Theory of Bernstein wave coupling with loop antennas

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Abstract
Coupling to ion Bernstein waves near the first harmonic of the ion cyclotron resonance with coil antennas is investigated by using a plane layered model of a tokamak plasma. The boundary conditions in vacuum are solved analytically for arbitrary orientation of the antenna and Faraday screen conductors, in terms of the surface impedance matrix of the plasma for plane waves. The latter is evaluated by solving the wave equations in the plasma by taking into account finite Larmor radius and finite electron inertia effects, cyclotron and harmonic damping by the ions, and Landau and collisional damping by the electrons. – Applications to the Alcator C tokamak give reasonable agreement between the calculated and measured radiation resistance when the first ion cyclotron harmonic is just behind the antenna; outside this range, the calculated resistance is lower than the experimental one. In general, the coupling efficiency is found to be very sensitive to the edge plasma density, good coupling requiring a low density plasma layer in the vicinity of the Faraday screen. Coupling also improves with increasing ion temperature in the scrape-off layer and is appreciably better for antennas with antisymmetric than with symmetric current distribution in the toroidal direction.