Budden tunnelling in parallel stratified plasmas

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Abstract
The process of Budden tunnelling of obliquely propagating extraordinary model is investigated in plasmas whose parameters vary along the magnetic field (parallel stratification). The wave tunnels through the evanescent region separating the right-hand cutoff layer from the electron cyclotron resonance. Coupled mode equations describing both ordinary and extraordinary waves are drove for arbitrary angle of incidence with respect to the magnetic field. Under appropriate conditions (nx and d In(B)/dx not too large) the coupling can be ignored, and the usual Whittaker equation is obtained for a linear magnetic field profile. It is shown that deviation from strictly parallel propagation (nx not=0) has a very small effect on tunnelling for a wide range of angle of incidence. The analytical results are verified by numerical integration of the field equations. The theory is applied to the propagation of extraordinary mode waves in the surface plasma of the ELMO Bumpy Torus devices. Fractions of incident power absorbed and transmitted to the high field region in the range of 2-15% occur for a broad spectrum of nx.