Measurement of Ion Mach Number of Arcjet Plasmas by a Directional Langmuir Probe Under High-Gas Pressure

Spatial profile of an ion Mach number for arcjet He and Ar plasmas are measured by using a directional Langmuir probe. The Ar arcjet has bright-dark emission structures, implying that expansion-compression waves are formed, whereas no structure is observed for the He discharge. It is found that for He plasma, the Mach number monotonically decreases along the jet axis, while in the Ar jet the prominent peaks appear. The peak positions are almost the same with the bright emission regions, indicating that there is a strong correlation between the plasma emission structure and the Mach number.