Efficient pre-ionization by direct X-B mode conversion in VEST

Abstract

Pre-ionization experiments with pure toroidal field have been carried out in VEST (Versatile Experiment Spherical Torus) to investigate the feasibility of direct XB mode conversion from perpendicular LFS (Low Field Side) injection for efficient pre-ionization. Pre-ionization plasmas are studied by measuring the electron density and temperature profiles with respect to microwave power and toroidal field strength, and 2D full wave cold plasma simulation using the COMSOL Multiphysics is performed for the comparison. It is experimentally figured out that exceeding the threshold microwave power (>3 kW), the parametric decay and localized collisional heating is observed near the UHR (Upper Hybrid Resonance), and the efficient XB mode conversion can be achieved in both short density scale length (L-n) and magnetic scale length (L-B) region positioned at outboard and inboard sides, respectively. From the 2D full wave simulations, the reflection and tunneling of X-wave near the R-cutoff layer according to the measured electron density profiles are analyzed with electric field polarization and power flow. Threshold electric field and wave power density for parametric decay are evaluated at least more than 4.8 x 10(4) V/m and 100 W/cm(2), respectively. This study shows that efficient pre-ionization schemes using direct XB mode conversion can be realized by considering the key factors such as L-n, L-B, and transmitted wave power at the UHR. Application to Ohmic start-up experiment is carried out to confirm the effect of the preionization schemes on tokamak plasma start-up in VEST. Published by AIP Publishing.