The behaviour of heavy particles in the expanding plasma jet in argon

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Abstract
This paper concerns the behaviour of heavy particles in an argon plasma expanding from a cascaded arc. The plasma is characterized using high-accuracy spatially resolved Thomson-Rayleigh measurements. It is shown that the expansion of the heavy particles, neutral particles and ions, is close to adiabatic and that three-particle recombination has a small effect on the ion density. The measurements are compared with an adiabatic model and are found to be in good agreement. The behaviour of the neutral particles in the shock front is independent from that of the ions. The neutral particle shock front is identical to the shock front found in neutral gases and is accordingly characterized by the neutral particle Mach number. It is experimentally confirmed that the shape of the shock front is of the Mott-Smith type. The motion of the ions in the shock front is influenced by the presence of the electrons in such a way that the electrons are compressed by the ions. The shape of the ion shock front is of the Mott-Smith type, however, with a different Mach number which includes the electron temperature.