Experimental investigations with respect to the applicability of the Bohm criterion

The streaming of a plasma in a magnetic nozzle field in front of a terminating target is studied by means of laser induced fluorescence. In contrast to expectations basing on Bohm's criterion it is found that the flow is generally sub-sonic in the region behind the last magnetic coil. Half-sided Maxwellian ion-distribution functions with Mach numbers around 0.5 are typical for standard recycling conditions within a distance of the mean free path length from the target. Larger flow velocities, possibly exceeding M = 1, are however observed under maximum gas pumping conditions. The importance of the recycling neutrals with respect to the streaming behavior is also corroborated by numerical simulations.