Research Program of Spherical Tokamak in China - SUNIST在中国研究项目SUNIST球形托卡马克.docx

2006 research report of “ECR plasma current startup with electrode discharge assistance” (research contract No.12935/R0)Recent Progress on SUNIST1, 2* HE Yexi, * TAN Yi, *XIE Lifeng, *GAO Zhe, *GUAN Xiaoyin, *LIU Fei, *WANG Wenhao, *XIE Huiqiao, *ZENG Long, *ZHANG Liang, **WANG Long, ** YANG Xuanzong, **FENG Chunhua, **FU Hongjun11 This work is supported by International Atomic Energy Agency (Research contract No. 12935/R0)2 This work is supported by the National Nature and Science Fund of China (Grant numbers: 10275041 and 10375089)* SUNIST Laboratory, Department of Engineering Physics, Tsinghua University, Beijing 100084, P.R.Cnina** SUNIST Laboratory, Institute of Physics, Chinese Academy of Sciences, Beijing 100080, P.R.ChinaIn 2005 - 2006, the main circuit of ohmic field magnet has been modified and tested. Continuously, better results from Electron Cyclotron wave Current Startup (ECCS) experiment, longer non-inductive plasma current, have been obtained too.By adding a turn-off circuit to the switch of the ohmic field main circuit, three Silicon Controlled Rectifiers (SCR) connected in parallel, will be turned off. Then the current in coil is discharge to fly-wheel diode branch with a 0.1Ohm, connected in series. The ohmic field decays at a smaller L/R time constant of 6ms than 30ms before modification. The test discharge of modified power supply is assisted by a 2.45GHz/1kW microwave pre-ionization for lower breakdown loop voltage. Results of the test showed the further modification is necessary for suitable ramp up rate of plasma current.In the preliminary experiments, 2.45GHz/60kW ECW on SUNIST can only startup a spike current with very short time duration (500A/0.3ms). The lack of density control was one of the reasons supposed to be responsible for the spike current driven. This year, we have taken most efforts to reduce background gas pressure and improve wall condition. First, a CRYO-PLEX 8LP high vacuum cryogenic pump was installed in SUNIST, lowering the background pressure from greater than 1×10-4 Pa to 5-8×10-5 Pa when operating with discharges. Second, the gas puffing valve was moved to the top of the device, direct to the vacuum chamber without long-slim-pipe, results in a notable shortening of puffing pulse from 8ms to 2ms and gas-filling latency from 30ms to 3ms. Third, the chamber wall was siliconized through a 30 minutes SiH4+He dc glow discharge. The duration of ECCS current has been promoted to 2ms. There’re possibilities for improving ECCS results still. The microwave was badly coupled to the plasmas until now. Large ratio of microwave power was reflected after production of plasma current and the ECCS current is decreasing from the start peak rapidly. This situation made us difficult to analyze the effect of electrode assistant discharge to ECCS current. So, reform of the microwave launch system will be the next step of SUNIST ECCD program.I Introduction to SUNISTSUNIST spherical tokamakThe mission of this program is to explore the spherical torus plasma on the SUNIST spherical tokamak [Fig.1], including discharge character exploration, edge plasma characteristic research, turbulence research, and plasma current startup and sustain without solenoid. The parameters of SUNIST device are as following:major radius R 0.3 mminor radius a ~ 0.23 maspect ratio A ~ 1.3Elongation k ~ 1.6toroidal field at R0 BT0 ~ 0.15 TPlasma current IP 0.05 MAflux swing DF 0.06 VsFig. 1. SUNIST spherical tokamak device Fig, 2. SUNIST vacuum vesselThe machine assembling has been completed in November 2002. The leak rate of cross seal was less than 2´10 -7 Pa·m3/s. Test discharge of SUNIST was completed at the end of 2002 and the plasma with ~ 50 kA of current was gotten. A series of experiments has been taken for edge plasma parameters, fluctuation and turbulence before and after power supply modification. Improvement of equilibrium quality decreased fluctuation level of edge plasma.The SUNIST vacuum vessel consists of outer shells with 6 mm of thickness and 0.5 mm thickness of central column. A viton cross seal ring, between two half outer shells, and central post, provides vacuum sealing and the electrical break along toroidal and poloidal directions (Fig. 2) for interrupting one of the eddy current during breakdown. Main parameters of SUNIST magnets and power supplies are listed in table I. systemIMAX kAVMAX VL /R mH /mWC banktoroidal10200521/5.32.56F/400Vohmic±134700532/18.33.4mF/5kVequilibrium22000/200697/15.51mF/2kV, 0.1F/250Vmicrowave23A25000125mF/25kVTable 1. Main parameters of SUNIST magnets and power suppliesPower supplies, diagnostics and other systems in SUNIST, limited by budget, were considered as simply as possible. Following the successful test operation, upgrades of the peripheral system and the improvement of ope。