Design and Construction of Versatile Experiment Spherical Torus at SNU Y.H. An, K.J. Chung, B.K. Jung, C. Sung, H.Y. Lee, Y.S. Na, T.S. Hahm 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 Background & Motivation Introduction Research Topics Partial Solenoid VEST : Versatile Experiment Spherical Torus Objectives Basic research on a compact, high- ST (Spherical Torus) with elongated chamber in partial solenoid configuration Study on innovative partial solenoid start-up, divertor, etc Spherical Torus (ST) : Low aspect ratio (A<2) fusion device Solenoid Start-up Non-inductive Current Drive Breakdown by Partial Solenoid The most effective start-up method High shaping and equilibrium ability Hard to keep low aspect ratio Difficult to apply to spherical torus ECRH in ST : Limited due to cutoff Electron Bernstein Wave (EBW) can propagate through mode conversion → No cutoff density. Study on EBW Heating & Current Drive (2.45GHz ECH system) Enough space for Divertor Large aspect ratio (Conventional Tokamak) Small aspect ratio (Spherical Torus) a Partial Solenoid Operation R Specifications Inherits the merits of solenoid start-up Possible to maintain low aspect ratio Effective Start-up method in ST 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] Aspect Ratio >1.3 Plasma Current [kA] 30 100 Safety factor*, qa 7.4 6.7 Innovative Divertor Concept Plasma Merging Advantages Weakness Control of heat flux is one of the important issues in fusion research ST : High heat density & Compactness → Appropriate for heat flux study VEST : Enough space for divertor + Suffcient no. of PF coils → Suitable for divertor research Innovative divertor concepts such as super-X and snow-flake divertors will be investigated. High performance : High β, High plasma current Compactness Difficulty in start-up & sustainment due to the lack of space for solenoid Sequential Tokamak Injection for Ramp-up Maintaining plasma currents through partial solenoid operations. Sequential injection of small tokamak plasmas by partial solenoid while the main plasma is sustained by thin solenoid or ECH Enough space for Divertor Innovative start-up method is critical issue for ST! By developing new start-up and non-inductive current drive methods, ST can be a high performance fusion research device. Breakdown by Partial Solenoid Partial Solenoid * Elongation : 3.3 assumed Status of VEST Vacuum Vessel & Center Stack PF2 (Upper partial solenoid) Vacuum Vessel Center Stack Design Cu block is brazed Nipples for water cooling Overall dimension of VEST Thickness of vacuum chamber wall Lower Chamber Upper Chamber Rectangular Port 12” Port 10” Port Main Chamber 4” Port 6” Port PF3 PF4 ~ 1.1 m PF2 17mm 15mm 13mm “Section A-A” 15mm PF5 PF1 Coil 2.4 m ~268mm 3.4mm 6mm G-10 Block PF6 Partial Solenoid (PF2) ~2.7m f87mm PF1 (Long solenoid) A PF7 A Epoxy molded 1.2 m 0.8 m PF8 5mm 15mm PF1 f165mm Center Stack Chamber Wall 15mm 13mm 87mm 0.6 m PF9 2.8mm 6mm Thin Solenoid (PF1) PF2 (Upper partial solenoid) PF10 TF Coils (24ea) 23 June 2011 VEST successfully installed at SNU Inner TF Coils Inner Pipe (PF1 Bobbin) Material : S/S 316L Very elongated vacuum chamber Plasma with extremely high elongation of up to 4 can be contained. Enough space for divertor study. Utilizing most effective ohmic solenoids without losing the low aspect ratio geometry TF Coil System Schematic of VEST PF Coil System MC Switch Pneumatic Switch Fuse 8.3kA for 4s Emergency Safety Breaker Battery Bank VEST TF Coil R = 15 mΩ L = 1 mH Designed to able to supply up to 8.3 kA Based on commercial deep-cycle battery 5 banks with 40 batteries for each bank (Total 200 batteries are used to make 0.1 T) Adv.: Cost effective and long flat-top Disadv.: Always contain large energy VEST TF Coil Power Supply PF3 Vertical stability & Control of small plasma TF Coils