Spatial and temporal growth and collapse in a PBII plasma

The spatial and temporal growth and collapse of the ion sheath around a spherical target during a negative high-voltage pulse have been studied in a uniform nitrogen plasma with a plasma density range of 109–1010 cm−3 and an electron temperature of 1.4 eV. The transient sheath dynamics were observed by the measurement of an electron saturation current to a Langmuir probe. The sheath expansion speed, measured using the Langmuir probe, was higher than the ion acoustic speed until the sheath width approached the steady-state extent determined by the Child–Langmuir law. After the pulse voltage was returned to zero (more exactly, the floating potential), the electron current begins to recover. When the pulse fall time was longer than the plasma transit time, which was approximately given by the propagating time of an ion acoustic wave, the ion sheath shrank in accordance with the reduction of the negative voltage. When the pulse fall time, however, was shorter than the plasma transit time, the electron saturation current overshot up to approximately twice the steady-state saturation current. The resultant excess of plasma density was similar to a tidal wave in the generation mechanism. This plasma tidal wave propagated into the plasma at the ion acoustic speed and its amplitude decreased exponentially with distance from the target.