1 Differential Residual Gas Analysis in the HT-7 Tokamak J.S Hu and HT-7 vacuum group Institute of Plasma Physics, Chinese Academic of Science, Hefei, Anhui , P.R.China th General Scientific Assembly of Asia Plasma & Fusion Association On New Development of Plasma Physics and Fusion Technology Hangzhou, China Oct 13 – 16, 2003 HT-7 RGA ASIPP

2 Abstract(1) This paper mainly discussed residual gas analysis during plasma discharge in HT spring campaign. The differential RGA mainly measured hydrogen (isotopes) and H 2 O, D 2 O, CO, CO 2, CH 3, O 2, which were main impurities in carbon limiter HT-7 device during plasma discharge. APFA 2003 HT-7 RGA

3 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. APFA 2003 HT-7 RGA

4 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 D 2 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 cleaning Conclusions ASIPP APFA 2003 HT-7 RGA

5 ASIPP 1.1 Introduce HT-7 vacuum system Two turbo molecular pumping stations; 2.Two cryo-pumps (pumping speed 10 4 l/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. APFA 2003 HT-7 RGA

6 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/m 2 heat loads for 100s with active cooling 200  m SiC gradient coatings Water cooled poloidal and toroidal belt graphite limiters Water cooled poloidal graphite limiter Water cooling system of the toroidal belt limiter ASIPP HT-7 RGA

7 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 100 o C 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 -7 Pa after baking. 5.After connection with main chamber, the ultimate pressure was 1.0×10 -6 Pa as the pressure in main chamber was 2.0×10 -5 Pa and bellows tube was baked to 100 o C. ASIPP APFA 2003 HT-7 RGA

8 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. ASIPP APFA 2003 HT-7 RGA

9 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 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 I P =140~160KA, ne≈1×10 19 /m 3, t=1~2s, during shot to and from to 56647, before the first boronization in this campaign the average value of the ratio is about ASIPP APFA 2003 HT-7 RGA

10 The ratio of H/(H+D) during plasma discharge 1. After boronization with C 2 B 10 H At early, the ratio was near 1. 3.Then the ratio decreased slowly 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. ASIPP APFA 2003 HT-7 RGA

Neutral D 2 behavior during long plasma discharge 1.At initial fueling, P D2 was high, which existed as molecules. 2.Goes down very fast at plasma startup. 3.Almost stable and only little higher than the background 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. ASIPP APFA 2003 HT-7 RGA

12 Typical time evolution of the deuterium partial pressure in the differential chamber during plasma discharge 1.The line averaged electrons density was 5×10 18 /m 3 in shot ×10 -5 Pa 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 -5 Pa three seconds later. ASIPP In some special shots, the pressure increased during whole plasma discharge! APFA 2003 HT-7 RGA

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. ASIPP APFA 2003 HT-7 RGA

14 As an example, the partial pressure of m/q=28 impurities, mainly CO was chosen in a 47.43s low plasma density (ne=1×10 19 /m 3 ) discharge (56781) Increased from 3×10 -8 Pa to 5×10 -8 Pa at the plasma startup, Stable at 4.5×10 -8 Pa for about 20s, Later grows up slowly to 7×10 -8 Pa before plasma termination, due to the thermal chemical erosion according to graphite limiter temperature. After plasma discharge termination, to 4.6×10 -7 Pa. 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. ASIPP APFA 2003 HT-7 RGA

15 Partial pressures of several impurities during the plasma discharge of shot for duration of 41.80s with low plasma density (ne=3×10 18 /m 3 ). 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 -7 Pa to a maximum value of 1.0× Pa in the first 25s. 3.Then it went down to a stabilized value of 8×10 -7 Pa. 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. ASIPP APFA 2003 HT-7 RGA

16 partial pressures of several impurities during the plasma discharge of shot for duration of 41.80s with low plasma density (ne=3×10 18 /m 3 ). The power of LHCD was 166KW. Special behavior of m/q=44 Increased higher than other particles at startup plasma, from 3×10 -8 Pa to 2.5× Pa, 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. ASIPP The partial pressure of impurities is corresponding to the thermal chemical erosion according graphite limiter temperature, which will be discussed in other paper. APFA 2003 HT-7 RGA

17 After the plasma termination The 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 CO 2 were formed. The partial pressure of CH 3 increased little. But the partial pressure of O 2 went down to background very fast. ASIPP 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 CO 2, so carbon-oxygen impurities partial pressure increased large at same time the oxygen pressure go down after plasma termination. APFA 2003 HT-7 RGA

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. ASIPP APFA 2003 HT-7 RGA

19 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 to shot Shot and plasma discharge happened disruption ASIPP APFA 2003 HT-7 RGA

20 Before the discharges with disruption, background at very low, 1×10 -8 Pa. After shot 58067, the maximum partial pressure after discharge termination increased to 1×10 -7 Pa and fast went down. The background increased to 3×10 -8 Pa. After the second shot discharge with disruption, 58068, the maximum partial pressure increased to 2×10 -6 Pa. During the plasma discharge, the pressure increased from 3×10 -8 Pa to 1×10 -7 Pa. 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 and 58070, the maximum pressure increased to about 2×10 -6 Pa. 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. ASIPP APFA 2003 HT-7 RGA

21 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 APFA 2003 HT-7 RGA

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23 Thank you! ASIPP APFA 2003 HT-7 RGA