Langmuir probe measurements of the secondary electron population across the cathodic pre-sheath of a DC argon discharge

Abstract

Cylindrical Langmuir probe measurements in a low-pressure DC argon discharge are used to obtain the spatial evolution of ion, cold and hot electron parameters across the cathodic pre-sheath. The cathodic pre-sheath is formed by a stainless steel plate allowed to float negatively with respect to the plasma. The velocity distribution function of the hot electron population in the pre-sheath is shown to be matched by a drifting Maxwellian that thermalizes across it. The source of the hot electron population is hypothesized to arise from secondary electron emission from the plate. A Bayesian estimation routine is proposed to compare and validate different Langmuir probe models as well as calculating the relative uncertainty between models. The results are analyzed using existing pre-sheath theory for which experimental evidence of the spatial influence of energetic electrons is lacking. The data are shown to follow Riemann's pre-sheath model that the ion-neutral mean free path λ i is proportional to the pre-sheath characteristic length l, and that the potential drop ϕ ( x ) with distance x from the sheath/pre-sheath boundary x0 follows ϕ ( x ) ∝ ( x 0 − x ) / l.