Quadrupole -induced resonant particle transport in a pure electron plasma

We performed experiments that explore the effects of a quadrupole magnetic field on a pure electron plasma confined in a Malmberg-Penning trap. Our simple model describes the shape of the plasma and shows that a certain class of resonant electrons follows trajectories that take them on large radial excursions, leading to enhanced transport. The quadrupole field destroys the cylindrical symmetry of the system, but our model predicts that if the electrons are not resonant with the quadrupole field, then diffusion will not be greatly enhanced. Our experimental results show that the plasma's shape agrees with our model, but that the diffusion does not. The plasma has the shape of a flux tube if the bulk rotation of the plasma is slower than the axial bounce motion. The plasma is cylindrical if the bulk rotation is faster than the axial bouncing. The measured diffusion scales as the square of the quadrupole field strength as expected. Some predictions of our model prove to be only approximate. The location of the resonance in parameter space scales roughly inversely with the length and proportional to the temperature of the plasma. Further, the temperature we use in fitting the data differs from an independently measured temperature by a factor of four. In addition to being an example of resonant particle transport, this effect is important for experiments that plan to use magnetic quadrupole neutral atom traps to confine antihydrogen created in double-well positron/antiproton Malmberg-Penning traps.