First demonstration of MHD power generation using externally ionized, cold, supersonic air as a working fluid

Magnetohydrodynamic (MHD) interactions present a tantalizing opportunity to control hypersonic flows without the need for strong shock waves, high entropy gain, and movable control surfaces. An MHD interaction, however, requires a conductive flowfield. At Mach numbers too low for thermal ionization, flow energies may still be sufficient to justify the cost of an efficient external ionization scheme.; In this work, MHD power is extracted from cold, supersonic air using short-duration, high-repetition-rate, high-voltage pulses (∼2 ns, 100 kHz, ∼5 kV/cm) to ionize a cold, Mach 3 (600 m/sec), 0.04 kg/m 3 flow. A few tens of milliwatts were extracted from the 3 cm cube region of ionization, which scales to hundreds of kilowatts of power in higher velocity, larger scale devices that would be appropriate for flight applications.; Because the electrical pulses used to ionize the flow greatly exceed the breakdown threshold, the efficiency of the pulsed ionization scheme is expected to greatly exceed that of DC, or steady RF discharges. To determine the ionization efficiency, two diagnostic methods were developed.; A microwave diagnostic technique, capable of measuring both the electron number density and electron collision frequency in small scale, weakly ionized, cold plasmas was developed for measurements in the pulser-generated plasmas. The technique involved broadcasting extraordinary waves through the plasma. By varying the intensity of an applied magnetic field, the upper hybrid resonance of extraordinary microwave propagation was shifted over a range of electron number densities. Electron number densities and collision frequencies were then determined based on the conditions corresponding to a complete cessation of microwave transmission across the plasma. Number densities in the range of 5 x 1011 per cm3 at collision frequencies in the neighborhood of 5 to 10 GHz are reported for measurements in a static discharge cell at conditions comparable to the supersonic MHD channel.; Power measurements via the Current/Voltage characteristics of the pulser driven discharge were investigated using Fourier analysis of multiple signals from several measurement points in the experiment. Combining these measurements with the aforementioned electron number densities yielded efficiencies of electron generation in the range of 70 to 170 eV per electron.