Sun-Ho Kim

The design of the KSTAR tokamak

Abstract The Korea Superconducting Tokamak Advanced Research (KSTAR) Project is the major effort of the Korean National Fusion Program (KNFP) to develop a steady-state-capable advanced superconducting tokamak to establish a scientific and technological basis for an attractive fusion reactor. Major parameters of the tokamak are: major radius 1.8 m, minor radius 0.5 m, toroidal field 3.5 Tesla, and plasma current 2 mA with a strongly shaped plasma cross-section and double-null divertor. The initial pulse length provided by the poloidal magnet system is 20 s, but the pulse length can be increased to 300 s through non-inductive current drive. The plasma heating and current drive system consists of neutral beam, ion cyclotron waves, lower hybrid waves, and electron–cyclotron waves for flexible profile control. A comprehensive set of diagnostics is planned for plasma control and performance evaluation and physics understanding. The project has completed its conceptual design phase and moved to the engineering design phase. The target date of the first plasma is set for year 2002.

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

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 (Ln) and magnetic scale length (LB) 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 measur...

Coupling study of fast wave near the lower hybrid frequency range in VEST

The fast wave branch in lower hybrid resonance frequency range, especially higher than 2 ω l h, has been proposed for the central or off-axis electron heating and current drive in higher density plasmas than the slow wave scheme. With a higher cutoff density for launching, efficient coupling between the antenna and plasma would be a priority issue for feasibility. The fast wave coupling characteristics with the wave frequency, gap size, and electron density profile of the Versatile Experiment Spherical Torus (VEST) device are investigated using a commercial full wave FEM solver, COMSOL. Maximum coupling between combline antenna and plasma is expected to be at ∼500 MHz with n ∥ ∼ 4.5. The coupled power ranges from 90% to 60% in the gap size between 0.5 cm and 1.5 cm. The relative power fraction of the fast wave is larger than 80% at these conditions. The propagation and coupling power of the fast wave is crucially dependent on the plasma density window by launching and confluence densities. Initial experimental result with low power shows that measured coupling efficiency starts to increase as electron density in front of antenna attains the level of cutoff density for the fast wave propagation. It varies from 30% to 90% with the edge density evolution, which is consistent with the coupling simulation using the measured edge density profile. Coupling simulation verified in this study will make it possible to predict and analyze the coupling characteristics of future lower hybrid fast wave experiments.The fast wave branch in lower hybrid resonance frequency range, especially higher than 2 ω l h, has been proposed for the central or off-axis electron heating and current drive in higher density plasmas than the slow wave scheme. With a higher cutoff density for launching, efficient coupling between the antenna and plasma would be a priority issue for feasibility. The fast wave coupling characteristics with the wave frequency, gap size, and electron density profile of the Versatile Experiment Spherical Torus (VEST) device are investigated using a commercial full wave FEM solver, COMSOL. Maximum coupling between combline antenna and plasma is expected to be at ∼500 MHz with n ∥ ∼ 4.5. The coupled power ranges from 90% to 60% in the gap size between 0.5 cm and 1.5 cm. The relative power fraction of the fast wave is larger than 80% at these conditions. The propagation and coupling power of the fast wave is crucially dependent on the plasma density window by launching and confluence densities. Initi...