0. Design Features and Commissioning of Versatile Experiment Spherical Torus (VEST) at Seoul National University
K. J. Chung, Y. H. An, B. K. Jung, H. Y. Lee, C. Sung, Y.S. Na and Y. S. Hwang
Center for Advance Research
in Fusion Reactor Engineering
Department of Nuclear Engineering Seoul National University
Hello,
My name is YongHwa An from Seoul National University, Korea.
Today topic of my talk is “Optimization of enhanced surface production by the negatively biased electrode in H- ion sources.”
The 8th General Scientific Assembly of
Asia Plasma and Fusion Association (APFA 2011)
November 1-4, 2011, Guilin, China

1. Outline Background & Motivation Introduction to VEST Device
Design Features
Key Design Points with Operational Scenario
Research Plans
Commissioning
Vacuum Chamber and Center Stack
TF Coil System
PF Coil System
ECH Pre-ionization System
Diagnostics Plan
Port Assignment
Summary

2. Background & Motivation Spherical Torus (ST)
Spherical Torus (ST) : Low aspect ratio (A<2) fusion device
Large aspect ratio
(Conventional Tokamak)
Small aspect ratio
(Spherical Torus) a R
Advantages
Weakness
High performance
: High β, High plasma current
Compactness
Difficulty in start-up & sustainment
due to the lack of space for solenoid
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.

3. Introduction VEST Status
Man Power, but NO Power yet
23 June 2011
VEST successfully installed at SNU

4. Introduction 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
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]
Aspect Ratio
>1.3
Plasma Current [kA] 30
100
Safety factor, qa
7.4
6.7
* Elongation : 3.3 assumed

5. Key Design Point Solenoid-Free Start-up
Plasma injection and merging is one approach for solenoid-free start-up
Existing injection scenario
Compression-Merging method
: Use in-vessel PF coil’s swing (START, MAST)
Drawbacks of in-vessel PF coil
Impurity
Engineering problems
Double Null Merging (DNM)
: Use Outer PF coil swing (UTST)
Limitation : Hard to get equilibrium

6. Key Design Point Double Null Merging Start-up with Partial Solenoids
Relatively smaller space for central solenoid in ST.
Hard to supply sufficient magnetic flux.
Breakdown
by Partial Solenoid
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
Plasma Merging
Partial Solenoid Operation
Inherits the merits of solenoid start-up
Possible to maintain low aspect ratio
Effective Start-up method
in Spherical Torus
Breakdown
by Partial Solenoid
Partial Solenoid

7. Operational Scenario Loop Voltage Calculation with Circuit Model
eddy currents are accounted in this model
Most severe at thick cover and wall close to PF2
Eddy current decay most slowly
in thick cover wall
Eddy Chamber wall
Loop voltage deceased
by eddy current
38% Reduced

8. Operational Scenario Field Null Formation and Lloyd Condition
Field null formation with circuit model
PF2, 3, 5, 8 Current Waveform
Field Null
Lloyd condition:
Lloyd condition Max. at ~15 ms
> 100 V/m for 0.7 ms
* Induced eddy current at the chamber wall considered.

9. Pressure Driven Current by ECH Heating Trapped Particle Configuration
Operational Scenario Start-up Scenario utilizing Pressure Driven Current
Breakdown
by Partial Solenoid
Pressure Driven Current by ECH Heating
2.45GHz
3kW ∙
Trapped Particle Configuration at midplane.
Pressure driven current by ECH heating
Opposite direction to the main plasma current induced by partial solenoid.
But, it can be used as virtual coil for further optimization of null formation.
2.45GHz
6kW X
Pressure driven current by ECH
Trapped Particle Configuration
2.45GHz
3kW ∙
Breakdown
by Partial Solenoid

10. Research Topics Sequential Tokamak Injection for Ramp-up
PF # kAt
PF # kAt
PF # kA
Small Plasma
Solenoid 0A
+40kAt 0A
PF # kAt
PF # kAt
PF # kA
PF # kAt
PF # kAt
PF # kA
PF # kAt
PF # kAt
PF # kA
Ip ~ 12kA
Ip ~ 12kA
Ip >12kA
Solenoid charging
Solenoid Swing Down
Plasma injection
+40kAt 0A
Solenoid 0A
Small Plasma
Study on the feasibility of a sequential tokamak plasma injection method
for maintaining plasma currents through partial solenoid operations.

