Experimental investigation of the plasma-wall transition.

We report on measurements of the ion velocity distribution as a function of distance to a target immersed in a magnetized argon plasma. Two situations are investigated: (a) practically the whole plasma streams onto a large target, and (b) the size of the target is significantly smaller than the plasma diameter. The Mach number M=u/c_{s} decreases from M=1 at the target surface to values around 0.5 at a typical scale of lambda_{a}=30 mm and lambda_{b}=5 mm, respectively. In order to explain these small decay lengths, the measurements of case (a) are compared with a source-diffusion model and those of (b) to Hutchinson's model. In (a) good agreement between modeling and experiment is obtained assuming a low neutral gas temperature. The data in (b) also agree excellently with modeled profiles, although the large fitting parameter D=20 m;{2}/s indicates that other processes than diffusion contribute significantly to the transport.