Diagnostics of short pulsed sustained plasmas in a cold air MHD channel

Magnetohydrodynamic (MHD) power extraction in a cold air channel with artificial ionization is attractive for hypersonic applications. A Mach 3 experimental facility has been developed to demonstrate nonequilibrium ionization for MHD power extraction. Short duration, high repetition rate, high voltage (2 ns, 100 kHz, 30 kV) pulses have been used to generate a plasma in the test section of the tunnel. For an accurate assessment of the MHD power extraction, it is important to characterize the plasma in the MHD channel. For this purpose, information on electron number density and electron collision frequencies is needed. As a continuation of previous work, microwave diagnostics has been applied to a short pulsed sustained plasma in a static cell at the conditions similar to those in MHD experiments. It has been shown that for the conditions existing in the cell, the electron collision frequencies lie between 2 to 10 GHz at 10-20 torr. The low values of collision frequencies were found consistent with electron temperature of the order of 0.1 eV or lower. Experimental data also revealed that electron number densities in the plasma were in the range between 2×10¹¹ to 7×10¹¹ /cm³. Emission spectroscopy was used to estimate the averaged rotational and vibrational temperatures.