Collisionality dependence of a shielding factor of a beam driven current

Honda, Mitsuru; Kikuchi, Mitsuru; Azumi, Masafumi*

The collisionality dependence on a shielding factor of a neutral beam driven current, which is generally neglected for most models currently used, is shown to be important especially for plasma parameters comparable to those of present-day tokamak experiments. Here, the shielding factor is defined as the ratio of the beam driven current to the fast-ion circulating current. An addition of friction coefficients involving beam ions to the matrix inversion method based on the moment approach readily makes it possible to estimate the collisionality-dependent shielding factor. The collisionality-dependent models proposed coincide with the collisionless model in the collisionless limit. They clearly elucidate the strong dependence of the shielding factor on collisionality: Increasing collisionality certainly decreases the shielding factor. This finding indicates that using a collisionless model always overestimates a beam driven current, more or less.