Hysteresis and mode transitions in plasma sheath collapse due to secondary electron emission

Abstract

In this experiment, hysteresis is observed in the floating potential of wall material samples immersed in a low-temperature plasma as the energy of a prevalent non-thermal electron population is varied from 30–180 eV. It is indicated that the hysteresis is due to secondary electron emission from the wall material surface. Measurements are performed in a filament discharge in argon gas pressure 10−4 Torr of order 107 cm−3 plasma number density. The primary ionizing electrons from the discharge filament make up 1%–10% of the overall plasma number density, depending on discharge voltage. Immersed LaB6-coated steel and roughened boron nitride (BN) wall material samples are mounted on the face of a radiative heater, and the wall temperature is controlled from 50–400 °C such that thermionic emission from the LaB6-coated sample is not significant. The energy of the primary plasma electrons from the discharge filament is varied and the floating potentials of the material samples are monitored. The floating potentials are observed to transition to a “collapsed” state as the primary electron energy is increased above 110 and 130 eV for the LaB6 and rough BN, respectively. As primary electron energy is subsequently decreased, the floating potentials do not “un-collapse” until lower energies of 80 and 100 eV, respectively. The hysteresis behavior agrees with a kinetic model. The results may help explain observations of global hysteresis and mode transitions in bounded plasma devices with dielectric walls, significant secondary electron emission, and departures of electron energy distribution function from a thermal Maxwellian.