The dynamic sheath: Objects coupling to plasmas on electron-plasma-frequency time scales

Abstract

The time-dependent interaction between solid objects and the unmagnetized plasmas in which they are immersed is investigated. To this end over 1900 high-resolution, one-dimensional particle-in-cell simulations of the plasmas surrounding cylindrical and planar objects are statistically analyzed. Numerical shot noise produces an electron-plasma-frequency ringing in the simulations, the amplitude of which is related to the plasma temperature and to the numerical system temperature. Whenever the potential of an object is rapidly biased, the surrounding plasma rings with a large amplitude at the electron-plasma frequency. During this ringing, a depletion layer forms around the object on ion-acoustic time scales. Positively charged objects discharge via plasma currents in about τpe /4 and negatively charged objects discharge in about τpi . Owing to charge separations in the plasmas, for the first few ion-plasma periods after a perturbation, the potential of an object is not directly related to the charge on it. The electron-plasma-frequency ringing drives large-amplitude Langmuir waves, which energize electrons and drive cavitation in the plasma. The fluxes of electrons reaching the objects are bursty at ωpe, and the energies of the ions striking the object slowly and systematically vary with time.

Fluid simulation of the collisionless plasma sheath surrounding an electric dipole antenna in the inner magnetosphere

“…However it was also suggested by Calder and Laframboise [1990] that plasma ringing exists due to the abrupt voltage changes which can affect the transient current collection on the electrodes for many plasma periods that cannot be accounted for in a fluid treatment. A similar analysis was made by Borovsky [1988] using a PIC approach in which he varied the potential on the electrode and noted the plasma ringing effects which were also amplified by the electron‐ion two‐stream instability.…”

Particle simulation of the auroral zone showing parallel electric fields, waves, and plasma acceleration

“…While the ion beams in the simulations are qualitatively similar to those observed in the auroral zone at high altitudes, the precipitating electron distribution at low altitudes took the form of a high-energy tail (Figure 9), rather than the sharp beam that is often observed. This is evidently the result of the high noise level that is inherent in one-dimensional PIC codes [Borovsky, 1988]; the noise can scatter electrons and thereby smooth out coherent beam-like structures. Since the noise is most intense at higher frequencies near the electron plasma frequency, its effect is strongest on electrons.…”

Transport of Current by Whistler Waves

“…Laboratory 2 and computer 3 experiments on timedependent currents have been performed; however, the steady state is assumed to have been reached once the sheath is established (typically, several plasma periods). As stated elsewhere, 3 these assumptions form the basis of many current systems studied in the laboratory and space (e.g., diagnostic probes, antenna response, beam emission, etc.). In this Letter we report in situ measurements of the magnetic-field perturbations caused by switching currents to cold electrodes in a magnetized afterglow plasma (magnetized electrons and weakly magnetized ions).…”