Beam experiments in space: how do we take the charge off the spacecraft?

Abstract:

The idea of using a high intensity electron beam to actively probe magnetic field line connectivity in space has been discussed since the 1970’s. However, its experimental realization onboard a magnetospheric spacecraft has never been accomplished because of serious spacecraft charging concerns. Unlike the case of beam experiments in the ionosphere, the tenuous magnetospheric plasma cannot provide the return current necessary to keep the spacecraft charging under control and alternative pathways must therefore be sought. One such pathway is the concept in which a high density contactor plasma emitted prior to the electron beam is used to aid beam emission.

In this work, we perform Particle-In-Cell simulations to investigate the conditions under which a high intensity electron beam can be emitted from a magnetospheric spacecraft. First, we show that the electron beam cannot simply be compensated for by an ion beam of equal current, because the Child-Langmuir law is easily violated under conditions of interest. Second, we study the release of a contactor neutral plasma before beam emission and show that the presence of the contactor plasma allows beam emission for longer times. This is consistent with a simple picture where the capacitance of the system consisting of the spacecraft and the ion contactor cloud is increased, as is the time necessary for the spacecraft to charge to the beam kinetic energy and call the beam back. Last, we perform simulations where both the electron beam and the contactor plasma are emitted simultaneously. We show that, after an initial transient controlled by the size of the initial contactor cloud, where the spacecraft potential rises, the spacecraft potential can settle into conditions that allow for electron beam emission. A physical explanation of this result is presented and its implications for beam experiments in space are discussed.