11. Research Topics EC/EBW H&CD
Use of ECRH is limited in ST device due to cutoff density.
However, Electron Bernstein Wave (EBW) heating through mode conversion is possible since EBW has no cutoff density.
7.5GHz ECR
5GHz ECR
2.45GHz FHM
5GHz SHM
7.5GHz THM
HFS Launching
O-mode
cutoff
2.45GHz System under Preparation
EBW heating and current drive will be studied
[V.F. Shevchenko, et. al., Nucl. Fusion 50 (2010) ]
Schematic of the EBW assisted plasma current start-up in MAST
LFS Launching
2.45GHz
UHR
Various EC Resonance Layer in VEST

12. Research Topics Innovative Divertor Concepts
Control of heat flux and neutron flux is one of important issue in fusion research
ST with high heat density and compactness is appropriate to heat flux study.
VEST with enough space for divertor and sufficient number of PF coils is suitable for divertor research
Innovative divertor concepts such as super-X and snow-flake divertors will be investigated.
8 pairs of PF coils
Enough Space
for Divertor
Single Null (Left) and Snowflake (Right) Configuration in TCV
[F Piras et al,Plasma Phys. Control. Fusion 52 (2010) ]

13. Vacuum Chamber and Center Stack Overall Dimensions of VEST
Overall dimension of VEST
Thickness of vacuum chamber wall
PF3
PF4
~ 1.1 m
PF2
17mm
15mm
13mm
15mm
PF5
0.6 m
3.4mm
6mm
PF6
~ 2.7 m
PF7
0.8 m
PF8
1.2 m
5mm
15mm
PF1
15mm
13mm
PF9
0.6 m
2.8mm
6mm
PF10

14. Center Stack and Bottom Chamber
Vacuum Chamber and Center Stack Design and Fabrication of Vacuum Chamber
Lower Chamber
Upper Chamber
Rectangular Port
12” Port
10” Port
Main Chamber
4” Port
6” Port
Middle Chamber
Center Stack Parts
Center Stack
V/V : S/S 316L
Center Stack and Bottom Chamber

15. Vacuum Chamber and Center Stack Design and Fabrication of Center Stack
Nipples for water cooling
Cu block is brazed
“Section A-A”
PF1 Coil
2.4 m
~268mm
G-10 Block
Partial Solenoid (PF2)
f87mm
Epoxy molded A A
f165mm
Center Stack
Chamber Wall
87mm
Thin Solenoid (PF1)
TF Coils (24ea)
Inner
TF Coils
Inner Pipe
(PF1 Bobbin)

16. TF Coil Design Parameters of TF Coil
Design values
BTF [T] at R0
0.1
Coil length [m]
2.7
Wire size [mm2]
Inner: 124 (12 x 12)
* Cooling channel : 6Φ
Outer: 500 (50 x 10)
No. of turns [#]
24 (12 x 2 pairs)
ITF [kA]
8.33
Driving circuit
Battery
R [mΩ]
15 (18.8 measured)
L [mH]
1.0 (0.93 measured)
Battery bank
100 Ah battery x 200 ea
Internal Resistance of battery bank
~ 10 mΩ
(200 ea Total)
V0 [V]
250
(20 batteries in series)
Switching
Magnetic Contactor
Strand Specification for TF Coils
Structure of VEST TF coil

17. TF Coil TF Coil Power Supply: Battery Banks
VEST TF Coil Power Supply
Pneumatic
Switch
MC Switch
Battery Bank
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
R = 15 mΩ
L = 1 mH
Emergency Safety Breaker
Fuse
8.3kA for 4s

18. Commissioning Battery-based TF Power Supply
TF coil current of 8.6 kA is achieved successfully by using 8 battery modules (Note: = 0.1 T for 8.3 kA TF coil current).
To minimize high current load on batteries during the turn-off, the sequential switch-off is adapted.

