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Choking effects due to Lorentz force in a gaseous MHD generator with segmented electrodes are analysed by means of the quasi-one-dimensional approximation. As an illustrative example, the flow of a perfect gas with constant electrical conductivity through a constant area and constant load factor duct is considered in detail. In the case of such a duct fed by an isentropic converging nozzle, maximum or choking flow and critical back pressure are evaluated in terms of electrical parameters, magnetic field strength and duct length, for fixed supply conditions. This allows one to acquire some quantitative grasp of choking and its relevant effects on the practical operation of an MHD generator.

The interaction of gas flow and electric current in the bulk of a subsonic linear generator for closed cycle is studied by means of a steady quasi-one-dimensional analysis. The characteristic of this interaction is the strong dependence of the gas electrical conductivity upon the flow Mach number, which can lead to rapid changes of electrical and gas properties along the channel. The influence of ionization turbulence on the electrical conductivity of the gas is taken into account by means of two different models based on the laminar electron energy balance (model I) and on the approximate average electron energy balance (model II), respectively. The theoretical results pertaining to the transverse current along the channel are compared with the experimental data obtained with the Frascati MHD facility. A good agreement is found with calculations based on model I, while model II leads to a substantial overestimation of the current and, hence, of the generator power output and electrical efficiency.

A theoretical model of the wall-free stream interaction, in MHD generators with turbulent non-equilibrium plasma, due to the variation of the gas parameters between opposite electrodes, is presented. The model allows one to evaluate, as a consequence of the gasdynamic boundary layers, some key parameters affecting the bulk of the generator, once the distribution of the gas properties in the cross-section of the duct is given. These parameters are the direction of the average current in the bulk and the total electrode voltage drops.

Ionization instability in a stratified non-equilibrium plasma subjected to crossed E and B fields is investigated by means of a simple model without diffusion processes and boundary effects. An inhomogeneous infinite plasma, consisting of alternate one-dimensional layers of high and low electron temperature (T02>T01) lying in the plane perpendicular to the magnetic field is taken as the unperturbed state. It is shown that, in order that a periodic stratified plasma be stable on the whole, some conditions on the thicknesses of the layers must be fulfilled and at least one of the two sets of layers (e.g. that with electron temperature T01) must be intrinsically stable (in the sense that a uniform infinite plasma with electron temperature T01 must be stable with respect to the ionization instability).

As an application of the general theory described in a previous paper (1), the evolution of a weakly unstable, infinite plasma, with immobile ions, and without external fields, is studied for the so-called bump-in-tail case. It is shown that, in the quasi-linear approximation, the equations of motion of the electron distribution function and the spectrum of plasma waves are equivalent to the usual adiabatic formulation. The present approach yields the equation of motion for the plasma waves in the form of a diffusion equation in wave-number space.

In deriving the dispersion equation of ionization waves in a (low temperature) seeded MHD plasma, allowance is made for an electron current along the impressed magnetic field. This leads to a critical value of the Hall which depends on the angle ( alpha ) between current and magnetic field by a factor 1/sin alpha . The most unstable waves no longer travel at right angles relative to the magnetic field direction; their increment and direction are given in terms of alpha and magnetic field strength.

The evolution of a weakly turbulent, unstable, spatially uniform plasma is considered by means of a kinetic theory based on the Klimontovich equation for a single species of interacting particles. The usual expansion in powers of the amplitude of the electric field is used. This leads to a quasi-linear analysis in which the effects of the discreteness of matter are explicitly taken into account. A pair of coupled equations for the one-particle distribution function and the field fluctuations is deduced. In the special case of small growth rates the latter is transformed into an equation for the spectrum of the fluctuating energy density. The results are compared with those of previous treatments.

The complete dispersion relation is analytically solved by neglecting the electron pressure term in the Ohm's law together with the heat convection in the electron energy equation. Within this limit, it results that the threshold of ionization instability does not depend on the relaxation rates within a broad range of operating conditions for the MHD energy conversion method.