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Mathematical models for the design of the linear channel of an MHD generator, are described. By means of a quasi-one-dimensional optimization's model the main design parameters of the channel, are calculated. The local phenomena effects are analyzed through a steady state two-dimensional model. This model is based on the description both of the fluid dynamic and of the electrodynamic in the plane containing the channel axis and normal to the magnetic induction. The transients caused by variations of the loading and by faults, are studied by means of a time dependent quasi-one-dimensional model. © 1992, Taylor & Francis Group, LLC. All rights reserved.

Mathematical models for the analysis of the linear MHD generator channel operation are described. The local phenomena are analyzed through a steady-state two-dimensional model which is based on a description of both the fluid dynamic and the electrodynamic behavior of the plasma. The transients caused by variations of the loading and by faults are studied by means of a time-dependent quasi-one-dimensional model. © 1992 IEEE

This paper gives an overview of the experimental work on the MHD-hypersonic flow interaction, carried out in the High Enthalpy Arc Heated hypersonic wind Tunnel of Alta, Pisa, Italy, in cooperation with the Department of Electrical Engineering of the University of Bologna, under the support of The European Space Agency and the Italian Space Agency. The aim of the experiment was to produce a database to be used for the validation of numerical codes for the analysis and the simulation of the agnetofluid-dynamic interaction in hypersonic flows. In order to do this a grat attention has been dedicated to the diagnostics and mainly to those which can deliver experimental data to be compared with numerical results. In the experiments reported sharp and blunt bodies have been investigated at Mach 6 and Mach 15. The experiments have shown large effect of the MHD interaction at each of the experimental conditions investigated.

This paper describes the results of an experimental investigation on the effect of the MHD interaction in the plasma of the shock layer above a test body immersed into a hypersonic Argon flow. The hypersonic flow was obtained by means of the high-enthalpy arc-heated wind tunnel of Alta SpA at Mach 6 and Mach 15. An extensive activity was carried out in order to quantitatively characterize the weakly ionized free stream, for pressures in the range between 0.5 and 1.5 bar. Two different models, with embedded high strength magnets have been utilised: a sharp cone and a truncated-cone blunt body. The models are equipped with surface pressure measurement devices and electrostatic probes. Two geometrically identical, non magnetic models were used to compare results with and without MHD interaction. This paper will describe the experimental setup, the characterization activity, and the test campaign results. A short critical analysis on the obtained results will be also present including comparison with the past experiments conducted in Alta by the same team.

This paper deals with the design of an experiment devoted to MHD flow control, to be performed in CIRA GHIBLI hypersonic facility. The aim of this work is to define the proper test conditions and to design and realize the model that will be tested in the facility. Numerical simulations have been carried out in order to have a preliminary estimate of the ionization degree and of the heat fluxes over the test article. Moreover the MFD code from Bologna University has been used in order to calculate the expected interaction effect, under the assumption of low Rem. Finally, the design of the test article has been carried out.

A method for the determination of the current sources of an axisymmetric magnetic field specified within a given domain of finite extent is presented. The currents must be outside the domain and must satisfy the specified field boundary conditions. Both the geometry of the regions where the currents flow and the current distribution in these regions are required. The formulated problem is a nonlinear problem of compound synthesis. >

In the present work an experimental investigation on the electrohydrodynamic (EHD) interaction effect, induced by means of a dielectric barrier discharge on a one atmos-phere subsonic air flow, is described. The air flow is obtained in an open circuit blowing wind tunnel at different air speeds. A plane plasma panel equipped with a dielectric dis-charge barrier system is immersed in the flow. A single phase sinusoidal power supply system, a superposition of a single phase sinusoidal system with a DC voltage supply, and a three phase symmetrical power supply configuration are considered. Electric, fluid dynamic and spectroscopic diagnostic techniques are utilized. Pitot probe meas-urements are performed in the boundary layer on the plasma panel surface. Schlieren imaging is done to visualize the plasma sheath. Vibrational and rotational temperatures are evaluated by means of spectroscopic techniques. The force induced on the flow by the dielectric barrier discharge acts on the ions of the non-neutral sheath ahead of the plasma filament traveling on the panel surface. This causes a reduction of the time in-terval during which the force is active. The EHD effect and the momentum transfer to the gas flow in the boundary layer are reduced when the flow speed grows. An increase of the supply frequency and of the supply voltage causes an increase of the momentum transfer. In these experiments no significant increase of the efficiency of EHD effect is observed when utilizing the superposition of a DC voltage to a single phase power supply or a three phase power supply.

