Mach probe interpretation in the presence of suprathermal electrons

The collisionless theory of Mach probes assuming isothermal, Maxwellian electrons is extended to include an isotropic, two-temperature electron distribution function. The kinetic equations for ion and electron motion in the probe wake are solved using a quasineutral particle-in-cell method, which compares qualitatively well with the results of a simple fluid model. The measured Mach number decreases slightly with increasing hot electron concentration, but the main effect is on the measured electron temperature. Due to the fact that the probe is sensitive to even a tiny population of hot electrons, the resulting ion sound speed can be overestimated by up to a factor of 2, leading to measurements of absolute flow speed that are too large.