  A high-beta, supersonic plasma flow plays a crucial role in MHD phenomena in
space and fusion plasmas. There are a few experimental researches on production
and control of a fast flowing plasma in spite of a growing significance in the
magnetized-plasma flow dynamics. A magneto-plasma-dynamic arcjet (MPDA) is one
of promising devices to produce a supersonic plasma flow and has been utilized
as an electric propulsion device with a higher specific impulse and a
relatively larger thrust. We have improved the performance of an MPDA to
produce a quasi-steady plasma flow with a transonic and supersonic Mach number
in a highly-ionized state. There are two methods in order to control an
ion-acoustic Mach number of the plasma flow exhausted from an MPDA: one is to
use a magnetic Laval nozzle to convert a thermal energy to a flow energy and
the other is a combined system of an ion heating and a divergent magnetic
nozzle. The former is an analogous method to a compressible air flow and the
latter is the method proposed in an advanced thruster for a manned
interplanetary space mission. We have clarified the plasma flow characteristics
in various shapes of a magnetic field configuration. It was demonstrated that
the Mach number of the plasma flow could increase up to almost 3 in a divergent
magnetic nozzle field. This paper reports recent results on the flow field
improvements: one is on a magnetic-Laval-nozzle effects observed at the muzzle
region of the MPDA, and the other is on ICRF (ion-cyclotron-range of frequency)
heating of a supersonic plasma by use of a helical antenna.