The MHD interaction around a conical test body in a hypersonic argon flow is experimentally investigated. The aim of the experiment is to produce a data base to be used for the validation of numerical codes for the analysis and the simulation of the magneto-fluid dynamics in hypersonic flows. In these experiments the flow is realized in the high-enthalpy arc-heated hypersonic wind tunnel - HEAT - of Alta (Pisa-Italy). Speeds at Mach 6 have been realized. In the present experiment the flow and the plasma characteristics have been determined. The MHD interaction is obtained in the shock layer of a conical test body placed at the exit of the hypersonic nozzle. The electrical configuration realized, allows enhancing the effect of the MHD interaction on the plasma parameters. This is done by utilizing the Hall field to generate the MHD interaction and by short-circuiting the Faraday current inside the plasma of the shock layer. The magnetic flux density is produced by an array of three magnets located in the test body. Test runs are performed at three different stagnation pressures. Fluid-dynamic, electrical and optical observations have been done. The experiment showed a large effect of the MHD interaction on the values of the measured quantities. In order to have a test body entirely contained in the region where a uniform hypersonic flow has been measured, a test body of smaller sizes has been constructed and tested. The results obtained in this case confirm the results of the tests with the large test body.

In this work we present a theoretical study concerning the Laser Induced Plasma (LIP) produced by the interaction between a nanosecond laser with a metallic titanium target in air and in water. The strong influence of chemical processes on the fluid dynamic expansion has been investigated. To this aim a 2D fluid dynamic code has been developed and coupled with a chemical equilibrium model.

The In the present work an experimental investigation on the EHD interaction effect, induced by means of a dielectric barrier discharge on a one atmosphere subsonic air flow, is described. The air flow is obtained in an open circuit blowing wind tunnel at different air speeds. A plane plasma panel equipped with a dielectric discharge barrier system is immersed into the flow. A single phase sinusoidal power supply system, a superposition of a single phase sinusoidal system with a DC voltage supply, and a three phase symmetrical power supply configuration are considered. Electric, fluid dynamic and spectroscopic diagnostic techniques are utilized. Pitot probe measurements are performed in the boundary layer on the plasma panel surface. Schlieren imaging to visualize the plasma sheath is done. Vibrational and rotational temperatures are evaluated by means of spectroscopic techniques. The force induced on the flow by the dielectric barrier discharge acts on the ions of the non-neutral sheath ahead of the plasma filament traveling on the panel surface. This cause a reduction of the time interval in which the force is active. The EHD effect and the momentum transfer to the gas flow in the boundary layer is reduced when the flow speed growths. An increase of the supply frequency and of the supply voltage cause an increase of the momentum transfer. In these experiments no significant increase of the efficiency of EHD effect is observed when utilizing the superposition of a DC voltage to a single phase power supply or a three phase power supply.

In this work a high voltage—high frequency generator for the power supply of a dielectric barrier discharge (DBD) plasma actuator for the aerodynamic control obtained by the electrohydro- dynamic (EHD) interaction is described and tested. The generator can produce different voltage waveforms. The operating frequency is independent of the load characteristics and does not require impedance matching. The peak-to-peak voltage is 30 kV at a frequency up to 20 kHz and time variation rates up to 60 kV μs−1. The performance of the actuator when supplied by several voltage waveforms is investigated. The tests have been performed in still air at atmospheric pressure. Voltage and current time behaviors have been measured. The evaluation of the energy delivered to the actuator allowed the estimation of the periods in which the plasma was ignited. Vibrational and rotational temperatures of the plasma have been estimated through spectroscopic acquisitions. The flow field induced in the region above the surface of the DBD actuator has been studied and the EHD conversion efficiency has been evaluated for the voltage waveforms investigated. The nearly sinusoidal multilevel voltage of the proposed generator and the sinusoidal voltage waveform of a conventional ac generator obtain comparable plasma features, EHD effects, and efficiencies. Inverse saw tooth waveform presents the highest effects and efficiency. The rectangular waveform generates suitable EHD effects but with the lowest efficiency. The voltage waveforms that induce plasmas with higher rotational temperatures are less efficient for the conversion of the electric into kinetic energy.

