1. 2010 Spring Korean Physical Society
Startup Preparation for Versatile Experiment Spherical Tokamak(VEST) at SNU
2010 Spring Korean Physical Society
Hyunyeong Lee, Choongki Sung, J. Kang,
S. Choi, K. J. Jung and Y. S. Hwang,
NUPLEX, Dept. of Nuclear, Seoul National University, San 56-1, Shillim-dong, Gwanak-gu, Seoul , Korea

2. 1 2 2 3 4 Contents Motivation & Introduction
Startup preparation – Null formation & Preionization 3
Pretest for ECRH system 4
Future Work & Summary

3. Worldwide Spherical Tokamaks(ST)
NSTX (US)
MAST (UK)
Globus-M (RF)
LATE (J)

4. Spherical Tokamak as an Alternative Concept
: Low aspect ratio (A<2) tokamak
overcomes tokamak’s drawbacks
(Low beta, large size)
Advantages of ST
Weakness of ST
High performance
(High plasma current, βlimit)
Compactness
Difficulty in start-up & sustaining
(Lack of space for solenoid)
Innovative start-up method is critical issue for ST!!
Once developing new start-up method,
high performance tokamak would be studied in ST.

5. Existing solenoid-free start-up concepts are
Motivation
Existing solenoid-free start-up concepts are
Compression-Merging method
: Use in-vessel PF coil’s swing
Disadvantage
Impurity due to in-vessel PF coil
Engineering constraints
Double Null Merging (DNM)
: Use Outer PF coil’s swing
Disadvantage : Hard to get equilibrium
[1]
Not effective as much as conventional solenoid start-up.
However, ST has not sufficient space for solenoid.
Thus, we need innovative concept for generating first target plasma.
[1] : P. Micozzi, the 11th workshop on spherical torus, St. Peterburg, 2005

6. Partial Solenoid Operation
PF coil
Solenoid start-up
The most effective method for start-up
High shaping and equilibrium ability by PF coils
Disadvantage
Hard to keep low aspect ratio
Incompatible to Spherical Tokamak
Partial solenoid operation
Keeping solenoid start-up’s merits
Maintaining low aspect ratio
Possible effective start-up in ST

7. Device Specifications
VEST in SNU
Device Specifications
Initial phase
Future
Chamber radius [m]
0.8m (middle)
0.6m (up & down)
Chamber height [m]
2.4m
Toroidal B field [T]
0.1T
0.3T
Major radius [m]
0.4m
Minor radius [m]
0.25m
0.3m
Aspect ratio
1.6
1.3
Plasma current [kA]
30kA
100kA
qedge
4.6
5.1

8. Null region formation in partial solenoid geometry.
Despite of asymmetry, high quality null is formed.
Partial solenoid can supply more volt-sec to null region than normal double null merging by PF coils.
B-null region (B < 20G)
: 0.25<r<0.5, 0.87<z<1.1
10G 5G
20G
15G

9. Start-up Analysis : Lloyd Condition for Breakdown
with pre-ionization (for 10% ionization, ECH used at r=0.33)
Lloyd Condition depending on time
t=0.1ms
t= 0.17ms
Z [m]
Z [m]
R [m]
R [m]
: Expected plasma center (R=0.33, Z=1m)
During 0.17ms, Lloyd condition is satisfied in large region if 10% ionization.
[3] B. Lloyd et al., Nucl. Fusion 31, 2031 (1991)

10. Start-up Analysis : Preionization
Lloyd condition for breakdown[3]
with pre-ionization (for 10% ionization, ECH used at r=0.33)
To meet 10% preionization, ECH preionization needs to be arranged appropriately.
Key points of the ECH preionization
Available stable ECH power
Synchronization of the 2.45GHz microwave injection
Development of ECRH system satisfying those conditions
[3] B. Lloyd et al., Nucl. Fusion 31, 2031 (1991)

11. The necessity of ECRH system
The reason of ECRH System
Necessity in preionization
Efficient and localized heating
Magnetron for the ECRH system
-> 2.45GHz microwave source is chosen.
Simple, safe, easily available and economical
-> household microwave oven
Need circuit modification!

12. The resonance zone and the ECRH system
2.45GHz
Microwave
2.45GHz
Microwave

13. The pretest system for ECRH : Helicon ion source
Waveguide(WR284)
Magnetron
The reason of choosing the helicon ion source chamber
Easy control of the magnetic field up to 900G
Available for various plasma diagnostics

14. Microwave plasma launched
In the magnetic field(875G), the plasma is launched using the magnetron
of the household oven.

15. Ion saturation current measured by Langmuir probe

16. A schematic diagram of the household oven circuit
Low voltage part: To heat filament to supply electrons to magnetron
High voltage part: To accelerate hot electron to generate microwave
Issues are
Maintaining sufficient pulse length of the microwave source
Triggering capability

17. Modification of the household oven circuit
To accomplish the goal(economical and efficient preionization)
-> To use DC power supply : not economical
-> To make a simply modified circuit.
Modification
High Voltage part
-> Use high voltage source with capacitor charged
-> Triggering microwave source by using switch
Low Voltage part
High Voltage part
Therefore, appropriate pulse length and start time can be obtained as we want.

18. Expectation of ECRH Power
duration
POWER
PULSE DURATION
FREQUENCY
MAJOR RADIUS
MINOR RADIUS
KAIST-TOKAMAK
2.5kW
450ms
2.45GHz
53cm
17cm
NSTX
15kW
50ms
18GHz
85cm
67cm
MAST
150kW
20ms
60GHz
65cm
START
1.4kW
3.5ms
Aspect ratio : 1.25
1.7kW
6GHz
SUNIST
20kW
10ms
30cm
23cm
PEGASUS
60kW
4ms
5.5GHz
33cm
28cm
In comparison with similar STs(SUNIST, PEGASUS), we estimate the appropriate ECRH power and pulse duration in VEST. : 200~250(Ws)??

19. Summary
Versatile Experimental Spherical Tokamak(VEST) in SNU is designed for partial solenoid operation.
For successful startup operation, null formation and preionization is being prepared.
ECR source will be used as a preionization tool with multiple units of microwave sources for economical and efficient startup.
Preliminary tests are ongoing.

20. Future Work
Simultaneous triggering of multiple units of magnetron sources for ECH preionization
To determine ECR power – Appropriate power for the startup of VEST
Preliminary plasma generation test – The ionization rate and the density of plasma in the pretest device
Various operation scenario for ECH-assisted startup in VEST