Full-wave feasibility study of anti-radar diagnostic of magnetic field based on O-X mode conversion and oblique reflectometry imaging

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, contains 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.

Prospects for a dominantly microwave-diagnosed magnetically confined fusion reactor

“…The more recent proposal and simulation of oblique reflectometry imaging [118] shows that the O-X conversion of an externally injected wave has advantages over the B-X-O conversion of internally emitted EBWs. The idea is now that, instead of a peak in transmissivity, the diagnostic characterizes a minimum in reflectivity, obtained for the same special, B-dependent direction.…”

Overdense microwave plasma heating in the CNT stellarator

“…At the same time, the plasma minor radius a was smaller, and the experiment had to be interpreted by full-wave modeling. For various physics reasons, full wave modeling in the electron cyclotron frequency range is also becoming increasingly popular in fusion devices of larger a and smaller or much smaller 0 l , where it is computationally more demanding [17,18]. Here it is argued that modeling small, low-field plasmas is a natural first step on the way to simulating larger ones.…”