Mode conversion processes for electron Bernstein wave excitation in spherical tori

Summary form only given. Transport in spherical tori should be reduced, compared to more traditional large aspect ratio tokamaks, because of the flow shear and geometric configuration. Moreover, because of the high safety factor, disruptions should be reduced. Neither the O-mode nor the X-mode are appropriate for heating or driving non-inductive currents in the plasma core at these high beta over-dense plasma conditions. O-X-EBW conversion is examined for frequencies under consideration for operation in MAST. The absorbed power is estimated to yield constraints on required beam alignment and polarization. For O-X-EBW mode conversion, the transition of the O-mode though the plasma resonance region at oblique incidence is crucial. Two different density profiles are considered: (a) a simple parabolic profile with exponential decay in the SOL, and (b) a flat-topped profile with sharp density gradients at the plasma resonance region and SOL. Neglecting magnetic shear and toroidal effects, a full-wave investigation of wave propagation in a warm inhomogeneous magnetized plasma is performed using finite-elements. At 60 GHz, the incident wave will pass through the plasma resonance only if the beam is extremely well collimated if there is an angular beam divergence of even several degrees there will be a very detrimental in power absorption efficient or EBW excitation efficiency. This result is also found for the density profiles with very steep gradients. On the other hand, the 28 GHz wave readily penetrates the plasma resonance region for arbitrary density profiles and for moderately divergent beams. Since the UHR is located near the plasma boundary, the toroidal effects and magnetic shear should have negligible effect.