1. JongGab Jo, H. Y. Lee, Y. H. An, K. J. Chung and Y. S. Hwang*
Effective pre-ionization using fundamental extraordinary mode with XB mode conversion in VEST
JongGab Jo, H. Y. Lee, Y. H. An, K. J. Chung and Y. S. Hwang*
Department of Nuclear Engineering, Seoul National University, Seoul , Korea

2. Contents Introduction Motivation & Objectives Experimental Setup
ECH system and diagnostics in VEST
Experimental Result
Heating effect with pure toroidal magnetic field
Comparison between O-mode and X-mode injection
Pre-ionization effect on trapped particle configuration start-up
Summary & Conclusion

3. Introduction Motivation & Objectives
<ECH>
<EBW>
Device
EC Mode
ASDEX-U X2
COMPASS-D
X1, X2, O1
DIII-D
X1, X2
FTU O1
JT-60U
O1, X2
T-10
TCV
X2, X3
TEXTOR
TORE SUPRA
KSATR
LHD
W7-X
ITER
Device
MC Scenario
MAST
OXB
NSTX
OXB. XB
CDX-U XB
LATE
TST-2
W7-AS
Conventional tokamak:
O1 mode or harmonics of X mode
Spherical torus:
EBW by XB or OXB mode conversion

4. Introduction Motivation & Objectives
Prater, Phys. Plasmas 11, 2349 (2004)
Polarization, cold plasma
X1 mode has large fraction of RH component at low density and cold plasma.
Electron cyclotron damping of O1 and X2 mode is FLR effect.
For effective pre-ionization in VEST,
X1 mode with XB mode conversion must be utilized.

5. Introduction Motivation & Objectives
H. Y. LEE
Bt center
2.45GHz microwave
LFS X-mode injection produces the largest electron density in preliminary experiment in linear device.
Production of overdense plasma by XB mode conversion.
ECH launching system of VEST has been designed in a low field side injection configuration by accounting the preliminary experimental results in linear device.

6. Experimental Setup ECH System and diagnostics in VEST
2.45GHz, 6kW, CW
2.45GHz, 3kW, pulse
Triple Probe
2.45GHz, 6kW microwave generator and 3kW magnetron is installed in main chamber of VEST.
Low field side X-mode injection configuration.
WR284 / WR340 rectangular waveguide for TE10 mode propagation.
Directional coupler and rf power meter for microwave power monitoring.
A triple probe is fabricated and installed to diagnose the time varying plasma density and temperature during discharges.

7. Experimental Result The effect of ECH power on pre-ionization with pure TF
UHR
Power absorption in UHR(ne) and ECR(Te).
Initial breakdown occurs in ECR, and then UHR move outward with electron density build-up.
Doppler shift and relativistic effect in wave-particle resonance condition.

8. Experimental Result The effect of TF strength on pre-ionization with pure TF (ne)

9. (k, Ln: evaluated at the R-cutoff)
Experimental Result The effect of TF strength on pre-ionization with pure TF (ne)
Distance between the UHR and R-cutoff can be expressed by density scale length and magnetic field within the limit of
(k, Ln: evaluated at the R-cutoff)
TF Current T R C
8.2kA
0.2754
0.5251
0.1995
6.7kA, 5.4kA
0.05
0.9
3.8kA
0.123
0.7691
0.1079
Budden Parameter
Budden analysis (UHR, R-cutoff doublet)
Steep density gradient and low magnetic field are favorable to XB mode conversion.
When the TF current is 3.8kA, reflected wave from inner wall of the chamber makes situation similar to triplet case increasing mode conversion efficiency.
High density plasma is produced when the peak of density profile is near the inner wall or outer wall with the aid of high X-B mode conversion efficiency.

10. Experimental Result The effect of TF strength on pre-ionization with pure TF (Te)
2nd
1st
2nd
1st
1st
2nd

11. Experimental Result Second harmonic heating
Te [eV]
TF Current: 3.8kA
1st ECR
2nd ECR
Electron temperature peak is located in the 1st ECR at the beginning of breakdown, and then another peak near the 2nd ECR layer appears at the ECH power ramp-up phase.
Second harmonic heating is observed when both 1st and 2nd ECR layer exist in chamber but X2 mode breakdown without 1st ECR layer is fail.
Pre-heated plasma will be needed for second harmonic heating (FLR effect)

12. Experimental Result Comparison between O-mode and X-mode injection
X wave ~ X wave
O wave X wave
RF power meter with directional coupler to collect the chosen wave polarization
X-mode injection is slightly better than O-mode.
Power meter data shows that many of injected O-wave is converted into X-mode in the chamber unlike X-mode injection.
X-mode has a high rate of single pass absorption while O-mode experiences multiple reflection and then converted X-mode is absorbed in the fundamental ECR and UHR layer.

13. Experimental Result Pre-ionization effect on plasma kick up
Trapped Particle Configuration by PF 3&4
PF 3&4 make trapped particle field structure and PF 1 provide loop voltage.
Check the plasma current kick up without vertical field for force balance.
More plasma current is generated when loop voltage is applied in trapped particle configuration.

14. Experimental Result Pre-ionization effect on plasma kick up
Enhancement of pre-ionization by trapped particle configuration in overall chamber makes plasma kick up with low loop voltage of ~1V.

15. Experimental Result Pre-ionization and EBW heating effect on plasma current
~400ms
~400ms
Plasma current of ~8kA is sustained using additional vertical field for force balance.
Enhanced pre-ionization plasma by trapped particle configuration.
Current ramp-up rate, maximum current and pulse length are increased as TF strength decrease.
Effect of pre-ionization and EBW heating.

16. Summary & Conclusion
Fundamental X-wave injected from low field side is absorbed in UHR (ne) and fundamental ECR (Te) layer.
High density plasma is produced when the peak of density profile is near the inner wall or outer wall with the aid of high X-B mode conversion efficiency.
O-wave injected from low field side is converted into X-mode in the chamber and then absorbed with lower absorption efficiency.
Plasma current ramp-up rate and pulse length are increased by effective pre-ionization and consequent higher heating efficiency.