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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 151-742, Korea. Contents. Introduction Motivation & Objectives

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JongGab Jo , H. Y. Lee, Y. H. An, K. J. Chung and Y. S. Hwang*

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  1. 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 151-742, 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. IntroductionMotivation & Objectives <ECH> <EBW> • Conventional tokamak: • O1 mode or harmonics of X mode • Spherical torus: • EBW by XB or OXB mode conversion

  4. IntroductionMotivation & Objectives Polarization, cold plasma Prater,Phys. Plasmas 11, 2349 (2004) • 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. IntroductionMotivation & Objectives H. Y. LEE Bt ~875G @ 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 SetupECH System and diagnostics in VEST Triple Probe 2.45GHz, 6kW, CW • 2.45GHz, 6kW microwave generator and 3kW magnetron is installed in main chamber ofVEST. • 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, 3kW, pulse

  7. Experimental ResultThe 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 ResultThe effect of TF strength on pre-ionization with pure TF (ne)

  9. Experimental ResultThe 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) • 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 tripletcase 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. Budden Parameter

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

  11. Experimental ResultSecond 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 plasmawill be needed for second harmonic heating (FLR effect)

  12. Experimental ResultComparison between O-mode and X-mode injection X 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. O wave X wave

  13. Experimental ResultPre-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.

  14. Experimental ResultPre-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.

  15. Experimental ResultPre-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.

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