Effects of surface roughness on plasma sheath

In this experiment, sheath potential profiles are measured over smooth (R_a <; 0.2 um) and rough (R_a = 9.08 um) wall material samples of AX05-grade boron nitride. Argon plasmas are generated at 1.0° 10^-4 Torr-Ar using a multidipole-type plasma device. Wall material samples are positioned within the plasma and heated using an enclosed radiative sample heater. The sheath potential profiles are measured with emissive probes, and electron number densities and temperatures are measured in the bulk plasma with a planar Langmuir probe. Plasma number densities are roughly 3 × 10¹² m_-3. The electron energy distribution of the plasma is non-Maxwellian, with isotropic and directed energetic electron populations from 70 - 150 eV and hot and cold Maxwellian populations from 3.4 - 6.4 eV and 0.3 - 1.3 eV, respectively. Plasma Debye lengths range from 3 to 6 mm and ion-neutral mean free paths are maintained at least two orders of magnitude greater in order to study the collisionless sheath regime. Sheath thicknesses range from approximately 30 to 60 mm, and are smaller in the collapsed sheaths. For both rough and smooth samples, increased primary electron energy is seen to affect a transition in the sheath structure to a collapsed profile. Secondary electron emission (SEE) is inferred as the mechanism for the transition in accordance with prevalent theory. Increased surface roughness causes the sheath to transition at higher plasma electron energies, believed due to decreased SEE from geometrical obstruction of escaping electrons. When rough and smooth wall are in the same sheath regime, sheaths are similar within the measured precision.