The supersonic expansion of an ionized gas through a converging-diverging nozzle is characterized by strong departure from equilibrium conditions. In this work, a complete plasma characterization has been carried out on the Alta High-Enthalpy Arc-Heated Tunnel (HEAT), as a preliminary activity of a test campaign focused on the effect of the MHD interaction with the plasma of the shock layer. After a standard aerothermodynamic characterization of the hypersonic flow carried out through pressure and heat flux measurements, two different plasmas diagnostic systems have been utilised to characterize the system: emission spectroscopy in the HEAT plenum chamber and microwaves at the exit of the nozzle. The plasma in the arc heater plenum chamber has been investigated by means of optical emission spectroscopy techniques relying on the high pressure and, thus, on the PLTE conditions before the expansion in the nozzle. Electron excitation temperature has been evaluated from continuum and Boltzmann plots; electron number density has been derived from H-alpha Stark broadening and continuum. Conversely, the plasma cools down throughout the expansion in the nozzle and electrons develop non equilibrium distribution, as demonstrated by the calculations done in order to simulate the process. Microwaves absorption diagnostic technique shows a very weak dependence on electron energy distribution, and therefore was chosen to probe the electron number density at the end of the nozzle. Probing the plasma at the same time both at the beginning and at the end of the nozzle leads to the correlation of the measures to the results of a quasi one-dimensional model of the plasma expansion code based on the state-to-state kinetics, that consider a kinetic equation for each internal state of the heavy particles, self consistently coupled with the Boltzmann equation of free electrons.

The MHD interaction around a conical test body in a hypersonic argon flow is experimentally investigated. The hypersonic flow is realized in the high-enthalpy arc-heated wind tunnel of Alta (Pisa-Italy) at Mach 6. The aim of the experiment is to produce a data base to be used for the validation of numerical codes which are utilized to analyze the magneto-fluid dynamics in hypersonic flows. In the present experiment the MHD interaction is obtained in the shock layer of a conical test body placed at the exit of the hypersonic nozzle. The electrical configuration realized, allows to enhance the effect of the MHD interaction on the plasma parameters. This is done by utilizing the Hall field to generate the MHD interaction and by short-circuiting the Faraday current inside the plasma of the shock layer. The magnetic flux density is produced by an array of three magnets located in the test body. Test runs are performed at three different stagnation pressures. Fluid-dynamic, electrical and optical observations have been done. The experiment showed a large effect of the MHD interaction on the values of the measured quantities.

In this work, the properties of a one atmosphere uniform three phase barrier discharge panel in a subsonic wind tunnel have been investigated. The power supply system has been specifically designed for this application by means of a PIC generated signal reference and audio MOSFET amplifiers. Electric, fluid-dynamic and spectroscopic diagnostic techniques were utilized to characterize the plasma. The discharge panel had been tested inside an open circuit blowing wind tunnel, at different power supply conditions and air flow speeds. Moreover, a phase transposition has been performed in order to investigate the influence of the three phase translating wave on the fluid acceleration. All diagnostics were applied to the different electrode connection schemes. Vibrational and rotational temperatures were evaluated by means of spectroscopic techniques. Active and reactive power have been monitored by means of voltage and current probes. The flow velocity on the board surface was measured by means of a Pitot probe.

In this paper the MHD interaction around a conical body in a in hypersonic argon flow is experimentally investigated. The hypersonic flow is obtained from the high-enthalpy archeated wind tunnel of Alta (Pisa) at Mach 6. The experimental activity are performed to produce a data base of measurement that can be conveniently be utilised for the validation of numerical codes developed for the analysis of the MHD interaction. Test are performed at various plenum chamber stagnation pressures. In order to characterise the plasma parameter in the free stream, microwave and emission spectroscopy has been utilised. The magnetic flux density is produced by an array of three magnets located in the test body.

This paper describes the results of an experimental investigation on the effect of magnetohydrodynamic (MHD) interaction with the plasma of the shock layer at a test body in a hypersonic argon flow. The hypersonic flow is obtained from the high-enthalpy arc-heated wind tunnel of Alta, Pisa, Italy, on Mach 6. Tests are carried out at heating chamber stagnation pressures of 0.65, 0.85, and 1 bar and magnetic fields of 0.15–0.35 T. The experimental observations are done by means of a set of electrical probes, an optical multichannel analyzer, and a fast shutter charge-coupled device camera. In order to maximize the effect of MHD interaction, the Faraday field is shorted, and a magnetic field perpendicular to the test body surface is used. An increase of the distance between the shock front and the body, owing to the MHD interaction, is observed. The MHD interaction effect is reduced by the low conductivity of the plasma in the boundary layer at the test body surface.

A model for the analysis of the magneto-plasmadynamic regime, devoted to MHD systems in a hypersonic flight vehicle, is presented. In the model the discrete formulation of fluid dynamics and electrodynamics are coupled. The Navier-Stokes equations are discretised by means of a finite volume formulation. The electrodynamics is discretised by means of a finite element method. The model has been utilised for the analysis of MHD interaction for the fluid control over a hypersonic body. Calculations have shown that MHD interaction can strongly affect the boundary layer fluid dynamic regime.

