Ion sheath expansion for a target voltage with a finite risetime

Properties of the ion sheath expansion in plasma are investigated for a tar get voltage with a finite rise time. A theoretical model for ion sheath exp ansion is developed when the negative target voltage increases linearly. Th e theory predicts that the sheath expansion is proportional to the square r oot of time at the beginning and is proportional to the 5/6 power of time l ater on. An experimental measurement has been carried out and the measured data are compared with theoretical results. It is shown that the sheath fro nt propagates very fast at the beginning and slows down later, even for con tinuously rising negative voltage on target. (C) 1999 American Institute of Physics. [S1070-664X(99)04410-9].