The ion fly-wheel effect on the electro-thermal instability in non-equilibrium MHD hall disc generators

Abstract

The electro-thermal instability has so far been studied in collisional time scales, neglecting the effects of the charge separation field which follows any perturbation in a non-equilibrium plasma used as the operating fluid in a closed-cycle MHD generator. In this paper, the electro-thermal instability has been treated in an approximately collisionless time scale. With the appearance of the charge separation field, some energy will be transferred from the electrons to the ions during the process of restoration of the charge neutrality. Part of the energy thus transferred can be stored in the azimuthal translational mode of the ions (referred to in this paper as “the ion fly-wheel”, for simplicity); the energy thus stored will be transferred to the neutrals by ion-neutral collisions, without triggering further instabilities. There will be a gain in the critical Hall parameter, since the energy causing the instability is now reduced. The conditions for the transfer of energy from the electrons to the ion fly-wheel are ideally available in a non-equilibrium MHD Hall disc generator operating with flow symmetry. Approximate calculations predict that the critical Hall parameter can be raised from 2 to 3 by virtue of the ion fly-wheel effect, if the pressure gradient is about 2 bars/m for the case of a potassium seeded argon plasma with T_e = 2500 K and B = 3 tesla.