No abstract available

[en] Ionization instabilities in MHD generators may be suppressed by the use of grids that short circuit the AC electric field component corresponding to the direction of maximum growth. An analysis of the influence of the corresponding boundary conditions has been performed in order to obtain more quantitative information about the stabilizing effect of this system

[en] A model, describing the time dependent behaviour of a noble gas MHD generator plasma, has been set up. With this model it is possible to calculate the relaxation for ionization or recombination as a response to a stepwise temperature development, once the initial and final conditions are given. In model radiative transitions and a deviation from Maxwellian electron distribution are included. Radiation causes an enhancement of both the ionization relaxation time and the recombination relaxation time. A non-Maxwellian electron distribution results in an increase of the relaxation time for an ionizing plasma because of an underpopulation of the high energy electrons. A decrease of the relaxation time for a recombining plasma is caused by an overpopulation of high energy electrons. The relaxation time is strongly dependent on the seed ratio and the temperature step. (Auth.)

[en] The onset of the nonequilibrium conductivity in a closed-cycle MHD generator is experimentally investigated. Conditions for deviations from a Maxwellian electron energy distribution are realized during an afterglow experiment. Here, values of the electron temperature and electron density correspond with those present at the entrance of an MHD generator working at low stagnation temperatures (TS less than or equal to 2000 K). Minimum currents are measured in MHD experiments. They correspond to the threshold values of the electron density (5 x 10 to the 18th per cu m) for the transition from a non-Maxwellian to a Maxwellian shape of the electron energy distribution. For low inlet electron densities (less than or equal to the threshold value) the onset of the nonequilibrium conductivity becomes difficult and relaxation ionization lengths up to the generator length have been shown to exist by the experiments. At low stagnation temperatures, higher inlet values of electron density and shorter relaxation lengths are realized by means of preionization. 12 references

[en] From continuity, momentum and energy equations for the different species of an MHD working fluid, a one dimensional analysis is obtained of a stationary argon flow in a linear Hall generator. Electron heating and non-equilibrium ionization are described by the equations. For several initial conditions numerical solutions are given. They show electron temperature relaxation lengths of several millimetres and ionization relaxation lengths between 20 and 45 cm. A disc generator experiment is prepared to measure nonequilibrium phenomena in non-seeded argon. In this experiment plasma pulses are produced by an inductive plasma gun. These plasma pulses are investigated with an image converter camera and double floating probes. These investigations confirm the possibility of using the inductive plasma gun in the experiments planned. (author)

[en] A numerical calculation has been developed to describe the two-dimensional, time-dependent behavior of an inhomogeneous plasma in nonequilibrium MHD generators. The development of 'streamers' as typical discharge structures is simulated. A cesium-seeded argon plasma is considered. The numerical conditions used in the calculations are set equal to the experimental conditions used in the Eindhoven shock-tube facility. The elliptic part of the equations is solved by the finite-element method and the hyperbolic part by integration along the characteristic lines. It is found that the current concentrations on each electrode edge grow within microseconds into discharge structures which are concentrated in nonstationary streamers transverse to the gas flow. After they are formed, these streamers are frozen in the gas and are convected downstream at the gas velocity. Inside the streamers the cesium is fully ionized and the electron temperature is in range of 4000-6000 K, and outside the streamers the electron density is about 10 to the 19th and the electron temperature is about 2500 K. These calculated results agree quite well with the experimental ones observed in the Eindhoven shock-tube facility

No abstract available

[en] The paper describes the results of analysis of the experimental data on the evolution of a current-carrying plasma clot in the channel of an experimental MHD generator. Analysis is performed by a one-dimensional nonstationary numerical model of the physical processes in an experimental MHD facility with a shock tube. A comparison of the calculation results with the experimental data enable one to identify the precursor of the plasma clot as a compression wave propagating from the site of its generation, as well as to identify the wall tails as a result of deformation of the clot in the nonuniform velocity field. An increase of generated electric current in the bottom half of the MHD channel indicates that the plasma clot is self-maintained

[en] At the present time the problem of the generation and dynamics of plasma clots in the low temperature plasma flow has attracted considerable interest. The special generated plasma clot can be used for the aerodynamic drag force decreasing, it can be employed as a projectile to destroy tile obstacle and it can be used for the generation of the electrical energy in the external magnetic field. The submitted paper presents results of the experimental and numerical research of the plasma clot generation by means of the capacitor discharge and analysis of the plasma clot evolution in tile supersonic channel in presence of the external magnetic field. It was found that the pressure wave created during the generation stage propagates ahead of the clot. Furthermore the clot moving through the channel takes the shape with the extended wall tails due to its deformation in the nonuniform velocity field. The experiments have been carried out in Eindhoven Shock Tunnel Facility. The experimental facility consists of a shock tube, a channel with a supersonic nozzle and a tank. The products of CO and O₂ combustion in the detonation wave which is seeded with Cs are used as a working body. The plasma clot is formed by means of the 10 mks capacitor discharge with energy about 120 J near the throat after the shock-wave startup of the nozzle. The clot transporting through the channel by the gas flow in tile presence of the external magnetic field of 3.3 T generates the electrical current which is measured on the galvanically separated electrode pairs. The streak photography in the middle of the channel is obtained in experiments also. The physical processes studied are simulated by proper numerical models. The modified one-dimensional time-dependent numerical model of the gasdynamic and electrodynamic processes in the experimental facility with the shock tube and the numerical solution of the approximate two-dimensional time-dependent model have been used in numerical analysis

[en] The possibility of deviations from local thermodynamic equilibrium of a cesium seeded argon plasma has been analyzed. A four level model of cesium has been employed. Overpopulations of the ground state and the first excited state as well as the corresponding reduction of the electron density are calculated for cylindrical discharge structures by solving stationary rate equations. Numerical results are presented. These results indicate that in a large regime of plasma conditions the LTE assumption is valid for electron temperatures larger than 3000 K. (orig.)