  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.
