EFFECT OF PITCH-ANGLE SCATTERING ON THE ALPHA-PARTICLE BOOTSTRAP CURRENT IN THE NEAR-AXIS REGION OF THE TOKAMAK

A kinetic equation is derived for the neoclassical distribution functi on of alpha particles with orbits intersecting or approaching the magn etic axis of a tokamak. This equation takes into account both the coll isional slowing down and the pitch-angle scattering of alpha particles . An equation with a simplified pitch-angle scattering term is solved analytically, and the distribution function obtained is used to find t he alpha particle bootstrap current at the magnetic axis. It is shown that the pitch-angle scattering leads to an alpha-particle current in the near-axis region that is larger than the one predicted from early neoclassical theory, which allows only for the slowing down of alpha p articles.