19. PF Coil Overall Features of Various PF Coils
Long Solenoid for plasma sustaining
Null formation
PF3
PF4
Vertical stability &
Control of small plasma
PF1
PF5
Null formation
Equilibrium
of small plasma
PF2
PF6
Vertical stability &
Control of small plasma
PF7
PF8
Null formation
PF9
Partial solenoid
for plasma start-up
Equilibrium of
main plasma
Strand Size - PF1 & PF2: 3.5mm*15mm
- PF3 ~ 10 : 6.5mm*6.5mm (3.5ф hole)
PF10
Pancake design of PF 3~10
Pancake module : 6*2 strands
4*2 strands for PF #3 & #4
Role of each PF coil

20. PF Coil Design Parameters for PF1 & PF2 Solenoid Coils
Initial Goal
Large plasma of 30 kA
Small plasma of 10 kA
Volt-sec [mV-s]
~ 55
~ 56 (28 x 2)
Required A-T [MA]
5.2
0.21 x 2
Rin / Rout [m]
0.045 / 0.063
0.08 / 0.125
Coil length [m]
2.4
0.5 x 2
Wire size [mm2]
56.0 (3.5 x 16)
N [#]
632 (4 x 158)
250 (10 x 25) x 2
IPeak [kA]
7.3
0.84
Driving Circuit
RLC double swing
R [mΩ] 68
104 (52 x 2)
L [mH]
1.6
7.4 (3.7 x 2)
C [mF]
200 / 1 / 500
10 / 0.2 / 50
V0 [kV]
+1.0 / +1.0 / -2.0
+0.7 / +0.7 / -0.5
Max. achievable V-s limit
130
545
IPeak limit
27.3
14.0
BPeak limit
7.4
8.0
Max. sustaining time
@thermal limit (90oC)
~ 50 ms
~ 180 ms
PF2 (Upper
partial solenoid)
PF1 (Long
solenoid)
PF2 (Lower
partial solenoid)
Stress limit:
Tensile strength of Cu ~ 70 MPa

21. Commissioning PF2 Coil Power Supply
Power Supply for PF2 Coils
Scheme: Double swing circuit
Switching: Thyristor with ferrite isolation
Control: Optical trigger
SW2
SW1
SW3
PF2 coil C1
(10 mF) C2
(0.2 mF) C3
(50 mF)
HV Relay Module
SW2
Gate trigger Module
Capacitor bank
Thyristor Module
SW1
SW3

22. Commissioning ECH Pre-ionization for VEST
Breakdown by Partial Solenoid
with ECH Pre-ionization
Magnetron
VEST Preionization
Upper & Lower Chamber
Two cost-effective homemade magnetron power supplies
(3kW, 2.45GHz)
Low field side launching
Main Chamber
Commercial microwave power supply (6kW, 2.45GHz )
2.45GHz
3kW
Merged
Plasma
2.45GHz
6kW
6” port
WR284
Waveguide
Vacuum Window
Reducer
WR340→WR284
WR340
3kW, 2.45GHz
Magnetron
ECH injection system for pre-ionization
2.45GHz
3kW
Breakdown by Partial Solenoid
with ECH Pre-ionization

23. Preparation for Operation 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 merging plasma measurements
Flux Loop
Initially 10 loops
Probe
Electrostatic Probe
Optical
OES
Monochromator
Interferometry
(Under design of phase comparator and supporting structure)
94 GHz
Fast CCD camera
Near Term
Ordered
20 kHz
Soft X-ray array
AXUV16ELG
Photodiode
Thomson Scattering
Long Term -
Nd:YAG Laser 1.2J/pulse 10ns

24. Preparation for Operation Port Assignment
MM 11 : Pumping Duct
MM12 : 7.5 GHz Klystron
2.45GHz ECH Power, 6kW
UU1/MM1/LD1 : Halpha Monitoring
UT1/ LB1 : ECH (Magnetron, 3kW)
MM10 : Vacuum BNC
MM2 : NBI In
UU3 : Interferometry
MM3 : Laser In (TS,LIF)
UU9/LD9 : Vacuum BNC
MM9 : Soft X-ray
UU8 : GDC
MM8 : Fast CCD
LD8 : Pressure Guage
UT4 : Piezoelectric valve
MM4 : Detector for CXS
UT7/LB7 : ECH (Power Supply)
MM7 : Laser Out (TS,LIF)
UU5/LD5 : Fast CCD
MM5 : Laser Detector (TS,LIF)
MM6 : NBI Dump
Yellow color in future plan.

