1 HT-7/EAST ASIPP Oxidation wall conditionings on HT-7 and EAST superconducting tokamaks J.S Hu, J.G Li, Y.P Zhao EAST Team Institute of.

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1 HT-7/EAST ASIPP Oxidation wall conditionings on HT-7 and EAST superconducting tokamaks J.S Hu, J.G Li, Y.P Zhao EAST Team Institute of Plasma Physics, Chinese Academy of Sciences, Hefei, Anhui , China PSI-19, May 24-28, 2010, San Diego, California, USA

2 Contributors EAST team –X.M Wang, Y.W. Yu, G.Z. Zuo, J. H, Li, J.H. Wu, Y. Chen, X.M Gu, Z.W Wu, J Huang, S.Y Lin, L Wang, B.L Ling, B.Shen, Y.Yang, X.Z Gong, L Hu, J.Y Zhao, X.Gao, X.D Zhang, B.N Wan et al. Partially supported by JSPS -CAS core university program –NIFS, Kyushu Univ., Shizuoka Univ. Co-operations in ITPA-DSOL group Funded by the National Nature Science Foundation of China under Contract No HT-7/EAST ASIPP

3 HT-7/EAST ASIPP 1.Introduction 2.Comparison of varios oxidations in HT-7 Thermo-oxidation, O-GDC O-ICR Sample analysis summary of oxidations Characteristics of oxidation techniques 3.Comparison of oxidations between HT-7 and EAST 4.Oxygen removal and plasma recovery 5.Prediction for ITER 6.Summary Outline

4 HT-7/EAST 1. Introduction The long-term retention of tritium fuel in fusion devices is one of the major problems in fusion technology. The ability to remove tritium from amorphous tritiated carbon layers in ITER will have an important impact on the machine operation and safety. Oxidation conditioning is one of potential solutions. On hot walls in tokamaks for in-situ co-deposit removal, such as in TFTR, TEXTOR, HT-7, AUG, EAST, DIII-D…… Lab. reserches on deposits removed from TFTR, AUG, JET, DIII-D. ASIPP

5 Motivations In a limiter machine - HT-7 with 20% carbon walls: Thermo-oxidation, O-GDC and O-ICR experiment on hot walls at K from 2004 to 2006 In an ITER-relevant full superconducting tokamak - EAST with divertor configuration He/O-ICR on full metal walls at 420K in 2007 He/O-ICR on full carbon walls at a room temperature in 2009 Mian efforts: In-situ co-deposit removal techniques; Plasma recovery after cleanup; Comparison of various techniques for oxidation wall conditioning Comparison of O-ICR between HT-7 and EAST machines, including the influence of plasma facing materials Oxygen removal and plasma recovery Prediction for ITER.

6 2. Various oxidations on HT Thermo-oxidation Low removal rate for D and C High O retention Oxidations is transient Not enough T walls and O 2 pressure Without pumping; Initial oxygen pressure is 0.7, 1.1, 9.3 and 32Pa. 1.1 Pa thermo-oxidation HT-7/EAST ASIPP J. Nucl. Mater., Vol.350(2006)9

7 2.2 O-GDC High pressure is beneficial for C and D removal. Compared to He-GDC cleaning, O-GDC wall conditioning has a higher removal rate for H by a factor of about 2~4 and for C by a factor of 25. In a long O-GDC operation in 2005, the removal rates for H and C sustained very high for 9.5hrs, whereas it decreased for D. HT-7/EAST ASIPP J. Nucl. Mater., Vol.349(2006)160

8 2.3 O-ICR in HT-7 Single pulse 2s on(20~100kW) Pulsed O-ICR(20kW) 0.3s on/ 1.5s off HT-7/EAST ASIPP O-ICR with pulsed waves (0.3s on and 1.2s off) seems better than with single wave. Compared to He-ICR, O-ICR has a higher removal rate for H-atoms by a factor of about 6 and for C-atoms by a factor of 20. Plasma Phys. Control. Fusion, Vol.47(2005)1271

9 Higher RF power Higher oxygen pressure Higher release of hydrogen and carbon; Higher oxides productions; He in He/O-ICR plasma, Promote oxygen reaction Reduce oxygen retention. 2.3 O-ICR in HT-7 HT-7/EAST ASIPP Plasma Phys. Control. Fusion, Vol.48(2006)807

Sample analysis By ~71mins O-ICR cleanings, Deposits were effectively removed. H and D retention reduced ~80%. Oxygen content increased <30%. Without exposure After 71min. He/O-ICR HT-7/EAST ASIPP Plasma Phys. Control. Fusion, vol49(2007)421

11 Thermo-OO-ICR4:1 He/O-ICRO-GDC Parameters32Pa40kW, 9 × Pa40kW, 9.8 × Pa1.5Pa, 250V/4A Highest removal rate (10 21 atoms/h) H-atoms~6 for C Only in <1min D-atoms C-atoms Oxygen absorbed rate (10 14 O-atoms/cm 2.min) Oxygen Coverage O-atoms/cm 2 ~ × × × Comparison of oxidations in HT-7 HT-7/EAST ASIPP 1.Thermo-O: with Bt; Access to all areas, specially to shadow ereas; low removal rate, but could be improved with high wall Temp.(e.g.350 o C) and higher oxygen pressure(e.g.>1000Pa) 2.O-GDC: without Bt; On Line-of-insight surface, effecitive area larger than O- ICR, specially to far positions, such as pump ducts; limited O 2 pressure; Continously High removal rate even on Low T walls, high O retention 3.O-ICR: with Bt; On Line-of-insight surface; limited O 2 pressure; Continously high removal rate even on Low T walls, low O retention

