arXiv subjects

Hyun-Kyung Chung

Publications and source records attributed to Hyun-Kyung Chung.

4 recordsLinked to original sources

arXiv2026

Collisional-radiative data for tokamak disruption mitigation modeling

Effective tokamak disruption mitigation is crucial for ensuring the safety and integrity of fusion power reactors. Accurate collisional-radiative (CR) modeling of a radiative plasma is a critical component in predictive disruption mitigation design. In this paper, we focus on quasi-steady-state CR modeling applicable to the current quench phase of a tokamak disruption. We employ the ATOMIC collisional-radiative code from the Los Alamos suite and the newly developed Fusion Collisional-Radiative (FCR) code to model the atomic processes, providing high-fidelity data for radiative power loss, as well as average and effective charge states for hydrogen, helium, neon, and argon plasma species over a wide range of tokamak-relevant electron temperatures and electron densities. Fine-structure-resolved CR models are used for hydrogen and helium plasma species, while configuration-average CR models are implemented for neon and argon plasma species. The calculated values are compared with the superconfiguration CR model (FLYCHK) and the commonly used coronal equilibrium approximation to demonstrate the advantages and limitations of each model. To facilitate coupling of high-fidelity CR data to plasma simulation models, we represent the ATOMIC/FCR results over the relevant plasma parameter range using a smooth tensor product B-spline surface in electron temperature and electron density. This approach yields compact coefficient tables that can be evaluated efficiently while preserving spline smoothness across the domain. These data were previously used to examine ways to minimize runaway electrons in a tokamak current quench, and they are now made available in easy-to-use forms for community use and benchmarking.

physics.plasm-ph↗

arXiv2022

The constraint of plasma power balance on runaway avoidance

In a post-thermal-quench plasma, mitigated or unmitigated, the plasma power balance is mostly between collisional or Ohmic heating and plasma radiative cooling. In a plasma of atomic mixture $\{n_α\}$ with $α$ labeling the atomic species, the power balance sets the plasma temperature, ion charge state distribution $\{n_α^i\}$ with $i$ the charge number, and through the electron temperature $T_e$ and ion charge state distribution $\{n_α^i\},$ the parallel electric field $E_\parallel.$ Since the threshold electric field for runaway avalanche growth $E_{av}$ is also set by the atomic mixture, ion charge state distribution and its derived quantity, the electron density $n_e,$ the plasma power balance between Ohmic heating and radiative cooling imposes a stringent constraint on the plasma regime for avoiding and minimizing runaways when a fusion-grade tokamak plasma is rapidly terminated.

physics.plasm-ph↗

arXiv2022

Non-thermal evolution of dense plasmas driven by intense x-ray fields

The advent of x-ray free-electron lasers (XFELs) has enabled a range of new experimental investigations into the properties of matter driven to extreme conditions via intense x-ray-matter interactions. The femtosecond timescales of these interactions lead to the creation of transient high-energy-density plasmas, where both the electrons and the ions may be far from local thermodynamic equilibrium (LTE). Predictive modelling of such systems remains challenging because of the substantially different timescales on which electrons and ions thermalize, and because of the vast number of atomic configurations that are required to describe the resulting highly-ionized plasmas. Here we explore the evolution of systems driven to high energy densities using CCFLY, a non-LTE, Fokker-Planck collisional-radiative code. We use CCFLY to investigate the evolution dynamics of a solid-density plasma driven by an XFEL, and explore the relaxation of the plasma to local thermodynamic equilibrium on femtosecond timescales in terms of the charge state distribution, electron density, and temperature.

physics.plasm-ph↗

arXiv2019

Impact of a minority relativistic electron tail interacting with a thermal plasma containing high-atomic-number impurities

A minority relativistic electron component can arise in both laboratory and naturally-occurring plasmas. In the presence of high-atomic-number ion species, the ion charge state distribution at low bulk electron temperature can be dominated by relativistic electrons, even though their density is orders of magnitude lower. This is due to the relativistic enhancement of the collisional excitation and ionization cross sections. The resulting charge state effect can dramatically impact the radiative power loss rate and the related Bethe stopping power of relativistic electrons in a dilute plasma.

physics.plasm-ph↗