In this paper the MHD interaction in the hypersonic flow around an axi-symmetric cone is investigated. The aim of the work is to provide a feasibility analysis of a proposed electrodynamic configuration suited for plasmas with high electron mobility. A numerical model coupling the Euler equations with the electrodynamics under the assumption of Rem <

A model for the analysis of the magnetoplasmadynamic regime, devoted to magnetohydrodynamic (MHD) systems in a hypersonic flight, is presented. In the assumption of a low-magnetic-Reynolds-number regime, the model couples a time-dependent formulation of fluid dynamics with steady-state electrodynamics. The Navier-Stokes equations are discretized by means of a finite volume formulation. The electrodynamics is discretized by means of a finite element method. The model has been utilized for the analysis of the MHD Interaction In the boundary layer of a test body in a wind tunnel. The activity Is aimed to design the experimental activity in the frame of an Italian Space Agency research project on the MHD interaction in hypersonic flows. Calculations have shown that MHD interaction can be greatly weakened by the effect of the Hall current.

The properties of a one atmosphere uniform barrier discharge panel in a subsonic wind tunnel, have been investigate. Electric, fluid-dynamic and spectroscopic diagnostic techniques have been utilized. The discharge panel was tested inside an open circuit blowing wind tunnel, at different power supply conditions and air speeds. The characteristic of the boundary layer on the plasma panel, have been investigated by means of a Pitot probe. Vibrational and rotational temperatures have been evaluated by means of spectroscopic techniques. The measured electric properties were correlated to spectroscopic data. Flow visualization was also performed by means of a Schlieren setup. In order to raise the thrust efficiency, the set up of the plasma panel has been varied. For each electrode pair, a high voltage DC electrode was added. This leads to a DC discharge on the plasma sustained by the AC barrier discharge. This discharge causes an aerodynamic effect originated by the associated ionic wind.

In plasma technological applications with high electrical conductivity, magnetic convection can become comparable to or even exceed magnetic diffusion. The physical formulation, derived from the Maxwell equations and Ohm's law, yields the convection-diffusion equation of the magnetic flux density. For high magnetic Reynolds numbers, the first order space derivatives dominate over the second order derivatives. Under these circumstances, special attention must be paid to the numerical solution of the problem. A numerical method based on a finite element formulation for the solution of the magnetic convection-diffusion equation has been developed. The method proposed utilizes an upwinding technique as in the finite difference methods mentioned above. A first order set of shape functions has been utilized to approximate the unknown function. In applying the weighted residual method, a second order weighting function has been adopted. An optimal value of the convexity of the weighting function is chosen to obtain an oscillation free solution with a good accuracy. The method has been utilized to solve the electrodynamics of a discharge in SF/sub 6/ at a pressure of 10 MPa and a current of 2 kA.

Summary form only given. A fully implicit method for the numerical solution of the model for the analysis of the discharge in an SF/sub 6/ circuit breaker is presented. The numerical model developed is an essential tool for the understanding of the operation of the device, to improve its efficiency and reliability during nominal operation and during faults. The physical formulation of the model (Borghi et al., 1997) is constituted by the continuity equation of mass, momentum and energy, by the state equations of gas and by the convection-diffusion equation of the magnetic flux density, derived from the Maxwell equations and Ohm's law. The main difficulties to deal with, are the presence of a moving boundary between hot gas and the surrounding cold gas, and the strong dependency of plasma conductivity and transport coefficients on temperature. When solving numerically the discharge model, due to the mentioned feature, attempts to explicitly perform the time integration, result in an excessively restricted stability condition on time step. An implicit method, which calculates at each time step the fluid dynamics and the electrodynamics, is needed to overcome the stability problem of the time integration.

Summary form only given. A model which includes electrodynamics, gas-dynamics, plasma physics and plasma chemistry is needed for the design of the SF/sub 6/ circuit breakers. The model is an essential tool for the understanding of the operations of the considered device, in order to improve its efficiency and reliability during nominal operation and during faults. The Lorentz force has been included in the momentum equation, and the radiation effects are taken into account in the energy equation. The equation describing the electrodynamics is the convection diffusion equation for the magnetic flux density, obtained from the Maxwell equations and Ohm's law. The induced electromotive force is neglected. A finite volume time dependent formulation has been adopted in order to numerically solve the fluid dynamic problem. The electrodynamics is solved by means of a finite element method. The fluid dynamic code is iteratively coupled to the electrodynamic code in order to obtain the time evolution of the discharge. The radiation term, the conductivity and the thermodynamic parameter of the plasma are evaluated assuming local thermal equilibrium. The high current phase of a discharge during the switch off process in an SF/sub 6/ circuit breakers has been considered. The magnetic convection effects on the discharge regime have been analyzed. The MHD interactions have an important role in the discharge behaviour. The values of the Lorentz force appear to be comparable to the applied electric field and the discharge is constricted in a channel of transverse dimension of about one centimeter.