12 3. Oxidations between HT-7 and EAST 3.1 O-ICRF in EAST full metal walls at 400K Full carbon walls at room Temp. Similar behaviors during oxidation, and similar influence of power, pressure, pumping speed and wall conditions. HT-7/EAST ASIPP J. Nucl. Mater., 390–391 (2009) 929

13 ICR power and Pressure: particle energy and plasma density Plasma volume:plasma power density PFCs materials and Plasma fuels : source of C, H, and D Configration and Plasma facing surfaces:cleaning area Pumping speed: particles exhaust RF antenna: coulpling power Wall temperatures: oxidation efficiency 3.2 1:1 He/O-ICRF betweenHT-7 and EAST HT-7/EAST ASIPP

Removal rates in 20kW 0.014Pa He/O-ICR HT-7EAST(metal walls)EAST carbon walls plasma volume2.616 plasma facing surfaces 1260 plasma-facing material 20% carbon +SSFull SSFull carbon plasma fuelD2D2 H2H2 D2D2 configurationLimiterdivertor AntennaSingle antenna at high field side both for ICWC and heating Two dedicated at low field side specially for ICWC Wall temperature K~400K~300K Removal rates for C1.9×10 21 atoms/h1.3×10 22 atoms/h1.5×10 22 atoms/h Removal rates for H2×10 21 atoms/h2.5×10 22 atoms/h4.1×10 22 atoms/h RR EAST /RR HT-7 =V HT-7 ×S EAST ×PS EAST /(V EAST ×S HT-7 ×PS HT-7 )~12.8 HT-7/EAST ASIPP

15 HT-7/EAST ASIPP Higher oxygen pressure, more oxides produced, however higher retention. Higher oxygen retention in O-GDC than in O-ICR; 4.Oxygen retention, removal and plasma recovery 4.1 Oxygen retention To reduce O-retention in O-ICR: Lower oxygen pressure High ratio of He:O 2 High ICR power

Oxygen removal Both He-GDC and He-ICRF were effective for oxygen removal. D 2 -ICR cleaning has a higher removal rate of O than He-ICR by a factor of 5-7. Oxygen removal depended on oxygen retention on the walls, cleaning parameters. HT-7/EAST ASIPP Fusion Eng.&Des., Vol.81/19(2006)2175 Fusion Eng.&Des., Vol.82/2(2007)133

Plasma recovery Plasma could be recovered, but tens disruptive shots(32-97) are required. After oxidation, impurities increased and Ha decreased. Impurities, such as C, O, decreased shots by shots. In 2007 experiments, 1g Boronization after He/O-ICR made plasma recovery easily, Zeff~2.5 was got in the recovered plasmas. Less O retention and boronization helps the plasma recovery. HT-7/EAST ASIPP Fusion Eng.&Des., Vol.83(2008)689

18 5. Prediction for ITER HT-7/EAST ASIPP DeviceSeriesParametersRemoval rates for C (atoms/h) Removal rates for H (atoms/h) Estimated C removal rate for ITER with 3MW ICR (atoms/h) HT-7 (20% carbon walls) Pure O-ICR (2004) 40kW,0.09 Pa1.5× × ×10 25 (358g/h) 4:1 He/O-ICR (2005) 40kW,0.098 Pa7.5× × ×10 24 (177g/h) 1:1 He/O-ICR (2005) 40kW, 0.06Pa4.2 × × ×10 24 (101g/h) Pure O-ICR (2006) 60kW, 0.05Pa1.1× × ×10 24 (175g/h) EAST (metal walls) 1:1 He/O-ICR (2007) 20kW, 0.07Pa4.2 × × ×10 24 (189g/h) EAST (carbon walls) 1:1 He/O-ICR (2009) 20kW, 0.1Pa2.6 × × ×10 24 (117g/h) To remove 700g T in ITER, it would require 0.5~7hrs of wall conditioning with 3MW O-ICR discharges. Assuming with a same oxygen pressure, mian factors, such as power density, cleaning surface and actively pumping, are linearity effect on the deposits removal.

19 HT-7/EAST ASIPP Series oxidation experiments were carried out on HT-7 and EAST. O-GDC appeared to be the most efficient on HT-7. However, O-ICR was almost as good and could be applied with the magnetic field in ITER. O-ICR on both EAST and HT-7 revealed that power density, working pressure, plasma facing area and materials, pumping speed and wall temperatures were main factors influencing on the deposits removal. High power, low oxygen pressure and He in O-ICR are beneficial for reducing oxygen retention. Plasma discharges could be recovered by effective cleanings and boronization. To remove 700g T in ITER, it would require 0.5~7hrs by 3MW O-ICR cleanings. Oxidation treatment is one of the most promising methods for amorphous tritiated carbon layers removal for ITER. summary

20 HT-7/EAST ASIPP Thank you for your attention!!