arXiv subjects

Eisung Yoon

Publications and source records attributed to Eisung Yoon.

5 recordsLinked to original sources

arXiv2026

Fast ion effects on the threshold conditions of ion temperature gradient mode and electron temperature gradient mode

We investigate the fast ion effects on the threshold conditions of ion temperature gradient (ITG) mode and electron temperature gradient (ETG) mode both analytically and numerically using gyrokinetic equation. The onset condition for ITG mode shows a strong and monotonic favorable dependence on the fraction of fast ions, and mostly favorable but non-monotonic dependence on the fast ions' normalized temperature $T_f/T_i$ ($T_f$ is the effective temperature of fast ions, $T_i$ is the temperature of thermal ions). Overall favorable parametric trends are consistent with those for the linear growth rate reported in previous papers, as they are largely determined by kinetic wave-particle resonance effects. While general analytic expressions for the critical normalized thermal ion temperature gradient scale length $(R/L_{T_i})_c$ are quite complicated, an explicit compact expression $\left(\frac{R}{L_{T_i}}\right)_c=\left(\frac{4}{3}+\frac{3}{2}\sqrt{\frac{\pi}{2}}\frac{|\hat{s}|}{q}\right)\left(1+\frac{T_i}{Z_i(1-f_h)T_e}\right)$ has been derived for the mode with its perpendicular scale larger than thermal ion gyroradius, but much smaller than the fast ion gyroradius so that finite Larmor radius effects are manifested in opposite asymptotic limits depending on ion species when $T_f\gg T_i$, and weak density gradient. Here, $q$ is safety factor, $\hat{s}$ is magnetic shear, $Z_i$ is thermal ions' charge, and $f_h$ is fast ion charge density fraction. In this limit, only the fast-ion-induced thermal ion dilution effects persist as fast ion density response becomes unmagnetized and negligible. On the other hand, the fast ion effects on ETG-threshold are found to be unfavorable.

physics.plasm-ph↗

arXiv2025

Data-Driven Reduced-Order Modeling of Phase Mixing Dynamics from Particle Kinetic Simulation

Phase mixing is a fundamental kinetic process that governs dissipation and stability in collisionless plasmas, but its inherent filamentation in velocity space creates major challenges for both high-fidelity simulations and reduced-order modeling. This work presents the first exploratory evaluation of a joint Proper Orthogonal Decomposition and Sparse Identification of Nonlinear Dynamics (POD-SINDy) framework applied to particle-in-cell simulations of phase mixing. Simulation datasets were generated under progressively complex conditions, starting from a passive kinetic case without self-consistent electric fields, extending to self-consistent simulations with nonlinear electric field feedback, and finally to a noisy dataset with reduced particle resolution. In the passive kinetic regime, POD-SINDy achieved near-optimal reconstructions with only five modes, reproducing filamentation with errors below four percent. In self-consistent electrostatic cases, variance spread across more modes due to nonlinear interactions and noise, slowing singular value decay and making strict low-rank embeddings more demanding. Nevertheless, retaining ten modes was sufficient to recover the dominant structures, yielding reconstruction errors of about seven percent for the low-noise case and thirteen percent for the noisy dataset. Across all scenarios, SINDy provided sparse and interpretable equations for modal amplitudes that remained predominantly linear despite the underlying nonlinear data, while POD truncation effectively filtered particle noise and preserved coherent dynamics. These findings demonstrate that POD-SINDy constitutes a compact and interpretable approach to reduced-order modeling of phase mixing, capable of retaining essential physics across regimes of increasing complexity while achieving data compression from three to five orders of magnitude depending on dataset complexity.

physics.plasm-ph↗

arXiv2025

Predictive study of non-axisymmetric neutral beam ion loss on the upgraded KSTAR plasma-facing components

We simulate ion loss induced by neutral beam injection (NBI) in three-dimensional (3D) space with high fidelity on the plasma-facing components (PFCs) of the Korea Superconducting Tokamak Advanced Research (KSTAR) device. Utilizing a 3D collision detection routine added to the NuBDeC code and computer-aided design data reflecting the recent upgrade to a tungsten divertor, we have characterized 3D heat flux distribution and patterns over PFCs due to the NB ion loss throughout parameter scans. First, we identify axial asymmetry in the heat flux distribution. The plasma-wetted areas extend along the direction of the plasma current and deviate diagonally in the direction of the NB ion's poloidal turn. Additionally, on the PFC surfaces, local heat flux peaks are observed along both poloidal and toroidal directions. These local peaks emerge on the surfaces of the PFCs that protrude toward the regions swept by the NB ions. Second, through a case study of NB1-C (beam source C of neutral beam 1) that results in the most loss, we analyze several changes in the heat flux patterns observed on the divertor and the poloidal limiter. Such analysis allows us to examine how variations in the parameters affect the movement of the peak heat flux position and the extent of the plasma-wetted area, if formed. We observe that the ion loss increases under several conditions: shallower beam deposition, higher beam energy, larger poloidal beta, and lower plasma current. Ions born near the plasma edge due to shallow beam injection follow orbits with a larger minor radius. Flux surface shifts and ion drifts from changes in beam energy, poloidal beta, and plasma current move these orbits closer to the wall. These factors increase the chance of ion loss through wall collisions. This study is believed to help optimize design and operation of NBI systems.

physics.plasm-ph↗

arXiv2024

Verification of Fast Ion Effects on Turbulence through Comparison of GENE and CGYRO with L-mode Plasmas in KSTAR

This study presents a cross-verification of fast ion effects on turbulence through a systematic comparison of two leading gyrokinetic codes, GENE [F. Jenko et al., Phys. Plasmas 7 1904-1910 (2000)] and CGYRO [J. Candy et al, J. Comput. Phys. 324 73-93 (2016)], using L-mode plasma profiles from KSTAR for local linear and nonlinear electromagnetic simulations. The focus is on the impact of fast ions and rotation effects on energy flux, aiming to identify the similarities and differences between these codes in the context of turbulence transport research. The analysis shows consistency in linear stability results, fractional changes in energy flux, changes in the distribution of energy fluxes, fluctuations and phase angle with fast ions, and zonal shearing between the codes. However, discrepancies arise in absolute thermal energy levels and rotation effects on energy transport, especially in the presence of fast ions. The study underscores the critical importance of phase angle analysis in gyrokinetic code verification, particularly when assessing fast ion effects on turbulence. Additionally, it highlights the need to examine quantities at lower levels of the primacy hierarchy, as discrepancies at lower levels can lead to divergent results at higher levels. These findings indicate the necessity for further investigation into these discrepancies and the novel phase angle structures observed, contributing to the advancement of accurate transport predictions in fusion plasmas.

physics.plasm-ph↗

arXiv2020

Effects of plasma turbulence on the nonlinear evolution of magnetic island in tokamak

Magnetic islands (MIs), resulting from a magnetic field reconnection, are ubiquitous structures in magnetized plasmas. In tokamak plasmas, recent researches suggested that the interaction between the MI and ambient turbulence can be important for the nonlinear MI evolution, but a lack of detailed experimental observations and analyses has prevented further understanding. Here, we provide comprehensive two-dimensional observations that indicate various effects of the ambient turbulence on the nonlinear MI evolution. It is shown that the modified plasma turbulence around the MI can lead to either destabilization or stabilization of the MI instability in tokamak plasmas. In particular, significantly enhanced turbulence at the X-point of the MI results in a violent disruption through the fast magnetic reconnection and magnetic field stochastization.

physics.plasm-ph↗