Potential performance of supersonic MHD power generators

A novel concept of hypersonic cold-air MHD power generators is discussed. Ionization of the cold air is shown to be a critical issue, determining overall design, geometry, operating conditions, and performance envelope. Analysis of various ionization methods, from electron beams to high-voltage repetitive pulses to alpha particles, shows that keV-class electron beams injected along magnetic field represent the most efficient and credible method of ionization. The requirement that the energy cost of ionization be lower than the extracted electric power reduces the conductivity in electron beam sustained MHD channels compared with that in conventional MHD generators. This restricts performance and calls for very strong magnetic fields and high Hall parameters, making ion slip and near-anode processes first-order issues. Possible problems that could be caused by hypersonic boundary layers and electrode sheaths, including anode sheath instability and ways to avoid it, are discussed. Calculations of hypersonic power generators for flight Mach numbers between 4 and 10 and altitudes of 15-40 km are quite promising. With 3-meter long channel of inlet cross section of 25×25 cm² and magnetic field of 7 Tesla, several megawatts of electric power could be generated, while spending only a few hundred kilowatts of that power on sustaining ionization.