Null formation PF4 PF5 Null formation Equilibrium of small plasma PF2 PF6 Vertical stability & Control of small plasma PF7 PF8 Null formation Strand Size - PF1 & PF2: 3.5mm*15mm - PF3 ~ 10 : 6.5mm*6.5mm (3.5ф hole) Partial solenoid for plasma start-up PF9 Equilibrium of main plasma Partial Solenoid Long Solenoid for plasma sustaining Pancake design of PF 3~10 PF1 PF10 Pancake module : 6*2 strands 4*2 strands for PF #3 & #4 Role of each PF coil Design parameters of PF1 and PF2 coils and their power supplies   Parameters PF1 PF2 Coil Coil length [m] 2.4 0.5 Coil radius [m] Inner: 0.045 Outer: 0.063 Inner: 0.08 Outer: 0.125 Strand size [mm×mm] 3.5×16 No. of turns [#] 632 250 Resistance [mW] 68 52 Inductance [mH] 1.6 3.7 Maximum flux limit [V∙s] 0.13 0.55 Maximum current limit [kA] 27 14 Power supply Scheme of circuit Double swing Energy storage device Capacitor Switch type Thyristor Voltage [V] 1,000 / -2,000 700 / -500 Peak current [A] 7,300 700 Estimated magnetic flux [V∙s] 0.055 0.056 Long Solenoid Design and measured parameters of TF coil and power supply   Parameters Designed value Measured value TF coil Overall coil length [m] 2.7 Overall coil radius [m] Inner: 0.07 / Outer: 1.1 Strand size [mm×mm] Inner: 12×12 (f6 cooling) / Outer: 50×10 No. of turns [#] 24 Resistance [mW] 18.8 Inductance [mH] 0.93 Power supply Scheme of circuit Battery-based DC power supply Power supply assembly 10 modules (parallel connection) No. of batteries per unit module 20 (series connection) Main switch per unit module Magnetic contactor Capacity of unit battery [A∙h] 100 Total No. of batteries 200 Internal resistance [mW] 10.0 6.2 Voltage [V] 240 250 Current [kA] 8.3 9.92 Estimated TF field at major radius [T] 0.1 0.12 Plasma Merging PF Coils Vacuum Chamber Power Supply for PF Coils Scheme: Double swing circuit Switching: Thyristor with ferrite isolation Control: Optical trigger HV Relay Module TF coil current of 9.92 kA is achieved successfully by using 10 battery modules (Note: BT@R0 = 0.12 T for 9.92 kA TF coil current). To minimize high current load on batteries during the turn-off, the sequential switch-off is adapted. Collaboration is strongly Welcome ! Gate trigger Module Capacitor bank Thyristor Module ECH Pre-ionization System Data Acquisition & Control Diagnostics Plan Diagnostic Method Installation Status Remark Magnetic Diagnostics Rogowski Coil Initial installation Fabricated Under fabrication 3 out-vessel 2 in-vessel Pick-up Coil Prototype Test Initially 16 pick-up coils Magnetic Probe Array Under design Vacuum field and plasma current density measurements Flux Loop Initially 10 loops Probe Electrostatic Probe Optical OES Monochromator Interferometer On installation 94 GHz Fast CCD camera Near Term On ordering process 20 kHz Soft X-ray array AXUV16ELG Photodiodes Thomson Scattering Long Term - Nd:YAG Laser 1.2J/pulse 10ns VEST Preionization Upper & Lower Chamber Two cost-effective household magnetron (3kW, 2.45GHz) Low field side launching Main Chamber Commercial microwave power supply (6kW,2.45GHz ) Breakdown by Partial Solenoid with ECH Pre-ionization Magnetron Power Supply Diagnostics TF Power Supply Magnetic Diagnostics PF Power Supply 2.45GHz 3kW Interferometry ECH Power Supply Monochromator Soft X-ray Gas Injection System Piezoelectric Valve Merged Plasma 2.45GHz 6kW Optical Receiver Monitoring Pressure Battery Voltage WR284 Waveguide Reducer WR340→WR284 3kw, 2.45GHz Magnetron 32ch Digital Output Optical Transmitter Serial Communication & Remote I/O modules 56ch Analog Input 2.45GHz 3kW User PC MDSPlus MySQL WR340 Waveguide 11 modules Ethernet Ethernet 6” port PXI System Breakdown by Partial Solenoid with ECH Pre-ionization Data Storage Vacuum Window ECH injection system for pre-ionization Design and Construction of Versatile Experiment Spherical Torus at SNU