Theory of ion-cyclotron resonances and diffusion at high energy in the 2XIIB magnetic mirror experiment

It is shown that a monochromatic ion-cyclotron wave can cause stochastic ion motion at high energy in a mirror plasma, as observed in the 2XIIB Magnetic Mirror Experiment. In earlier studies Rosenbluth, Timofeev, and Smith predicted regular (superadiabatic) motion at energies above 10 or 20 keV, in a slab (or small Larmor radius) model. Particle motion is described here by a Hamiltonian valid for arbitrary Larmor and guiding-center radii, in cylindrical geometry, and in either axisymmetric or quadrupole magnetic fields. The weakness of the self-dug magnetic well at moderate plasma pressure is used as an expansion parameter. Analytical and numerical calculations show large, stochastic changes in the particles’ kinetic energy, in the range of parameters observed in the experiment.