Simulations of pulsed electron beam injection during active experiments

Abstract

During electron beam injection the spacecraft can become positively charged, reaching potentials as high as the beam energy, particularly at currents exceeding about 100 mA. After beam turnoff the spacecraft can become negatively charged and the current system around the spacecraft can be strongly modified. The dynamics of such an alternating current system are of particular importance in understanding results from pulsed beam injection. Two‐dimensional electromagnetic particle simulations are used to investigate the characteristics of the spacecraft charging, particle acceleration, and wave emissions during beam injection, turnoff and subsequent pulsing of the beam. It is shown that during beam injection the beam current is neutralized by a spatially separate return current region extending several tens of meters from the beam region, with the currents being closed across the field lines by the perpendicular acceleration of ambient plasma ions into the beam region. After beam turnoff this current system reverses after a time lag of about an ion plasma period. The current reversal is accompanied by (1) prolonged electron collection by the spacecraft near the beam region, (2) preferential ion collection by sections of the spacecraft magnetically connected to the initial return current regions, and (3) the creation of hot plasma extending well into the return current regions. Because of the time lag, the currents induced in the plasma during periodic beam injection can be strongly modified from the imposed beam current. The resultant wave spectrum is dominated by (1) the fundamental pulsing frequency if the pulse frequency ωp is much greater than the ion plasma frequency ωpi, (2) odd harmonics if ωp ≳ ωpi, and (3) even harmonics if ωp ≲ ωpi. This dependence on ωp/ωpi is proposed as the reason for the different spectra during VLF pulsing of an electron beam during the shuttle STS‐3 experiments.