Full-Wave Feasibility Study of Anti-Radar Diagnostic of Magnetic Field Based on O-X Mode Conversion and Oblique Reflectometry Imaging

TL;DR

This paper proposes a millimeter wave diagnostic method using O-X mode conversion and oblique reflectometry imaging to measure magnetic fields in tokamak plasmas, supported by 2D simulations confirming its feasibility.

Contribution

It introduces a novel diagnostic approach combining mode conversion physics and reflectometry to measure magnetic field profiles in tokamaks, with initial simulation validation.

Findings

Reflectivity minima depend on magnetic field at the cutoff

Mode conversion physics confirms diagnostic feasibility

Simulations show potential for high signal-to-noise ratio

Abstract

An innovative millimeter wave diagnostic is proposed to measure the local magnetic field and edge current as a function of the minor radius in the tokamak pedestal region. The idea is to identify the direction of minimum reflectivity at the O-mode cutoff layer. Correspondingly, the transmissivity due to O-X mode conversion is maximum. That direction, and the angular map of reflectivity around it, contain information on the magnetic field vector B at the cutoff layer. Probing the plasma with different wave frequencies provides the radial profile of B. Full-wave finite-element simulations are presented here in 2D slab geometry. Modeling confirms the existence of a minimum in reflectivity that depends on the magnetic field at the cutoff, as expected from mode conversion physics, giving confidence in the feasibility of the diagnostic. The proposed reflectometric approach is expected to yield superior signal-to-noise ratio and to access wider ranges of density and magnetic field, compared with related radiometric techniques that require the plasma to emit Electron Bernstein Waves. Due to computational limitations, frequencies of 10-20 GHz were considered in this initial study. Frequencies above the edge electron-cyclotron frequency (f>28 GHz here) would be preferable for the experiment, because the upper hybrid resonance and right cutoff would lie in the plasma, and would help separate the O-mode of interest from spurious X-waves.