Paper

SUNIST Program and Improvement of Operation on SUNIST Spherical Tokamak

∗ He Yexi Non-member ∗ Gao Zhe Non-member ∗ Wang Wenhao Non-member ∗ Xiao Qiong Non-member ∗ Xie Lifeng Non-member ∗ Zeng Li Non-member ∗ Zhang Guoping Non-member ∗ Feng Chunhua Non-member ∗ Wang Long Non-member ∗∗ Yang Xuanzong Non-member

Spherical tokamak program in China was started up from 1999. The Sino United Spherical tokamak (SUNIST) has been assembled in November 2002. Test discharge of SUNIST completed at the end of 2002. We got the plasma with about 50 kA of current in test discharge without flattop on plasma current. After modification of the power supply of vertical field in 2003, we obtained fine equilibrium plasma current on SUNIST with about 2 ms flattop. The SUNIST laboratory has been founded in 2004, consisted of Depart- ment of Engineering Physics, Tsinghus University (DEPTS.) and Institute of Physics, Chinese Academy of Sciences (IOPCAS). A series of experiments has been taken on edge plasma parameters, fluctuation and turbulence before and after power supply modification. At the end of 2003, we tried to deposit siliconized film on vacuum vessel. After siliconization, plasma current flattop could extend to the regime where the signal of loop flux had fell down to zero.

Keywords: spherical tokamak, discharge, vertical field, siliconization

1. The SUNIST program was originally sponsored by Introduction National Nature Science Fund, Subject Development of In past thirty years, Chinese fusion research activi- Tsinghua University Project and Innovation Fund of In- ties have been concentrated in tokamak configuration stitute of Physics, Chinese Academy of Sciences. The machines, such as CT-6, HT-6B/M, HL-1/M, KT-5, research activities in first period include establishing a and HT-7 with superconducting toroidal field magnet. spherical tokamak device, named SUNIST located at Now two tokamak programs are being developed in Tsinghua University, a united laboratory (consisted China. One is the EAST superconducting tokamak of Department of Engineering Physics, Tsinghua program (1) in Institute of Plasma Physics, Chinese University and Institute of Physics, Chinese Academy Academy of Sciences (IPPCAS), which aims at steady of Sciences) established in 2004 and the preliminary state advanced tokamak (AT) operation, and the other experiments of spherical tokamak, which is introduced is the HL-2A tokamak program (2) in Southwestern In- in Sec. I. The improvement of operation on SUNIST stitute of Physics (SWIP), which is motivated by the re- is described in Sec. II. Sec. III gives the vertical search of high performance plasma, reversed shear and field effect on discharge, and Sec. IV gives the vacuum AT operation mode. The Sino United Spherical toka- conditioning and its influence on discharge. Finally, a mak (SUNIST) program is the only research program of summary is given. (3) spherical tokamak in China. 2. SUNIST Program (4)

∗ SUNIST Laboratory, Department of Engineering Physics, The mission of this program is to explore the spherical Tsinghua University torus plasma on the SUNIST spherical tokamak (Fig. 1), Beijing 100084, P.R.Cnina including discharge character exploration, edge plasma ∗∗ SUNIST Laboratory, Institute of Physics, Chinese Academy of Sciences characteristic research, turbulence research, and plasma Beijing 100080, P.R.China current startup and sustain without solenoid.

電学論 A，125 巻 11 号，2005 年 925 Fig. 2. SUNIST vacuum vessel

Table 1. Main parameters of SUNIST magnets and power supplies

system I(kA) V(kV) L/R (µH/mΩ) C(mF/kV) toroidal 10 0.2 521/5.3 2560/0.4 ohmic ±13 4.7 532/18,3 3.4/5 vertical 2 2/0.2 697/15.5 1/2, 450/0.25 microwave 0.023 25 0.125/25

