RF-generated trapped plasmas: Partial neutralization, vortex formation and active control

Simultaneous confinement of multiple species in a Penning-Malmberg trap, possibly with opposite sign of charge, is sometimes unavoidable or desired. This may be the case in applications like sympathetic cooling and crystallization, pair plasmas or production of neutral antimatter atoms. With respect to single-species plasmas, whose dynamics and equilibrium properties are more easily diagnosed and manipulated, partial neutralization significantly complicates the overall plasma evolution, generating or enhancing instability phenomena. We present an experimental study concerning a radio-frequency (RF) generated electron plasma, where the in-trap formation implies the presence of positive ions. Indeed, the overall dynamics towards an equilibrium configuration is affected by the continuous application of the relatively strong RF field, and the accumulation of the electron plasma is accompanied by the presence of a significant fraction of co-trapped ions (Ni/Ne≃ 10-2-10-1). We discuss the observation of new features with respect to conventional single-species plasmas, most notably non-trivial equilibrium states involving the formation of coherent structures that may persist indefinitely as long as the excitation is maintained. We show that resonant excitation schemes may be applied together with the generation drive to control the properties of the RF-produced electron plasmas. In particular the modulation of the l = 1 diocotron amplitude can be exploited to influence both positioning and charge of the electron column, or excitation of higher diocotron modes can alter the density profile increasing the peak density value.