Logo
ScienceStack
Table of Contents
Fetching ...
Sign In

Physics insights from a large-scale 2D UEDGE simulation database for detachment control in KSTAR

Menglong Zhao, Xueqiao Xu, Ben Zhu, Thomas Rognlien, Xinxing Ma, William Meyer, KyuBeen Kwon, David Eldon, Nami Li, Hyungho Lee, Junghoo Hwang

TL;DR

This work builds a large-scale 2D UEDGE database (~70,000 steady states) incorporating cross-field drifts to study detachment physics and enable real-time detachment control for KSTAR. It identifies robust detachment indicators, notably $T_ ext{e,osp} \approx 3-4\ \mathrm{eV}$ at onset, derives detachment scaling that shows weaker impurity sensitivity than 1D models, and reveals distinctive in–out asymmetries linked to KSTAR geometry. Time-dependent UEDGE simulations quantify intrinsic plasma response delays (roughly 5–15 ms at the OSP, ~25–30 ms at midplane/ISP, and ~40 ms for the LFS radiation front), which are captured by FOPDT models to support control-oriented reduced representations. Collectively, the steady-state mappings, scaling laws, and reduced-order dynamic models provide a physics-based foundation for surrogate-driven, latency-aware detachment control in KSTAR and inform extrapolations toward reactor-scale devices with careful consideration of regime validity. The results enable robust, real-time control design and surrogate-model development while highlighting areas for future extension, such as multi-charge-state impurities and experimental validation.

Abstract

A large-scale database of two-dimensional UEDGE simulations has been developed to study detachment physics in KSTAR and to support surrogate models for control applications. Nearly 70,000 steady-state solutions were generated, systematically scanning upstream density, input power, plasma current, impurity fraction, and anomalous transport coefficients, with magnetic and electric drifts across the magnetic field included. The database identifies robust detachment indicators, with strike-point electron temperature at detachment onset consistently Te around 3-4 eV, largely insensitive to upstream conditions. Scaling relations reveal weaker impurity sensitivity than one-dimensional models and show that heat flux widths follow Eich's scaling only for uniform, low D and Chi. Distinctive in-out divertor asymmetries are observed in KSTAR, differing qualitatively from DIII-D. Complementary time-dependent simulations quantify plasma response to gas puffing, with delays of 5-15 ms at the outer strike point and approximately 40 ms for the low-magnetic-field-side (LFS) radiation front. These dynamics are well captured by first-order-plus-dead-time (FOPDT) models and are consistent with experimentally observed detachment-control behavior in KSTAR [Gupta et al., submitted to Plasma Phys. Control. Fusion (2025)]

Physics insights from a large-scale 2D UEDGE simulation database for detachment control in KSTAR

TL;DR

This work builds a large-scale 2D UEDGE database (~70,000 steady states) incorporating cross-field drifts to study detachment physics and enable real-time detachment control for KSTAR. It identifies robust detachment indicators, notably at onset, derives detachment scaling that shows weaker impurity sensitivity than 1D models, and reveals distinctive in–out asymmetries linked to KSTAR geometry. Time-dependent UEDGE simulations quantify intrinsic plasma response delays (roughly 5–15 ms at the OSP, ~25–30 ms at midplane/ISP, and ~40 ms for the LFS radiation front), which are captured by FOPDT models to support control-oriented reduced representations. Collectively, the steady-state mappings, scaling laws, and reduced-order dynamic models provide a physics-based foundation for surrogate-driven, latency-aware detachment control in KSTAR and inform extrapolations toward reactor-scale devices with careful consideration of regime validity. The results enable robust, real-time control design and surrogate-model development while highlighting areas for future extension, such as multi-charge-state impurities and experimental validation.

Abstract

A large-scale database of two-dimensional UEDGE simulations has been developed to study detachment physics in KSTAR and to support surrogate models for control applications. Nearly 70,000 steady-state solutions were generated, systematically scanning upstream density, input power, plasma current, impurity fraction, and anomalous transport coefficients, with magnetic and electric drifts across the magnetic field included. The database identifies robust detachment indicators, with strike-point electron temperature at detachment onset consistently Te around 3-4 eV, largely insensitive to upstream conditions. Scaling relations reveal weaker impurity sensitivity than one-dimensional models and show that heat flux widths follow Eich's scaling only for uniform, low D and Chi. Distinctive in-out divertor asymmetries are observed in KSTAR, differing qualitatively from DIII-D. Complementary time-dependent simulations quantify plasma response to gas puffing, with delays of 5-15 ms at the outer strike point and approximately 40 ms for the low-magnetic-field-side (LFS) radiation front. These dynamics are well captured by first-order-plus-dead-time (FOPDT) models and are consistent with experimentally observed detachment-control behavior in KSTAR [Gupta et al., submitted to Plasma Phys. Control. Fusion (2025)]
Paper Structure (26 sections, 14 equations, 27 figures, 2 tables)

This paper contains 26 sections, 14 equations, 27 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Geomtry, deuterium plasma and impurity models
  3. Characteristics of KSTAR detachment physics
  4. Role of $T_\mathrm{et}$ in detachment
  5. $T_\mathrm{et}$ and Radiation front at detachment onset.
  6. Correlation of $T_\mathrm{et}$ and momentum and power loss.
  7. Detachment scaling laws
  8. Comparison with Eich’s $\lambda_q$–$B_p$ scaling
  9. Characteristics of in-out asymmetry
  10. In-out asymmetry in divertor detachment.
  11. Dependence of in-out asymmetry on control parameters.
  12. Time-Dependent UEDGE Simulations and Intrinsic Detachment Dynamics
  13. Simulation setup
  14. Plasma response to actuator timescales
  15. Impurity model validation
  16. ...and 11 more sections

Figures (27)

  • Figure 1: KSTAR equilibrium from the shot #22849@56 is used to construct the base UEDGE case.
  • Figure 2: (a) Base profiles of D, $\chi_i$ and $\chi_e$ from SOLPS-ITER simulations; (b) Examples of applying scaling factors of $0.2$ and $5.0$ to the base profiles.
  • Figure 3: Outer midplane separatrix electron temperature $T_\mathrm{e,omp}$ as a function of outer strike point electron temperature $T_\mathrm{e,osp}$ for all cases converged with $I_p = 700\,\mathrm{kA}$. The cases at outer divertor detachment onset are colored in green. Colormap represents input power.
  • Figure 4: (a) The poloidal location of the LFS radiation front as a function of $T_\mathrm{e,osp}$ for all cases with $I_p = 700\,\mathrm{kA}$. The distance away from the outer target plate is normalized by the poloidal length of the outer leg. The dashed line denotes the location of the X-point. (b) The radial location of the LFS radiation front as a function of $T_\mathrm{e,osp}$. The dashed line denotes the location of the separatrix ($\psi_N = 1.0$). For both plots, detachment onset cases are colored in green and colormap represents input power.
  • Figure 5: Outer strike point electron temperature $T_\mathrm{e,osp}$ at detachment onset as a function of outer midplane separatrix electron density $n_\mathrm{e,sep}$ (a), power flowing through the separatrix $P_\mathrm{SOL}$ (b), D scaling factor (c), and carbon fraction (d). The darkness represents the number of the converged cases. All the cases shown are with $I_p = 700\,\mathrm{kA}$. The temperature of $3.5\,\mathrm{eV}$ is denoted as the dashed line, around which most of the cases are located.
  • ...and 22 more figures