1. 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 , Korea,
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 [
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 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
Driving method : Battery (12 V, 100 Ah) Bank
Main Switch: Commercial Magnetic contactor
Preliminary ECH experiment using 2.45GHz Commercial Magnetron
Jitter ~ 5 turn-on
12 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