Fast measurement of picoamp plasma flows using trapped electron clouds

We demonstrate that magnetized electron clouds can diagnose picoamp ion currents (or equivalent neutralized plasma flows) on a kHz time scale. This could be used to measure the dynamics of neutral plasma losses to the walls, e.g., along divertor field lines. In essence, a current passing through an electron cloud in a Penning trap transfers angular momentum to the cloud, driving an easily measured orbital “diocotron” instability (from ion currents) or orbital damping (from electron currents). With neutralized plasma flows, the predominant effect is from the lower velocity (i.e., higher density) charge species. Experiments with electron, ion, and neutralized currents have fully characterized this collective (collisionless) electrostatic interaction, and demonstrate the picoamp and kHz resolutions.