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Yong-Seok Hwang

Publications and source records attributed to Yong-Seok Hwang.

2 recordsLinked to original sources
arXiv2026

Mechanism of Ionization Avalanche in Tokamak Microwave Gas Breakdown

Microwave breakdown driven by electron cyclotron (EC) waves provides a non-inductive route to plasma initiation in reactor-scale tokamaks. We introduce a three-dimensional Monte Carlo simulation that, for the first time, self-consistently treats nonlinear wave-particle interactions, atomic collisions, and guiding-center transport. The Monte Carlo simulation unveils the key role of parallel Brownian motion in the ionization avalanche mechanism. The predicted breakdown boundary is validated against KSTAR experiments. This work concludes that microwave gas breakdown will be successful under ITER-relevant conditions at a D$_2$ prefill pressure near 2 mPa with 1 MW of injected EC power.

physics.plasm-ph↗
arXiv2026

Bright Spot Characterization of Low dI/dt X-pinch Plasmas using Soft X-ray Spectroscopy with Bennett Relation

This study investigates the characteristics of X-pinch plasmas driven under low current rise rate ($dI/dt$) conditions using soft x-ray spectroscopy combined with the Bennett relation. X-pinch experiments were conducted on the SNU X-pinch device using copper wires at a low $dI/dt$ of 0.2-0.3 kA/ns. The resulting 1-10 keV soft x-ray signals, measured by an x-ray filtered AXUV photodiode array (XFPA), exhibit significant nonlinear effects due to the high intensity of the soft x-ray pulses. This work characterizes the nonlinear behavior of the AXUV-HS5 Si PIN photodiodes under intense pulsed radiation using a pulsed laser. We identified a charge conservation property that the total collected charge remains proportional to the incident pulse energy despite temporal profile distortion. Based on this diagnostic finding, we developed and applied a new framework for plasma parameter estimation. By combining a spherical emission model with the Bennett equilibrium, this approach determines that the soft x-ray source plasma is a 'bright spot', characterized by a plasma density $n_e \sim 10^{21} \text{ cm}^{-3}$, size $d \sim 30-40\ μ\text{m}$, electron temperature $T_e \sim 1 \text{ keV}$, and an emission duration $t_B \sim 1 \text{ ns}$, rather than an extremely compressed 'hot spot'.

physics.plasm-ph↗