International Youth Conference on Fusion Energy Conference The Development of Pre-ionization System for Versatile Experiment Spherical Tokamak(VEST) at SNU International Youth Conference on Fusion Energy Conference Hyunyeong Lee, Junghoon Yang, Choongki Sung, J. Kang, S. Choi, K. J. Jung and Y. S. Hwang, NUPLEX, Dept. of Nuclear, Seoul National University, San 56-1, Shillim-dong, Gwanak-gu, Seoul 151-742, Korea brbbebbero@snu.ac.kr

1 2 2 3 4 Contents Motivation & Introduction Modification of the operation circuit 3 Pretest for estimation of the power for pre-ionization 4 Summary & Future Work

Spherical Tokamak as an Alternative Concept : Low aspect ratio (A<2) tokamak overcomes tokamak’s drawbacks (Low beta, large size) Advantages of ST Weakness of ST High performance (High plasma current, βlimit) Compactness Difficulty in start-up & sustaining (Lack of space for solenoid) Innovative start-up method is critical issue for ST!! Once developing new start-up method, high performance tokamak would be studied in ST.

Innovative Startup Method - Partial Solenoid Operation PF coil Solenoid start-up The most effective method for start-up High shaping and equilibrium ability by PF coils Disadvantage Hard to keep low aspect ratio Incompatible to Spherical Tokamak Partial solenoid operation Keeping solenoid start-up’s merits Maintaining low aspect ratio Possible effective start-up in ST

Versatile Experiment Spherical Tokamak(VEST) in SNU Using new startup method, VEST has been designed. Device Specifications of VEST Initial phase Future Chamber radius [m] 0.8m (middle) 0.6m (up & down) Chamber height [m] 2.4m Toroidal B field [T] 0.1T 0.3T Major radius [m] 0.4m Minor radius [m] 0.25m 0.3m Aspect ratio 1.6 1.3 Plasma current [kA] 30kA 100kA qedge 4.6 5.1

Lloyd Condition for Breakdown with pre-ionization (for 10% ionization, ECH used at r=0.33) Lloyd Condition depending on time t=0.1ms t= 0.17ms Z [m] Z [m] R [m] R [m] : Expected plasma center (R=0.33, Z=1m) During 0.17ms, Lloyd condition is satisfied in large region if 10% ionization. [1] B. Lloyd et al., Nucl. Fusion 31, 2031 (1991)

Null region formation in partial solenoid geometry. To accomplish 10% ionization rate, high quality null is essential. Despite of asymmetry, high quality null is formed. B-null region (B < 20G) : 0.25<r<0.5, 0.87<z<1.1 10G 5G 20G 15G

The ECRH for pre-ionization The reason of ECRH pre-ionization Necessity in pre-ionization Efficient and localized heating Magnetron for the ECRH -> 2.45GHz microwave source is chosen. Simple, safe, easily available and economical -> Multiple units of household microwave oven[2] Objective The development of pre-ionization system using the multiple units of magnetrons for household microwave oven. [2] http://blog.naver.com/rlhyuny27

The pretest chamber : Helicon Plasma Chamber Preliminary Experiment : To confirm the feasibility of pre-ionization system using the household microwave oven Stainless Steel Tube Waveguide(WR284) Magnetron The reason of choosing the helicon plasma chamber[3] Easy control of the magnetic field up to 875G Available for various plasma diagnostics [3] S.H. Kim et al., Plasma Phys. Control. Fusion. 50, 035007 (2008)

Hydrogen plasma with 60Hz frequency The result of the pretest using the original circuit of microwave oven -> The hydrogen plasma is generated at 60Hz. -> AC operation(220V, 60Hz, 8ms) -> Half wave rectifier in the circuit makes this flickering signals.

Modification of magnetron operation circuit Isolate Transformer DC Power Supply Negative PS Original operation circuit[4] Low voltage part: To heat electrode for thermionic emission High voltage part: To accelerate hot electrons Modifications DC power supply to heat electrode provides 10A. Isolate transformer makes the whole circuit of the magnetron independent. Negative high voltage power supply provides -3.5kV to the circuit. In negative PS, the microwave power is controlled by the emission current. [4] W. Choi et al., Rev. Sci. Instrum. 71, 2728 (1999)

The Result of the Preliminary Experiment Hydrogen plasma density vs. microwave power under varying the pressure Features The lower pressure is, the higher plasma density is acquired. The higher the microwave power gets, the more density is acquired.

Estimation of the whole power for pre-ionization system Pressure (mTorr) The ionization rate per unit power (#/W∙m^3) 0.4 3.78ⅹ1014 2.9 1.21ⅹ1014 8.5 3.76ⅹ1013 The volume of the hydrogen plasma The pretest chamber: cylinder shape, diameter(8cm), length(55cm) -> VEST: torus shape, major radius(40cm), minor radius(15cm) To accomplish 10% ionization rate Above 3.8kW of RF power is necessary(0.4mTorr).

Modification of the circuit for Pulse Operation Low Voltage part High Voltage part The necessity of the modification 1. VEST will be operated in pulse mode(10ms). 2. Triggering the signal is essential using the multiple units of magnetron(1kW) High Voltage Capacitor ( ) will be determined by the operation time. This modified operation circuit will soon be completed.

Summary Versatile Experimental Spherical Tokamak(VEST) in SNU is designed for partial solenoid operation. For successful startup operation, pre-ionization system is prepared using modified magnetron circuit of household microwave oven. Above 3.8kW of microwave power is estimated for 10% pre-ionization through the ionization rate per unit power. The more modification of the circuit is necessary for pulse operation.

Future Works To complete the whole pre-ionization system for pulse operation in VEST Various operation scenario for ECH-assisted startup in VEST For successful startup, the research of the null quality The generation of the first plasma in VEST finally