A13(5), Sep/Oct 1995;[6] W.Poschenrieders, J.Vac.Sci.Technol. A5, 2265(1987);[7]V.Pilipps,et al, J.Nucl. Mater.162&164(1989) 520[8]B.N Wan et al,. J.Nucl. Mater. 313-316(2003)127[9] J.Li, et al,. J.Nucl. Mater. 313-316(2003)1188.[10]B.N Wan et al,. Plasma Science and Technologr. Vol4, NO.4 (2003) 1375.[11]Horn, A.,et al., Chem. Phys.Lett. 231(1994)193

Reference

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Thank you!

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