2010 Spring Korean Physical Society Startup Preparation for Versatile Experiment Spherical Tokamak(VEST) at SNU 2010 Spring Korean Physical Society Hyunyeong Lee, 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 Startup preparation – Null formation & Preionization 3 Pretest for ECRH system 4 Future Work & Summary

Worldwide Spherical Tokamaks(ST) NSTX (US) MAST (UK) Globus-M (RF) LATE (J)

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.

Existing solenoid-free start-up concepts are Motivation Existing solenoid-free start-up concepts are Compression-Merging method : Use in-vessel PF coil’s swing Disadvantage Impurity due to in-vessel PF coil Engineering constraints Double Null Merging (DNM) : Use Outer PF coil’s swing Disadvantage : Hard to get equilibrium [1] Not effective as much as conventional solenoid start-up. However, ST has not sufficient space for solenoid. Thus, we need innovative concept for generating first target plasma. [1] : P. Micozzi, the 11th workshop on spherical torus, St. Peterburg, 2005

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

Device Specifications VEST in SNU Device Specifications 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

Null region formation in partial solenoid geometry. Despite of asymmetry, high quality null is formed. Partial solenoid can supply more volt-sec to null region than normal double null merging by PF coils. B-null region (B < 20G) : 0.25<r<0.5, 0.87<z<1.1 10G 5G 20G 15G

Start-up Analysis : 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. [3] B. Lloyd et al., Nucl. Fusion 31, 2031 (1991)

Start-up Analysis : Preionization Lloyd condition for breakdown[3] with pre-ionization (for 10% ionization, ECH used at r=0.33) To meet 10% preionization, ECH preionization needs to be arranged appropriately. Key points of the ECH preionization Available stable ECH power Synchronization of the 2.45GHz microwave injection Development of ECRH system satisfying those conditions [3] B. Lloyd et al., Nucl. Fusion 31, 2031 (1991)

The necessity of ECRH system The reason of ECRH System Necessity in preionization Efficient and localized heating Magnetron for the ECRH system -> 2.45GHz microwave source is chosen. Simple, safe, easily available and economical -> household microwave oven Need circuit modification! http://blog.naver.com/rlhyuny27?Redirect=Log&logNo=30029307561

The resonance zone and the ECRH system 2.45GHz Microwave 2.45GHz Microwave

The pretest system for ECRH : Helicon ion source Waveguide(WR284) Magnetron The reason of choosing the helicon ion source chamber Easy control of the magnetic field up to 900G Available for various plasma diagnostics

Microwave plasma launched In the magnetic field(875G), the plasma is launched using the magnetron of the household oven.

Ion saturation current measured by Langmuir probe  

A schematic diagram of the household oven circuit Low voltage part: To heat filament to supply electrons to magnetron High voltage part: To accelerate hot electron to generate microwave Issues are Maintaining sufficient pulse length of the microwave source Triggering capability

Modification of the household oven circuit To accomplish the goal(economical and efficient preionization) -> To use DC power supply : not economical -> To make a simply modified circuit. Modification High Voltage part -> Use high voltage source with capacitor charged -> Triggering microwave source by using switch Low Voltage part High Voltage part Therefore, appropriate pulse length and start time can be obtained as we want.

Expectation of ECRH Power duration POWER PULSE DURATION FREQUENCY MAJOR RADIUS MINOR RADIUS KAIST-TOKAMAK 2.5kW 450ms 2.45GHz 53cm 17cm NSTX 15kW 50ms 18GHz 85cm 67cm MAST 150kW 20ms 60GHz 65cm START 1.4kW 3.5ms Aspect ratio : 1.25 1.7kW 6GHz SUNIST 20kW 10ms 30cm 23cm PEGASUS 60kW 4ms 5.5GHz 33cm 28cm In comparison with similar STs(SUNIST, PEGASUS), we estimate the appropriate ECRH power and pulse duration in VEST. : 200~250(Ws)??

Summary Versatile Experimental Spherical Tokamak(VEST) in SNU is designed for partial solenoid operation. For successful startup operation, null formation and preionization is being prepared. ECR source will be used as a preionization tool with multiple units of microwave sources for economical and efficient startup. Preliminary tests are ongoing.

Future Work Simultaneous triggering of multiple units of magnetron sources for ECH preionization To determine ECR power – Appropriate power for the startup of VEST Preliminary plasma generation test – The ionization rate and the density of plasma in the pretest device Various operation scenario for ECH-assisted startup in VEST