SIMULATION OF PULSED ELECTROPOSITIVE AND ELECTRONEGATIVE PLASMAS

A particle-in-cell code with nonperiodic boundary conditions, including ionization and ion motion, is used to simulate the approach to equilibrium and the decay in the postdischarge of model electropositive and electronegative plasmas in a symmetric RF diode. In the electropositive plasma the density decreases to 1/e in approximately 10-mu-s. The electronegative ion plasma density is about four times higher and decreases to 1/e in, approximately 50-mu-s, with the electron temperature and density decreasing to zero in a few microseconds. During this latter time scale, a weak field is set up to drive the negative ions to the boundaries at the same rate as the ion diffusion velocity. In the postdischarge this is close to thermal, hence some hundreds of microseconds are required to remove most of the negative ions from the system. As negative ions are thought to be the precursors for particulate formation, plasmas pulsed at around 1 KHz will severely decrease the lifetime of the negative ions and thereby reduce the possibility of particulate growth.