Yi-Wei Zhang

Active control of scrape-off-layer (SOL) density is emerging as a critical requirement for improving ion cyclotron resonance heating (ICRH) and enabling high-performance steady-state operation in future magnetic-confinement fusion devices. Helicon wave excitation offers a promising physics-based approach to generating high-density boundary plasmas with high ionization efficiency and low impurity release. In this work, we develop THEMIS code, a fully three-dimensional (3D) multiphysics model of helicon wave propagation and power deposition in a toroidal fusion-relevant configuration, employing a finite-temperature thermal dielectric tensor and full-vessel electromagnetic simulation constrained by experimentally measured magnetic-field and density profiles. The code quantifies the relative contributions of Doppler-shifted cyclotron damping, anomalous Doppler damping, collisional damping, and Landau damping, and demonstrates that slow-wave (Trivelpiece–Gould/TG) propagation and electron Landau damping dominate the accessible heating regime in Helimak device. A comparative study of four planar antenna geometries under the present protruding-window configuration shows that geometric cutoffs and SOL density gradients severely limit power penetration into the core-accessible region. To address this constraint, we introduce a recessed-window launch scheme that positions the dielectric window inside the vacuum vessel and perform systematic parameter scans of window position, antenna geometry, and installation orientation. From these analyses, we identify the key physics-driven principles governing efficient helicon wave coupling in toroidal geometry: the importance of open-circuit termination, maximized strap length and width for enhanced slow-wave excitation, controlled inter-turn spacing, and sufficient clearance from metallic walls to avoid near-field suppression. Guided by these principles, we designed an optimized racetrack spiral antenna that increases coupling efficiency by more than an order of magnitude compared with conventional short-circuited rectangular spiral antenna. The results provide new insights into helicon wave interaction with toroidal boundary plasmas and establish a validated antenna–window co-design strategy for future helicon-assisted SOL density control and RF coupling enhancement in tokamaks.

This study explores the transitional characteristics of blue-core mode helicon discharge, which to our knowledge was not particularly focused before. Parameters are measured on a recently built advanced linear plasma device, i.e., a multiple plasma simulation linear device [Sun et al., Fusion Eng. Des. 162, 112074 (2021) and Wu et al., Plasma Sources Sci. Technol. 33, 085007 (2024)] by various diagnostics including the Langmuir probe, an optical emission spectrometer, and a standard high-speed camera. It is found that the jump direction of electron density (from low level to high level) is opposite to that of electron temperature (from a high level to a low level). Electron density increases significantly, and the radial profile becomes localized near the axis when the blue-core mode transition occurs. With increased field strength, electron density increases whereas electron temperature drops. The radial profile of electron temperature looks like a “W” shape, i.e., minimizing around the edge of the blue-core column. Electron density increases with background pressure, while electron temperature peaks around certain pressure value. High-speed videos show that the plasma column oscillates radially and experiences azimuthal instabilities with high rate once entered the blue-core mode. An electromagnetic solver based on Maxwell's equations and a cold-plasma dielectric tensor is also employed to compute the wave field and power absorption during blue-core mode transition, to provide more details that are valuable for understanding the transitional physics but not yet available in experiment. The results show that the wave field in both radial and axial directions changes significantly during the transition, its structure differs from antenna to downstream, and the power dependence of the wave magnetic field is overall opposite to that of a wave electric field. This work presents comprehensive characteristics of the transitional blue-core mode discharge and is important to both physics understanding and practical applications.

To enhance power absorption efficiency in ion cyclotron resonance heating and lower hybrid current drive systems for advanced tokamaks, this study investigates helicon wave utilization through electromagnetic simulations of wave penetration and power deposition in plasmas with sharp density and temperature gradients. Employing an axisymmetric Maxwell-equation solver electromagnetic solver incorporating cold plasma dielectric tensor, we model tokamak conditions through radial profiles combining uniform core parameters and exponentially decaying edge distributions. The analysis systematically examines wave frequency, profile gradients, and antenna lengths effects on electromagnetic field patterns and power deposition characteristics, incorporating an equivalent collision frequency model to capture Landau damping and related dissipation mechanisms. Simulation results demonstrate that maximum power absorption occurs near the last closed flux surface, primarily governed by Landau damping spatial distribution. A frequency-dependent enhancement of absorbed power emerges, accompanied by undamped zones at larger radii whose spatial extent scales with both wave frequency and plasma gradient magnitude, leading to axial standing wave formation. Extended antenna lengths promote bidirectional wave propagation. Although excluding ionization dynamics and transport processes beyond the immediate scope, these findings establish fundamental insights into helicon wave penetration and power deposition within scrape-Off Layer plasmas. This work provides critical foundations for experimental optimization of radiofrequency heating schemes in high-density tokamak regimes.