Problems associated with the injection of a high-energy neutral beam into a plasma

Several problems associated with the heating of toroidal plasmas by high-energy neutral beams are discussed. The high-energy particles slow down classically on the background plasma and the distribution function of these particles is evaluated as a function of time by solving a simplified Fokker-Planck equation. The stability of this distribution is then examined both for times before and after it reaches its equilibrium configuration. It is found that in both cases the high energy distribution is unstable unless the angle of injection is almost parallel to the field lines. Finally, the problem of the electric field which is produced by an injected neutral beam is discussed.