Collisionality dependence of a shielding factor of a beam driven current

The collisionality dependence of 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. 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. The fraction of beam-ion density influences the shielding factor, a fact which has been neglected previously. This finding indicates that using a collisionless model always overestimates a beam driven current, more or less.