Anomalous inflow into rarefied plasma expansion

In this paper the physics of deep plasma expansion in the rarefied regime is studied in the framework of fast deposition, surface modification and molecule conversion. A large plasma flow, produced in a cascade arc plasma source, expands into a low pressure background. The density, temperature and velocity evolution are studied with laser scattering and fluorescence techniques. The velocity distribution functions are measured, and can accurately be described by a bimodal distribution. This bimodal velocity distribution function is interpreted in terms of an unchanged expanding component that is gradually converted into a slower component by collisions with molecules or atoms from outside the barrelshock. These particles can enter the expansion due to the rarefied conditions.