Quadrupole induced resonant particle transport in a pure electron plasma

Abstract

We performed experiments that explore the effects of a quadrupole magnetic field on a pure electron plasma confined in a Malmberg-Penning trap. This work is important both as an example of resonant particle transport and for antihydrogen (H̄). The H̄ experiments plan to use magnetic quadrupole neutral atom traps to confine H̄ atoms created in double-well positron/antiproton Malmberg-Penning traps. Our results show that a quadrupole field of only 0.020 G/cm can cause significant transport when applied to a 1 cm radius plasma confined by an axial field of 100 G. Our model describes the shape of the plasma and shows that resonant electrons follow trajectories that take them on large radial excursions, leading to enhanced transport. If the electrons are off resonance, then diffusion will not be greatly enhanced. The measured diffusion scales like the square of the quadrupole field strength, inversely like the square of the axial magnetic field and, below resonance, like the square of the E×B rotation frequency. The location of the resonance in parameter space scales accordingly as we vary the length and temperature of the plasma. However, the temperature used in fitting the data differs from the independently measured temperature by a factor of four, suggesting that our description of the effect as purely diffusive is not correct.