The sheath around biased objects immersed in streaming vacuum arc plasmas

Sheaths around negatively biased substrates immersed in vacuum arc plasmas were investigated using positively biased probes. Due to the high plasma density and velocity, the sheath is very thin at the upstream side of the substrate. Its thickness scales approximately with 1 mm/kV for the conditions investigated but the data can also be fitted with the theoretical (bias voltage)^4/3 law. The sheath very quickly reaches (<3 μs) its average quasi-stationary position. The actual position of the sheath edge is subject to fast fluctuations which are correlated with fluctuations of the plasma density. There is evidence that no presheath exists on the upstream side because the Bohm condition is oversatisfied