1. 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 , Korea,
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: = 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