1. International Youth Conference on Fusion Energy Conference
The Development of Pre-ionization System for Versatile Experiment Spherical Tokamak(VEST) at SNU
International Youth Conference on Fusion Energy Conference
Hyunyeong Lee, Junghoon Yang, 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
Modification of the operation circuit 3
Pretest for estimation of the power for pre-ionization 4
Summary & Future Work

3. 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.

4. Innovative Startup Method - 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

5. Versatile Experiment Spherical Tokamak(VEST) in SNU
Using new startup method, VEST has been designed.
Device Specifications of VEST
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

6. 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.
[1] B. Lloyd et al., Nucl. Fusion 31, 2031 (1991)

7. Null region formation in partial solenoid geometry.
To accomplish 10% ionization rate, high quality null is essential.
Despite of asymmetry, high quality null is formed.
B-null region (B < 20G)
: 0.25<r<0.5, 0.87<z<1.1
10G 5G
20G
15G

8. The ECRH for pre-ionization
The reason of ECRH pre-ionization
Necessity in pre-ionization
Efficient and localized heating
Magnetron for the ECRH
-> 2.45GHz microwave source is chosen.
Simple, safe, easily available and economical
-> Multiple units of household microwave oven[2]
Objective
The development of pre-ionization system using the multiple units of magnetrons for household microwave oven.
[2]

9. The pretest chamber : Helicon Plasma Chamber
Preliminary Experiment : To confirm the feasibility of pre-ionization system using the household microwave oven
Stainless Steel Tube
Waveguide(WR284)
Magnetron
The reason of choosing the helicon plasma chamber[3]
Easy control of the magnetic field up to 875G
Available for various plasma diagnostics
[3] S.H. Kim et al., Plasma Phys. Control. Fusion. 50, (2008)

10. Hydrogen plasma with 60Hz frequency
The result of the pretest using the original circuit of microwave oven
-> The hydrogen plasma is generated at 60Hz.
-> AC operation(220V, 60Hz, 8ms)
-> Half wave rectifier in the circuit makes this flickering signals.

11. Modification of magnetron operation circuit
Isolate Transformer
DC Power Supply
Negative PS
Original operation circuit[4]
Low voltage part: To heat electrode for thermionic emission
High voltage part: To accelerate hot electrons
Modifications
DC power supply to heat electrode provides 10A.
Isolate transformer makes the whole circuit of the magnetron independent.
Negative high voltage power supply provides -3.5kV to the circuit.
In negative PS, the microwave power is controlled by the emission current.
[4] W. Choi et al., Rev. Sci. Instrum. 71, 2728 (1999)

12. The Result of the Preliminary Experiment
Hydrogen plasma density vs. microwave power under varying the pressure
Features
The lower pressure is, the higher plasma density is acquired.
The higher the microwave power gets, the more density is acquired.

13. Estimation of the whole power for pre-ionization system
Pressure
(mTorr)
The ionization rate per unit power
(#/W∙m^3)
0.4
3.78ⅹ1014
2.9
1.21ⅹ1014
8.5
3.76ⅹ1013
The volume of the hydrogen plasma
The pretest chamber: cylinder shape, diameter(8cm), length(55cm)
-> VEST: torus shape, major radius(40cm), minor radius(15cm)
To accomplish 10% ionization rate
Above 3.8kW of RF power is necessary(0.4mTorr).

14. Modification of the circuit for Pulse Operation
Low Voltage part
High Voltage part
The necessity of the modification
1. VEST will be operated in pulse mode(10ms).
2. Triggering the signal is essential using the multiple units of magnetron(1kW)
High Voltage Capacitor ( ) will be determined by the operation time.
This modified operation circuit will soon be completed.

15. Summary
Versatile Experimental Spherical Tokamak(VEST) in SNU is designed for partial solenoid operation.
For successful startup operation, pre-ionization system is prepared using modified magnetron circuit of household microwave oven.
Above 3.8kW of microwave power is estimated for 10% pre-ionization through the ionization rate per unit power.
The more modification of the circuit is necessary for pulse operation.

16. Future Works
To complete the whole pre-ionization system for pulse operation in VEST
Various operation scenario for ECH-assisted startup in VEST
For successful startup, the research of the null quality
The generation of the first plasma in VEST finally