Fig. 1. SUNIST spherical tokamak device In coming years, SUNIST will start ECR plasma cur- rent startup and upgrade for coaxial helical injection The parameters of SUNIST device is as followings: (CHI) experiment. Peripheral systems would be modi- fied according to the demands of these experiments, such R major radius 0.3 m as power supply and control system, magnetic probes, spectroscopes, microwave interferometer, high speed im- a minor radius 0.23 m age recording and so on. A aspect ratio 1.3 k elongation 1.6 3. Improvement of SUNIST Operation BT toroidal field 0.15 T During the test discharge period on SUNIST, plasma IP plasma current 50 kA current could reach 50 kA easily, but there was no any ∆Φ flux swing 0.06 Vs flattop on plasma current curve. There were two pos- sible reasons, bad condition of vacuum vessel wall or The machine assembling has been completed in lack of basic equilibrium condition of plasma current col- November 2002. The leak rate of cross seal was less umn. It was difficult to distinguish without believable −7 3 than 2 × 10 Pa，m /s. Test discharge of SUNIST was information about plasma horizontal shift and impuri- (5) −5 completed at the end of 2002 and the plasma with ties. However, 6 × 10 Pa of vacuum vessel pressure 50 kA of current was gotten. A series of experiments was not too bad, and mass spectrum analysis also sug- has been taken for edge plasma parameters, fluctuation gested that vacuum should not to be the main reason and turbulence before and after power supply modifi- for the plasma current without flattop. On the other cation. Improvement of equilibrium quality decreased hand, comparing the plasma current with the currents fluctuation level of edge plasma. of ohmic and vertical field (Fig. 3) gave us useful in- The SUNIST vacuum vessel consists of outer shells formation. The current of vertical field dropped down with 6 mm of thickness and 0.5 mm thickness of cen- after touching the top when the plasma current reached tral column. A viton cross seal ring, between two half its peak also. We noticed the maximum value of the outer shells, and central post, provides vacuum sealing vertical field current and the decreasing subsequently and the electrical break along toroidal and poloidal di- were quite different with the behavior of the vertical rections (Fig. 2) for interrupting one of the eddy current field when it tested only. Associated with mutual in- during breakdown. ductivities between plasma and poloidal field (Table 2), Main parameters of SUNIST magnets and power sup- tight coupling between ohmic and vertical field might be plies are listed in Table 1. responsible for the hollow of vertical field current. Sim- Power supplies, diagnostics and other systems in ple analysis identified the similar in value of maximum SUNIST, limited by budget, were considered as simply value and dropped subsequently with test discharge. as possible. Following the successful test operation, up- Several methods could eliminate the couple effect com- grades of the peripheral system and the improvement of pletely. We selected a simplest one that is, using stor- operation become more important. age energy of capacitor banks instead of drawing from

926 IEEJ Trans. FM, Vol.125, No.11, 2005 SUNIST Program and Improvement of Operation on SUNIST

Fig. 5. Vertical effect on discharge, with 1 kG of toroidal field, 10 kA of ohmic field current and charging Volts of vertical field capacitor banks as separately; A: 250/160, B: 300/210 and C: 400/240

Fig. 3. Typical discharge curves, current of plasma (IP ), vertical field (IV ), ohmic (Iohm) and flux loop signals at different positions (Vp1- inside, Vp2- outside) Fig. 6. Vertical field effect on magnetic surface of plasma with 30 kA of IP andA:0.2,B:0.4andC: Table 2. Mutual inductivities between poloidal 0.8 kA of vertical field current in SUNIST spherical field and plasma tokamak

M(µH) Bohm Bvertical plasma Bohm 519 Bvertical 124.3 684 cal field current (charging volt of capacitor banks) with plasma 2.3 5.54 0.33 fixed toroidal field and ohmic field current. This phe- nomenon suggests that vertical field is not only to affect plasma horizontal position but also shape and plasma current value. We identified qualitatively the action of vertical field on plasma by mean of equilibrium calcula- tion with a simple single filament mode, shown in Fig. 6, the magnetic surface of plasma under same plasma current changes with vertical field acted on. It suggests that it is easy to keep plasma “equilibrium” by adjust- ing vertical field according to horizontal shift signal, but we would not only adjust the plasma position but also change other important parameters simultaneously. This phenomenon of plasma has influenced our re- search program from two aspects, one is we should per- Fig. 4. Typical discharge, reconfigured vertical field capacitor bank form integrality control of plasma current, position and shape to get a reproducible plasma for physics research; another, the contribution of vertical field on flux will be vertical field magnet energy to provide the coupling en- a interesting topic for research. ergy demand. We also planned to solve the problem 5. Vacuum Conditioning completely by real time control of poloidal field current later. After modifying the configuration of vertical field Vacuum condition is a special important issue in con- capacitor banks, from 1 mF/2 kV, 0.45 F/200 V to 2 trolled fusion research. There is a nature risk of vacuum mF/ 1 kV, 4.7 mF/900 V (or 18.8 mF/450V), we ob- leakage in SUNIST device, four positions of cross vac- tained desirable effect (Fig. 4). Current of vertical field uum seal. We got a calibrated leak rate of less than has been adjustable easily by charging voltage of capac- 10−7 Pa·m3/s on cross seal positions in vacuum test of itor banks and there has been about 2 ms of flattop on SUNIST vacuum vessel and monitor these position any- plasma current that extended to the maximum of ohmic time the vacuum condition changed. field current when there is no any Volt-second available. Typical methods of vacuum conditioning include bak- ◦ 4. Vertical Field Effect on Discharge ing (about 100 C) with positive temperature coefficient of resistance elements, glowing discharge with helium or We have noticed the effect of vertical field on discharge hydrogen and training discharge in SUNIST. We could when we scanned the vertical field for better equilibrium. summary glowing discharge effect with helium definitely Plasma current has been changed obviously in this scan- that it would decrease water partial pressure in vessel ning (Fig. 5). obviously and easy to get well controlled plasma, but no From the sequences of discharge in Fig. 5, one can see evident effect on the gas with mass/charge of 28, one of that the plasma current is very sensitive to the verti- the main residual gas in SUNIST (Fig. 7).

