In a transonic steady flow through a tube which has a varying cross-sectional area the sonic condition can be found at the smallest cross-section if the fluid is isentropic. When viscosity and heat addition are added, the fluid is no longer isentropic. The sonic condition moves downstream.
In a plasma the heat capacities at constant pressure and at constant volume include an additional internal energy term due to ionisation. The isentropic exponent is lower and the velocity of sound is defined different in comparison to a gas. The plasma Mach number should be lower than the corresponding gas Mach number. This implies that for a transonic plasma the Mach number at the smallest cross-section should be less than one.