Invasion of background atoms into hot rarefied expanding plasma jets

The expansion of hot plasmas into a low pressure background is investigated in detail by laser induced fluorescence on argon metastables. The measured velocity distributions contain two components: a cool, supersonic expanding one and a hotter slow component resulting from back ground gas invasion. It is found that the invading component appears at the outside of the barrel shock and that it gradually migrates inward into the expanding jet. The supersonic component dominates the beginning of the expansion and shows all characteristics of rarefied shocks: acceleration to two times the source acoustic speed, adiabatic cooling, and a parallel temperature, which remains higher than the perpendicular. However, the invading component is much slower, but also hotter due to collisions in the expanding flow. It is present already far before the shock front