Spatial and temporal growth and collapse in a PBII plasma

The spatial and temporal growth and collapse of the ion sheath around a sph erical target during a negative high-voltage pulse have been studied in a u niform nitrogen plasma with a plasma density range of 10(9-)10(10) cm(-3) a nd an electron temperature of 1.4 eV. The transient sheath dynamics were ob served by the measurement of an electron saturation current to a Langmuir p robe. The sheath expansion speed, measured using the Langmuir probe, was hi gher than the ion acoustic speed until the sheath width approached the stea dy-state extent determined by the Child-Langmuir law. After the pulse volta ge was returned to zero (more exactly, the floating potential), the electro n current begins to recover. When the pulse fall time was longer than the p lasma transit time, which was approximately given by the propagating time o f an ion acoustic wave, the ion sheath shrank in accordance with the reduct ion 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 e xcess of plasma density was similar to a tidal wave in the generation mecha nism. This plasma tidal wave propagated into the plasma at the ion acoustic speed and its amplitude decreased exponentially with distance from the tar get. (C) 2001 Elsevier Science B.V. All rights reserved.