Mach probe measurements in unmagnetized plasmas with subsonic and supersonic flow

Characteristics of an up-down type Mach probe are investigated by using a directional Langmuir probe (DLP) in unmagnetized plasmas with subsonic and supersonic flow produced by a magneto-plasma-dynamic arcjet (MPDA). The ion acoustic Mach number Mi is derived from plasma flow velocity U_p and ion temperature T_i measured by spectroscopy and electron temperature T_e by Langmuir probe. The obtained values of M_i are compared to the DLP data in various conditions of plasma flow and the coefficient of M_c in the Mach probe formula, M_i = M _c ln(J_up/ J_down), is evaluated. The obtained data are also compared with Hutchinson's PIC simulation results in an unmagnetized plasma and are in good agreement with each other. The dependence of in(J_up/J_down) on cosφ can be expressed as a quadratic function under subsonic (M_i < 1) and supersonic (M_i > 1) flow, so that the up-down type Mach probe can detect the components of plasma flow vector even when the probe collection surface inclines to plasma flow direction.