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 (Toroidal Helicon ElectroMagnetic Integrated Solver) model and 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 along the direction parallel to the magnetic field, and landau damping (LD), and demonstrates that slow-wave (Trivelpiece–Gould/TG) propagation and electron LD 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 (SA) that increases absorption efficiency by more than an order of magnitude compared with conventional short-circuited rectangular SA. 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 radio-frequency coupling enhancement in tokamaks.
Three-dimensional structures of ICRF waves in tokamak plasmas