A supersonic plasma is produced quasi-steadily by use of a magneto-plasma-dynamic arcjet (MPDA) in various shapes of an external magnetic field configuration. An ion acoustic Mach number Mi of the plasma flow is limited to be nearly unity in a uniform magnetic field configuration, while it increases up to almost 3 in a divergent magnetic nozzle configuration. Spatial variations of Mi is well predicted by an isentropic model for a compressible gas. The Mach number decreases in the far downstream region due to charge-exchange collisions between flowing ions and neutral atoms which are produced through surface-recombination on the end wall. Ion heating of the fast flowing plasma has been successfully demonstrated for the first time. This success is mainly due to the plasma density is high enough to reduce the penetration of neutral gases which cause the charge-exchange energy loss. It is found that an asymmetric RF wave with an azimuthal mode number m= ± 1 is most effective to heat the ions.