25. Fist plasma is coming soon !
Summary
Summary
A new spherical torus named as VEST (Versatile Experiment Spherical Torus) has been built to be a low cost, compact, educational fusion research device at Seoul National University.
Operation scenarios with two partial solenoid coils are prepared by using a simple circuit model accounting eddy currents.
Various research plans such as double null merging start-up, sequential tokamak injection, innovative divertor concepts and non-inductive heating & current drive with EBW are considered.
Coil and heating powers with basic diagnostics are under preparation.
Discharge cleaning and breakdown-test are ongoing.
Fist plasma is coming soon !

26. VEST Installation Movie Clip
Thank you for your attention !
VEST Installation Movie Clip

27. Thank you for your attention !
Collaboration is strongly Welcome !

28. Center for Advance Research in Fusion Reactor Engineering (CARFRE)
Lead the development of key technologies crucial for continuous and stable operation of fusion reactors
Develop analysis tools and compile experimental database for fusion reactor design and systems integration
Foster well-trained fusion research personnel
Promote international collaborations in fusion research
<Group 1>
Fusion Reactor
Systems Integration and Plasma Control Technologies
<Group 2>
Fusion Reactor
Edge Plasma Technologies
<Group 3>
Advance Technologies of Fusion Energy Conversion System
Organization: 10 Projects with 16 Principal Researchers
from 5 Major Universities and 2 National Institutes
Funding: ~1M US$/yr for 6.5years(+ 3 years optional) 28

29. Systems Integration and Plasma Control Technologies
Phase 1 (Sep Feb 2012): Key Technology Development with Research Infrastructure
Group 1 Fusion Reactor
Systems Integration and Plasma Control Technologies
Analysis tool for the
integrated system
Core plasma models
통합시스템 해석체계
노심 플라즈마 모델
Heat and particle flux, System integration data
Neutron flux, System integration data
Group 2 Fusion Reactor Edge Plasma Technologies
Group 3 Advance Technologies of Fusion Energy Conversion System
Edge plasma models
PFCs property tests
Blanket analysis model
Tritium behavior analysis
PFCs, Blanket High temp/ Low activation material 29

30. Systems Integration and Plasma Control Technologies
Phase 2 (Sep Feb 2015): Integration of Key Technologies for Applications
Group 1 Fusion Reactor
Systems Integration and Plasma Control Technologies
Analysis tool for the
integrated system
Core plasma models
통합시스템 해석체계
노심 플라즈마 모델
Heat and particle flux, System integration data
Neutron flux, System integration data
Group 2 Fusion Reactor Edge Plasma Technologies
Group 3 Advance Technologies of Fusion Energy Conversion System
Edge plasma models
PFCs property tests
Blanket analysis model
Tritium behavior analysis
PFCs, Blanket High temp/ Low activation material 30

31. Research Topics Sequential Tokamak Plasma Injection
Suppose that 12kA main plasma initiated by merging two 6kA plasmas ① ② ③ ④
① : Solenoid charging-up phase (12kA main plasma sustaining)
② : Plasma current ramp-up by merging (ramp up from 12kA to 21kA)
③ : Solenoid charging-up phase (21kA main plasma sustaining)
④ : Plasma current ramp-up by merging (ramp up from 21kA to 30kA)

32. List of Research Topics for VEST
1. Fusion Engineering
Plasma startup and ramp-up
Innovative divertor concept
PFC surface coating and material test
2. Plasma Transport and Stability
Confinement scaling
Effect of SOL flow on plasma rotation
Alfven wave characteristics in spherical torus
Magnetic reconnection mechanism during merging phase
Bootstrap current in spherical torus
3-D physics by internal coils like magnetic perturbations.
Asymmetric plasma characteristics in same magnetic flux surfaces
Transport & Stability studies during merging phase
Shaping effect on plasma including negative triangularity
Isotope effect as working gas
3. Heating and Current Drive
ECH&CD including mode conversion
Synergetic effect among heating schemes
4. Plasma-Wall Interactions
SOL physics
Dust