TRANSPORT OF ENERGETIC IONS BY LOW-N MAGNETIC PERTURBATIONS

The stochastic transport of MeV ions induced by low-n magnetic perturb ations is studied, focusing chiefly on the stochastic mechanism operat ive for passing particles in low-frequency perturbations. Beginning wi th a single-harmonic form for the perturbing field, it is first shown numerically and analytically that the stochastic threshold of energeti c particles can be substantially lower than that of the magnetic field , contrary to earlier expectations, so that magnetic perturbations cou ld cause appreciable loss of energetic ions without destroying the bul k confinement. The analytic theory is then extended in a number of dir ections, to clarify the relation of the present stochastic mechanism t o instances already found, to allow for more complex perturbations, an d to consider the more general relationship between the stochasticity of magnetic fields and that of particles of differing energies (and pi tch angles) moving in those fields. It is shown that the stochastic th reshold is, in general, a nonmonotonic function of energy, whose form can to some extent be tailored to achieve desired goals (e.g., burn co ntrol or ash removal) by a judicious choice of the perturbation. Illus trative perturbations are exhibited that are stochastic for low- but n ot for high-energy ions, for high- but not for low-energy ions, and fo r intermediate-energy ions, but not for low or high energy. The second possibility is the behavior needed for burn control; the third provid es a possible mechanism for ash removal.