Effects of wall material, wall temperature, and surface roughness on the plasma sheath

The effect of wall material, wall temperature, and surface roughness on the near-wall plasma sheath region that exists in confined plasma devices is experimentally investigated in a multidipole-type plasma device. Smooth and rough wall material samples are immersed in a quiescent argon plasma and heated using an enclosed radiative sample heater. This will all be new data, which the authors have not yet presented or published. It is hypothesized that elevated wall material temperature may increase secondary electron emission (SEE) from the wall material due to the increased initial energy of wall electrons. SEE has been observed in previous work to decrease the magnitude of the sheath potential drop and to effect a transition to an inverse sheath potential profile. Material samples of magnesium oxide (high SEE,) HP-grade boron nitride (medium SEE,) and quartz (low SEE) are studied at temperatures from 20 to 800 °C to characterize the effect of temperature and SEE on the sheath. Plasma Debye lengths are from 0.5-5.0 mm and ion-neutral mean free paths are maintained at least two orders of magnitude greater than the Debye length in order to study the collisionless sheath regime. The potential profile of the sheaths above the wall material samples are measured using an emissive probe, employing the technique of inflection points extrapolated to zero emission. Emissive probe measurements of plasma potential are obtained at 1-5 mm intervals, and the electron current collected by the emissive probe is reported as a surrogate for electron number density in the sheath. Bulk plasma parameters are monitored using a cylindrical Langmuir probe. The derivative of the Langmuir probe characteristic is obtained to characterize the electron energy distribution function (EEDF). The plasma EEDF is manipulated by controlling the bias voltage of the emissive tungsten filaments in the plasma device, which emit and accelerate the ionizing electrons.Increased SEE wall material is se- n to decrease the magnitude of the sheath potential drop at a given electron number density and EEDF, and with increasing electron energy to effect a transition to an inverse sheath potential profile.Increased wall temperature is observed to effect the transition to an inverse sheath regime at lower plasma electron energies, consistent with an increase in SEE. Increased surface roughness also allows the sheath to transition at lower plasma electron energies, believed due to increased SEE from the increased effective surface area.