Radial profile of alpha -particle heating in a Tokamak

Abstract

An appreciable fraction of the alpha -particles produced by fusion in next generation Tokamaks or in a reactor, will be trapped in the toroidal magnetic field. The radial excursion of their orbits will exceed the width of the production profile. The broadening effect of finite orbit size on the profile of alpha -particle density, and hence of the energy transferred to the thermal plasma, is evaluated. Existing orbit analysis is not valid in the conditions of interest ( epsilon 1/2 rho theta >r, where rho theta is the Larmor radius in the poloidal field) and the orbit characteristics must first be derived. The maximum orbit width is found to be 1.64 R (2q rho /R)2/3, instead of epsilon 1/2 rho theta as predicted by the small rho theta analysis. The broadening of the alpha -heating profile will have a modest effect on the temperature distribution.