Simulation studies of the behavior of positrons in a microtrap with long aspect ratio

Abstract

The charged particles storage capacity of microtraps (micro-Penning-Malmberg traps) with large length to radius aspect ratios and radii of the order of tens of microns was explored. Simulation studies of the motions of charged particles were conducted with particle-in-cell WARP code and the Charged Particle Optics (CPO) program. The new design of the trap consisted of an array of microtraps with substantially lower end electrodes potential than conventional Penning-Malmberg traps, which makes this trap quite portable. It was computationally shown that each microtrap with 50 micron radius stored positrons with a density (1.6x10^11 cm^-3) even higher than that in conventional Penning-Malmberg traps (about 10^11 cm^-3) while the confinement voltage was only 10 V. It was presented in this work how to evaluate and lower the numerical noise by controlling the modeling parameters so the simulated plasma can evolve toward computational equilibrium. The local equilibrium distribution, where longitudinal force balance is satisfied along each magnetic field line, was attained in time scales of the simulation for plasmas initialized with a uniform density and Boltzmann energy distribution. The charge clouds developed the expected radial soft edge density distribution and rigid rotation evolved to some extent. To reach global equilibrium (i.e. rigid rotation) longer runs are required. The plasma confinement time and its thermalization were independent of the length. The length-dependency, reported in experiments, is due to fabrication and field errors. Computationally, more than one hundred million positrons were trapped in one microtrap with 50 micron radius and 10 cm length immersed in a 7 T uniform, axial magnetic field, and the density scaled as r^-2 down to 3 micron. Larger densities were trapped with higher barrier potentials.