Electron Hole Oscillations below the Plasma Frequency in a Planar Gap

The paper treats an initially cold, uniform, quasi-neutral plasma within a planar gap with walls that are perfectly absorbing but incapable of emission. In the idealized case where a voltage source is instantaneously applied to the gap, we find that at early times, while ions remain virtually motionless, some electrons are first expelled from the gap, after which the remaining electrons oscillate within the gap. The oscillation frequency lies below the electron plasma frequency and is shown to correspond to the plasma frequency of the expelled electrons if their density is averaged over the full gap.