1. ASIPP HT-7/ EAST Overview of PSI activities at ASIPP G. –N. Luo, J. G. Li, and PSI Group Institute of Plasma Physics, Chinese Academy of Sciences P. O. Box 1126, Hefei, Anhui, 230031 China PRC-US Fusion Magnetic Collaboration Workshop May 18-19, 2006, Dalian, China

2. HT-7/ EAST ASIPP May 18-19, 2006PRC-US Magnetic Fusion Collaboration Workshop1 Introduction PSI studies play a significant role in achieving high performance and long pulse or steady-state operation in future tokamaks like EAST, ITER and beyond. Goals of the currently-running HT-7 superconducting tokamak at ASIPP are steady- state advanced operation and related physics ( I p >100kA, n e >1.0 ×10 19 m -3, T e >1keV, t>100s). Recently, some activities related to PSI issues have been carried out on the HT-7 tokamak.

3. HT-7/ EAST ASIPP May 18-19, 2006PRC-US Magnetic Fusion Collaboration Workshop2 Limiter system of HT-7 In 2004 and 2005, the limiter area was enlarged and covered by SiC coated graphite to hold plasma from all direction. And an active cooling system with Cu heat sink were equipped. The plasma facing surface area of limiters was 2.35m 2 (2004) plus 1.88m 2 (2005). The effective plasma facing area of limiters and liners is about 12m 2. Upper toroidal limiter lower toroidal limiter poloidal limiter belt limiter

4. HT-7/ EAST ASIPP May 18-19, 2006PRC-US Magnetic Fusion Collaboration Workshop3 PSI activities RF wall conditioning like boronization; Oxygen wall cleaning for deposits removal and hydrogen release; In-time retention evaluation by particle balance analysis; Dust measurements; Carbon based materials with doping and coating; Thick tungsten coatings (>1mm) on copper heat sink prepared by vacuum plasma spraying (VPS); Testing and PSI issues of the thick VPS-W/Cu plasma facing components in HT-7 tokamak. ------------------------------------------------------------------------------------ Future plan and collaboration opportunities

5. HT-7/ EAST ASIPP May 18-19, 2006PRC-US Magnetic Fusion Collaboration Workshop4 RF wall conditioning (1) RF wall conditioning like boronization and siliconization is used for recycling control, isotropic control, H removal in HT-7. Now the boronization using carborane (C 2 B 10 H 12 ) has become a routine conditioning before long pulse operation in HT-7. The boron film of ~350nm thick may last for 1500~2000 shots. RF-system f =15~30MHz P RF =10~100kW Working gas: D 2, He Pvv = 8×10 -4 ~0.2Pa B T = 0.5~1.8T RF plasma parameters Te: He (4~10eV) H 2 (2~5eV) Ti: H 2 (0.5~ 2keV) D 2 : 0.3~0.5 keV n e : ~ 0.5~ 3×10 17 m -3

6. HT-7/ EAST ASIPP May 18-19, 2006PRC-US Magnetic Fusion Collaboration Workshop5 RF wall conditioning (2) Improved performance Zeff close to 1; P r < 15% P OH ; n e max ~ 1.2 n e GW ; Higher LHCD efficiency; Higher heating efficiency; Extended operation limit. 60 min. RF He cleaning, T w =100  C, T liner = 200  C Boronization:1.5~2 hours with He:Caborane =1:1 30~60 min. RF cleaning (He) to remove hydrogen after boronization

7. HT-7/ EAST ASIPP May 18-19, 2006PRC-US Magnetic Fusion Collaboration Workshop6 Oxygen wall cleaning (1) High inventory of tritium is unacceptable for fusion reactors due to issues of safety and limited operation. The dominant mechanism for hydrogen retention is co-deposition with carbon. Important to develop techniques of removing co-deposits and hydrogen, and recovering plasma performance after cleanup. Application of O-ICR with permanent magnetic field in HT-7.

8. HT-7/ EAST ASIPP May 18-19, 2006PRC-US Magnetic Fusion Collaboration Workshop7 Oxygen wall cleaning (2) Oxygen removal After removing oxygen, plasma can be recovered satisfactorily with controllable density, after some initial disruptions.

9. HT-7/ EAST ASIPP May 18-19, 2006PRC-US Magnetic Fusion Collaboration Workshop8 In-time retention evaluation (1) Absolute evaluation Error of Q puff could be limited lower than 10%. Error of Q extract could be limited lower than 35%. Error of the evaluation with particle balance method could be limited <50% after careful design of gas injection system and regular calibration of gauges on HT-7. Relative evaluation Error of Q puff < 7% (from DAQ). Error of Q extract < 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. 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.

10. HT-7/ EAST ASIPP May 18-19, 2006PRC-US Magnetic Fusion Collaboration Workshop9 In-time retention evaluation (2) In HT-7, the 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.

11. HT-7/ EAST ASIPP May 18-19, 2006PRC-US Magnetic Fusion Collaboration Workshop10 Dust measurements (1) Dust inventory Scale up by 2 or 3 orders of magnitude in a next step device along with the erosion and the discharge duration; In accidental scenarios, chemical reactions with steam and air create potential explosion and dispersal of radioactivity hazards; Reliable measurement of dust and reliable methods to remove dust are necessary. In principle Photo of a detector for HT-7 Size: 150×150 mm 160VDC

12. HT-7/ EAST ASIPP May 18-19, 2006PRC-US Magnetic Fusion Collaboration Workshop11 Dust measurements (2) Dust signal Dust signal measured near the edge of HT-7 tokamak plasma in long pulse discharge. From top to bottom are plasma current, plasma electron density, Ha, ECE radiation, and dust signal.

