Arc transition in MHD generator

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Abstract
The current-voltage characteristics of a plasma (seeded argon) in a magnetohydrodynamic (MHD) electric power generator, reveal a sudden transition in the plasma current from a low current (thermal) to a high current (low-voltage arc) mode at applied voltages between 20 V and 50 V for an electrode separation of approximately 25 cm. In the experiments described, the high electron current densities at the transition originate at a relatively cold graphite cathode (< 1500 K) and the electric field intensity E at the cathode before transition is only of the order of 105 V cm−1, for argon seeded with alkali compounds and at gas temperatures between 2000 and 3000 K. Taking the experimental conditions and results, it is suggested that the arc transition is due to a thermal instability in the relatively cool gas layer adjacent to the cathode. The gas layer acts like a resistive layer of high thermal capacity subjected to potential and temperature gradients.

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