Plasma, presheath, collisional sheath and collisionless sheath potential profiles in weakly ionized, weakly collisional plasma

Potential variations in bulk plasma, presheath and sheath describe the plasma potential profile responsible for ion acceleration out of weakly ionized, weakly collisional plasma. Experiments with emissive probes, Langmuir probes, laser induced fluorescence (LIF) and Mach probes in a multi-dipole plasma show that the presheath potential near a negatively biased plate scales as -e phi/T-e = root(x(0)-x)/lambda and is insensitive to the value of the plate bias. Plasma parameters were chosen so that epsilon =lambda(D)/lambda approximate to 0.02 -> 0.06 and 0.2 < lambda/L < 0.6, where; is the ion-neutral mean free path, lambda(D) is the Debye length and L is the plasma length. A Child-Langmuir-like sheath with width scaling as lambda(D)(e phi/T-e)(3/4) with a T-e/lambda(D) initial boundary electric field has been measured. A transition region, in which the plasma starts to deviate from being quasi-neutral, is observed between the presheath and the Child-Langmuir sheath. The transition region is approximately 2 lambda(D) or lambda(1/5)lambda(4/5)(d). d Mach probe data, calibrated by LIF data, suggest that the average ion velocity reaches the Bohm velocity inside the transition region and significantly exceeds the Bohm velocity at the Child-Langmuir sheath/transition boundary.