Design and Fabrication of Versatile Experiment Spherical Torus (VEST) YoungHwa An, Kyoung-Jae Chung, BongKi Jung, HyunYeong Lee and Y.S. Hwang Dept. of Nuclear Eng., Seoul National Univ., 599 Gwanak-ro, Gwanak-gu, Seoul 151-742, Korea, email:ayh1800@snu.ac.kr Introduction Partial Solenoid Start-up Partial Solenoid VEST : Versatile Experiment Spherical Torus Spherical Torus (ST) : Low aspect ratio (A<2) fusion device Objectives Breakdown by Partial Solenoid Study on ST (Spherical Torus), the compact and high performance fusion research device Study on innovative partial solenoid start-up Solenoid Start-up The most effective start-up method High shaping and equilibrium ability Hard to keep low aspect ratio Difficult to apply to spherical torus Specifications Initial Phase Future Chamber Radius [m] 0.8 : Main Chamber 0.6 : Upper & Lower Chambers Chamber Height [m] 2.4 Toroidal B Field [T] 0.1 0.3 Major Radius [m] 0.4 Minor Radius [m] 0.25 Aspect Ratio 1.6 1.3 Plasma Current [kA] 30 100 Plasma Merging [http://www.ccfe.ac.uk/ST.aspx] Advantages Weakness Partial Solenoid Operation High performance : High plasma current, High βlimit Compactness Difficulty in start-up & plasma sustaining due to the lack of space for solenoid Inherits the merits of solenoid start-up Possible to maintain low aspect ratio Effective Start-up method in Spherical Torus Breakdown by Partial Solenoid Innovative start-up method is critical issue for ST! By developing new start-up method, ST can be a high performance fusion research device. Partial Solenoid Engineering Design of VEST Start-up Simulation Governing equation in matrix form Lower Chamber Upper Chamber Rectangular Port 12” Port 10” Port Main Chamber 4” Port 6” Port TF Coils PF #2 and PF #3 are serially connected. PF #5 & PF #8 are used for null formation. Main Chamber under fabrication Time marching using Runge-Kutta 4th order with given voltage profiles + Required PF Currents for null formation are found by linear least square solver Loop voltage calculation Partial Solenoid Field Null PF2, 3, 5, 8 Current Waveform Material : SUS 316L Vacuum Chamber Upper Chamber under fabrication Long Solenoid Partial Solenoid (PF2) Lloyd condition: Lloyd condition Max. at ~15 ms > 100 V/m for 0.7 ms Thin Solenoid (PF1) PF Coils Eddy current simulation Top cover under fabrication TF Coils Eddy current @ Chamber wall Eddy current decay most slowly in thick cover wall TF Coils Thin Solenoid (PF1) Most severe at thick cover and wall close to PF2 Loop voltage degradation by eddy current Supporting Legs 38% Reduced PF Coil module under fabrication Partial Solenoid (PF2) Power Supply Development ECH Pre-ionization Diagnostics Summary of VEST diagnostics plan Power Supply for TF Coils Preliminary study on ECH pre-ionization Diagnostic Method Installation Status Remark Rogowski Coil Initial installation Under fabrication 3 in-vessel 4 out-vessel Pick-up Coil Prototype Test Initially 16 pick-up coils Flux Loop Initially 10 coils Electrostatic Probe Under design OES Monochromator Interferometry Under design of phase comparator and supporting structure 70GHz Fast CCD camera Near Term 20kHz Soft X-ray Thomson Scattering Long Term Nd:YAG Laser 1.2J/pulse 10ns Single module characteristic Driving current : 8.3 kA (BT = 0.1 T @R0) Driving method : Battery (12 V, 100 Ah) Bank Main Switch: Commercial Magnetic contactor Preliminary ECH experiment using 2.45GHz Commercial Magnetron Jitter ~ 5 ms @ turn-on 12 ms @ turn-off Electron density measurement with various configuration TF Coil Driving Power Supply Magnetron Battery Bank (20ea* 10 modules) HFS : High-field side LFS : Low-field side Experimental Setup Safety Switch Current degradation ~ 1% / hr Main Switch Most effective heating with X-mode launching at High Field Side(HFS) *Battery Bank : Currently 4 modules are fabricated (capable of ~3.2 kA current) X-mode HFS launching is required for efficient pre-ionization 70GHz Interferometry System Prototype pick-up coil Power Supply for PF2 (Partial Solenoid) X-mode HFS launching under VEST geometry Circuit Diagram SW1 SW3 Driving method : Double swing circuit Main switch : Thyristor switch Summary & Future work SW2 Cut-off with LFS launching Too small volume at inboard side Launching from top side is the only possible option Summary PF2 driving power supply A new, small ST named Versatile Experiment Spherical Torus (VEST) has been designed and under fabrication at Seoul National University for the investigation of low aspect configuration as well as double null merging start-up with innovative partial solenoid concept and the novel sequential double null merging operation. The loop voltage for start-up is calculated with consideration of the eddy-current induced at the wall. The power supply for TF coils is fabricated utilizing commercial deep-cycle batteries which can generate B-field of 0.1T at R=0.4m. A double-swing power supply for partial solenoid is fabricated, which can supply current up to several kA required for plasma start-up. The ECH pre-ionization system utilizing cost-effective commercial magnetron is under construction. Various diagnostics for VEST are under preparation. The whole setup and installation of VEST is to be completed this spring. BT contour plot BT profile along the MW propagation path Port position Port position (R=0.4m, Z=1.2m) HV Relay Module Dummy Load Test Gate trigger Module Resonance region Capacitor bank Thyristor Module Resonance region (R=0.46m, Z=0.98m) Future Work Initial test of fabricated device such as vacuum test and coil test. Initial test and calibration of magnetic diagnostic devices. Start-up experiment using partial solenoid. X-mode HFS launching is possible under VEST geometry Design and Fabrication of Versatile Experiment Spherical Torus (VEST) at Seoul National University