電学論 A，125 巻 11 号，2005 年 927 6. Summary Since 1999, SUNIST program has been developed for 6 years. SUNIST spherical tokamak has been built up in 2002 and introduced preliminary experiments from 2003. SUNIST Laboratory has been established in 2004 based long-term collaboration between Department of Engineering Physics, Tsinghua University and Institute Fig. 7. Mass spectrum before (left) and few min- of Physics, Chinese Academy of Sciences. utes after (right) glowing discharge with helium The strong coupling effect between ohmic and verti- cal fields was the main problem to influence pulse length and plasma current flattop. The method we used is a simplest way, but it’s not an ideal way. We recognized vertical field effect on SUNIST plasma is not only to control position, but also to influence current value and shape. In this sense, we should find a way to eliminate coupling effect and control plasma current, shape and position. The discharge after siliconization has showed us an attractive performance. However, after siliconiza- tion major disruption was observed, which is supposed to be triggered by strong recycling. It might be avoided using pulse fueling in siliconized vessel. Acknowledgment This work is supported by JSPS-CAS Core-University Program on Plasma and Nuclear Fusion, the National Nature and Science Fund of China (Grant numbers: 10275041 and 10375089) (Manuscript received Jan. 23, 2005, revised Aug. 4, 2005)

Fig. 8. Discharge after siliconization (left, just References after siliconization, right, few shots later)

( 1 ) W. Yuanxi: “Overview of steady state operation of HT-7 and present status of the HT-7U project”, Nuclear Fusion, Vol.6, No.40, p.1057 (2000) ( 2 ) F. Engelmann: “Steady-state operation of magnetic fusion We had tried siliconization in January 2004, and ob- devices - plasma control and plasma facing components”, Nuclear Fusion, Vol.6, No.40, p.1030 (2000) tained very interesting discharges (Fig. 8). In conse- ( 3 ) Y-K. Peng and M. D. J. Strickler: “Features of spherical torus quent discharge just after siliconization, plasma current plasmas”, Nuclear Fusion, Vol.26, p.769 (1986) extended to the regime where the flux loop signal down ( 4 ) H. Yexi, et al.: “A Research Program of Spherical Tokamak to zero, but after few shots, hard disruptions on plasma in China”, Plasma Sci. Tech., Vol.4, p.1355 (2002) ( 5 ) W. Ying, et al.: “Initial Plasma Startup Test on SUNIST current appeared frequently. It is noted that the hard Spherical Tokamak”, Plasma Sci. Tech., Vol.5, p.2017 (2003) disruption event is never observed before. After sev- eral ten minutes helium GDC, we could obtain good discharges again. Few shots later, hard disruption came HE Yexi (Non-member) was born in Chongqing, China, on again. The continuous fueling with pressure feedback January 15, 1946. He graduated from Uni- should be reason. More and more hydrogen absorbed versity of Science and Technology of China on siliconization film would release too much hydrogen in 1968. He has begun to magnetic confine- ment fusion research since 1973 in Institute of in discharge and then triggered hard disruption. In sin- Plasma Physics (former name ”Laboratory of gle swing mode of ohmic field discharge, it’s difficult to Controlled Thermonuclear Reaction”, Chinese adjust discharge during ohmic field current going down. Academy of Sciences in Hefei. He has moved We plan to research this kind discharge again after mod- to Tsinghua University for the first Chinese ification of ohmic field power supply with pulse fuelling program of spherical tokamak in Beijing since 2000. in 2005.

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