13. HT-7/ EAST ASIPP May 18-19, 2006PRC-US Magnetic Fusion Collaboration Workshop12 Carbon based materials The graphite PFCs in HT-7 The graphite PFCs have been developed and used for the main belt toroidal and poloidal limiter in HT-7. Helpful to achieving better control of plasma and in turn longer pulse duration. Doped graphite The GBST1308 graphite (1%B, 2.5%Si, 7.5%Ti) samples coated with SiC of ~100  m thick have high thermal conductivity of 180 W/m.K (RT). The samples were mechanically joined to copper heat sink with a super carbon sheet as a compliant layer. The e-beam test shows encouraging results that the surface temperature is less than 1000 ºC even when the heat flux is about 3MW/m 2.

14. HT-7/ EAST ASIPP May 18-19, 2006PRC-US Magnetic Fusion Collaboration Workshop13 Thick tungsten coatings VPS-W coatings of 1 mm thick have been developed on Cu heat sink with a gradient Cu – W transition layer to decrease the property mismatch between the two materials. The W/Cu PFC passed an e-beam test of 20 cycles, each lasting 100 s at heat flux > 10 MW/m 2 (4 kW power onto area < 4 cm 2 in the test). W Cu

15. HT-7/ EAST ASIPP May 18-19, 2006PRC-US Magnetic Fusion Collaboration Workshop14 W/Cu movable limiter for HT-7 The W/Cu PFC was welded to a motion mechanism to serve as a movable limiter in HT-7 tokamak. The limiter is actively cooled through a co- axial water cooling system at a flow rate of ~ 2 m 3 /hr. The W/Cu PFC can be monitored using an IR-camera and six thermocouples inserted into Cu to different depths. The limiter has been mounted in HT-7, and now the testing is still underway.

16. HT-7/ EAST ASIPP May 18-19, 2006PRC-US Magnetic Fusion Collaboration Workshop15 Plasma-tungsten interactions Material issues: Component integrity under high performance and/or long pulse plasmas exposure. Surface and cross-section microstructure analysis and deuterium retention and distribution analysis. Heat load measurements: Evaluation on the heat deposited onto the movable limiter. Contribution to estimating the power/energy distribution inside HT-7. Impurity behavior: Study on W impurity generation and transport, and ways to hold it back. Test on capability of the now-equipped diagnostics in detection of the impurity in the core and edge plasmas.

17. HT-7/ EAST ASIPP May 18-19, 2006PRC-US Magnetic Fusion Collaboration Workshop16 Preliminary result of ML by IR-camera Shot 86308 (OH, I p ~130kA, n e ~1.5e19m -3, T e ~500eV, t=860ms) (r=254.5mm, a=270mm) 200ms 400ms 0ms160ms

18. HT-7/ EAST ASIPP May 18-19, 2006PRC-US Magnetic Fusion Collaboration Workshop17 Future plan (1) Plasma–wall interactions To understand the flows and exchanges of fuel and impurity particles between the plasma and the facing materials for fuel and impurity control; To understand the erosion and redeposition, and the lifetime of graphite and tungsten under steady state operation; To develop RF conditioning techniques in divertor device for ITER (cleaning, tritium removal, boronization, isotropic control,etc.).

19. HT-7/ EAST ASIPP May 18-19, 2006PRC-US Magnetic Fusion Collaboration Workshop18 Future plan (2) Plasma-facing materials / components development strategy To make the best of HT-7 as a test bench in developing different materials for divertor / first wall of EAST and beyond. EAST first wall materials: < 2MW/m 2 –SiC coated graphite + bolted Cu heat sink, technically ready; –W coatings on the reduced activation ferritic steel. EAST divertor: 4-6 MW/m 2 (8-12 MW/m 2 ) –W coatings (1-2 mm thick) on Cu heat sink, under development; –SiC coated graphite + brazed Cu heat sink, under development; –W coatings (mm thick) on the RAFS; –W coated graphite + brazed Cu heat sink. EAST may switch from the initial PFC (screw-fastened graphite + Cu heat sink) to the directly-cooled tungsten-coated PFC after the first some years’ operation to withstand higher heat flux with increasing the heating power, and finally it may become a whole tungsten PFC machine.

20. HT-7/ EAST ASIPP May 18-19, 2006PRC-US Magnetic Fusion Collaboration Workshop19 Collaborations Seeking for collaborations in PSI studies, especially issues related to plasma-tungsten interactions, e.g., –Methods to prepare for the analysis samples without contamination to the surfaces; –Ion beam analysis of the irradiated large W-PFCs in a non- destructive way; –Detection of tungsten impurity in tokamak plasma; –Behavior of the W PFCs under normal operation conditions and in the off-normal events like disruptions and VDEs, by means of simulation devices and tokamaks; –Modeling and computer simulation of the plasma-tungsten interactions.

21. HT-7/ EAST ASIPP May 18-19, 2006PRC-US Magnetic Fusion Collaboration Workshop20 Thank you !