TIME-DEPENDENT SHEATH RESPONSE TO ABRUPT ELECTRODE VOLTAGE CHANGES

Results are presented of particle simulations of the time-dependent behavior of the plasma sheath surrounding an electrode whose potential is abruptly changed from 0 to some positive value. The surrounding plasma is assumed isotropic and collisionless and the electrode is a sphere or infinite cylinder. Potential steps in the range 10 to 104kT/e are considered. The transient behavior of the collected electron current is examined. For small (≲25kT/e) potential steps, the current quickly settles to a value consistent with existing static probe theory. Persistent Langmuir oscillations launched by larger potential steps significantly perturb the collected current, with the result that static probe theory predictions are in some cases exceeded for times as long as 1000ωp-1 after the step. Trapping of electrons during all or part of an oscillation cycle is evident in many cases. The oscillations themselves are examined and evidence for beating between modes at ωp and at the sheath-plasma parallel resonance frequency is discussed. In some cases, the oscillations are driven to large amplitudes by the ion-electron two-stream instability. © 1990 American Institute of Physics.