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 Department of Nuclear Engineering, Seoul National University, Seoul , Korea

1 /15 1.Introduction Motivation & Objectives 2.Experimental Setup ECH system and diagnostics in VEST 3.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 4.Summary & Conclusion Contents

2 /15 Introduction Motivation & Objectives DeviceEC Mode ASDEX-UX2 COMPASS-DX1, X2, O1 DIII-DX1, X2 FTUO1 JT-60UO1, X2 T-10X2 TCVX2, X3 TEXTORX2 TORE SUPRAO1, X2 KSATRX2 LHDO1, X2 W7-XX2 ITERO1 DeviceMC Scenario MASTOXB NSTXOXB. XB CDX-UXB LATEOXB TST-2XB W7-ASOXB  Conventional tokamak: O1 mode or harmonics of X mode  Spherical torus: EBW by XB or OXB mode conversion

3 /15  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. Introduction Motivation & Objectives Polarization, cold plasma Prater, Phys. Plasmas 11, 2349 (2004)

4 /15  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. Introduction Motivation & Objectives B t center 2.45GHz microwave H. Y. LEE

5 /15 Experimental Setup ECH System and diagnostics in VEST  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. 2.45GHz, 6kW, CW 2.45GHz, 3kW, pulse Triple Probe

6 /15  Power absorption in UHR(n e ) and ECR(T e ).  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. Experimental Result The effect of ECH power on pre-ionization with pure TF UHR

7 /15 Experimental Result The effect of TF strength on pre-ionization with pure TF (n e )

8 /15 Distance between the UHR and R-cutoff can be expressed by density scale length and magnetic field within the limit of.  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. TF CurrentTRC 8.2kA kA, 5.4kA kA (k, L n : evaluated at the R-cutoff) Budden Parameter Experimental Result The effect of TF strength on pre-ionization with pure TF (n e )

9 /15 1 st 2 nd 1 st 2 nd 1 st 2 nd Experimental Result The effect of TF strength on pre-ionization with pure TF (T e )

10 /15  Electron temperature peak is located in the 1 st ECR at the beginning of breakdown, and then another peak near the 2 nd ECR layer appears at the ECH power ramp-up phase.  Second harmonic heating is observed when both 1 st and 2 nd ECR layer exist in chamber but X2 mode breakdown without 1 st ECR layer is fail.  Pre-heated plasma will be needed for second harmonic heating (FLR effect) Experimental Result Second harmonic heating Te [eV] TF Current: 3.8kA 1 st ECR2 nd ECR

11 /15 Experimental Result Comparison between O-mode and X-mode injection X wave ~ X wave O wave X wave  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. RF power meter with directional coupler to collect the chosen wave polarization

12 /15 Experimental Result Pre-ionization effect on plasma current 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.

13 /15 Experimental Result Pre-ionization effect on plasma current kick up  Enhancement of pre-ionization by trapped particle configuration in overall chamber makes plasma current kick up with low loop voltage of ~1V.

14 /15  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. ~400ms Experimental Result Pre-ionization and EBW heating effect on plasma current

15 /15 Summary & Conclusion  Fundamental X-wave injected from low field side is absorbed in UHR (n e ) and fundamental ECR (T e ) 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.