Decay of trapped-particle asymmetry modes in non-neutral plasmas in a Malmberg–Penning trap

The mechanism for the strong damping of diocotron-like azimuthal trapped-particle asymmetry modes in a Malmberg–Penning trap is investigated with a detailed three-dimensional particle-in-cell computer simulation. The m=1,kz≠0 modes are created by a voltage squeeze from a mid-detector ring followed by a displacement of trapped particles in opposite directions on either side of the ring. The voltage squeeze creates a population of particles confined to half the trap length (trapped) and a population of particles that move longitudinally along the full length of the cylinder (untrapped). The damping of the modes is found to be the result of radial transport relative to the m=1 mode (charge) center caused by transitions of particles from untrapped-to-trapped states induced by diffusion of the particles in velocity space. The transport is the immediate consequence of a difference in dynamical orbits for trapped and untrapped particles. The random walk in velocity space results in particles repeatedly changing state from trapped to untrapped and back. The dependence of the mode frequency and the exponential decay constant are explored as a function of squeeze voltage, magnetic field, and temperature in order to establish scaling behavior.