  We report experimental results on the parameters, structure, and evolution of
high-Mach-number (M) argon plasma jets formed and launched by a
pulsed-power-driven railgun. The nominal initial average jet parameters in the
data set analyzed are density \approx 2 x 10^(16) cm^(-3), electron temperature
\approx 1.4 eV, velocity \approx 30 km/s, M \approx 14, ionization fraction
\approx 0.96, diameter \approx 5 cm, and length \approx 20 cm. These values
approach the range needed by the Plasma Liner Experiment (PLX), which is
designed to use merging plasma jets to form imploding spherical plasma liners
that can reach peak pressures of 0.1-1 Mbar at stagnation. As these jets
propagate a distance of approximately 40 cm, the average density drops by one
order of magnitude, which is at the very low end of the 8-160 times drop
predicted by ideal hydrodynamic theory of a constant-M jet.
