RDHWT/MARIAH II MHD modeling and experiments review

Magnetohydrodynamic (MHD) acceleration can potentially be used to augment the flow enthalpy in an electron beam heated hypersonic wind tunnel. Preliminary modeling shows that an MHD accelerator placed downstream of the electron beam heated region and a subsequent expansion can increase the test section Mach number. Due to the low static temperature, an efficient artificial ionization in the MHD channel is necessary, for which purpose arrays of electron beams would be used. Similar cold-air hypersonic MHD devices with artificial ionization are attractive for power extraction and hypersonic flow control application. A Mach 3 experimental facility has been developed at Princeton University to demonstrate MHD effects with nonequilibrium ionization. Short duration, high repetition rate, high voltage (2 ns, 100 kHz, 30 kV) pulses have been used to successfully generate a volume-filling cold nonequilibrium plasma in the test section of the tunnel placed in a strong (up to 6.5 Tesla) magnetic field. To characterize the plasma in the MHD channel, information on electron number density and electron collision frequencies is needed. Microwave diagnostics has been applied to short pulsed sustained plasmas 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 between the pulses. Experimental data also revealed that electron number densities in the static cell plasma were in the range between 2×10¹¹ to 7×10¹¹ cm^-3. Emission spectroscopy was used to estimate the averaged rotational and vibrational temperatures. Peak electron densities between 5×10¹¹ and 10¹² per cm³ were reached in the supersonic flow experiments with 5 Tesla magnetic field. The electrical properties of the resultant MHD channel were investigated. Faraday MHD effect was demonstrated for the first time in the cold, supersonic, unseeded air flow. The Hall parameter was estimated from the electrical properties of the MHD channel. Modeling predictions were found to be in very good agreement with experimentally measured Faraday current between the pulses. Modeling showed that between pulses the electrons have low energy, electron and ion currents are comparable, and the cathode fall is very small.