The Electro-Hydro-Dynamics (EHD) interaction, induced in atmospheric pressure still air by a surface dielectric barrier discharge (DBD) actuator, had been experimentally studied. A plasma aerodynamic actuator array, able to produce a vectorized jet, with the induced airflow oriented toward the desired direction, had been developed. The array was constituted by a sequence of single surface DBD actuators with kapton as dielectric material. An ac voltage in the range of 0–6 kV peak at 15 kHz had been used. The vectorization had been obtained by feeding the upper electrodes with different voltages and by varying the electrical connections. The lower electrodes had been connected either to ground or to the high voltage source, to produce the desired jet orientation and to avoid plasma formation acting in an undesired direction. Voltage and current measurements had been carried out to evaluate waveforms and to estimate the active power delivered to the discharge. Schlieren imaging allowed to visualize the induced jet and to estimate its orientation. Pitot measurements had been performed to obtain velocity profiles for all jet configurations. A proportional relation between the jet deflection angle and the applied voltage had been found. Moreover, a linear relation had been obtained between the maximum speed in the jet direction and the applied voltage. The active power of the discharge is approximated by both a power law function and an exponential function of the applied voltage.

In this paper, a short review of the work done in the framework of a nation-wide research programme on 'Models and Methods for Plasma Control in Magnetically Confined Fusion Experiments' is presented. The broad aim of the overall programme is to develop and propose a new effective and reliable approach to the on-line plasma control for future fusion experiments, starting from the today's theoretical background, validated by experimental evidence from a number of tests performed on existing experiments. The proposed formulation to approach the control problem is a linearized model in terms of suitable state variables and input/output relationships. The basic project has been subdivided into four major areas of investigation: the linearized response plasma model, the three-dimensional electromagnetic model, the identification techniques and finally the plasma control requirements. The most remarkable results, achieved so far in each area above, are presented in the paper. (C) 2001 Elsevier Science B.V. All rights reserved.

The Electro-Hydro-Dynamics (EHD) interaction induced by a surface dielectric barrier discharge in the aerodynamic boundary layer at one atmosphere still air has been investigated. Three different geometrical configurations of the actuator have been utilized. In the first configuration, an electrode pair separated by a 2 mm dielectric sheet has been used. The second and the third configurations have been obtained by adding a third electrode on the upper side of the dielectric surface. This electrode has been placed downstream of the upper electrode and has been connected to ground or has been left floating. Three different dielectric materials have been utilized. The high voltage upper electrode was fed by an a.c. electric tension. Measurements of the dielectric surface potential generated by the charge deposition have been done. The discharge has been switched off after positive and negative phases of the plasma current (the current phase was characterized by a positive or a negative value, respectively). The measurements have been carried out after both phases. The charge distribution strongly depended on the switching off phase and was heavily affected by the geometrical configuration. A remarkable decrease of the charge deposited on the dielectric surface has been detected when the third electrode was connected to ground. Velocity profiles were obtained by using a Pitot probe. They showed that the presence of the third electrode limits the fluid dynamics performance of the actuator. A relation between the charge surface distribution and the EHD interaction phenomenon has been found. Imaging of the plasma has been done to evaluate the discharge structure and the extension of the plasma in the configurations investigated.

The electric potential distribution induced on the surface of an aerodynamic plasma actuator, operating by means of a surface dielectric barrier discharge (DBD), has been studied both numerically and experimentally. Three actuators made with three different dielectric materials (Teflon, Plexiglas, and glass) have been used. The geometric configuration of the three actuators is the same one. An electrode pair separated by a 2 mm thick dielectric sheet constitutes the DBD actuator. The exposed high voltage electrode has been fed by a 5 kHz a.c. electrical signal. Voltage values between 7.5 and 15 kVp have been used. Measurements of the distribution of the electrical potential in the dielectric surface, generated by the charge deposited on it, have been done. Numerical simulations allowed to evaluating the charge distribution on the dielectric surface. The discharge has been switched off after positive and negative plasma currents. The measurements have been carried out after both phases. The potential distribution is always positive. The charge build up takes place several centimeters downstream of the upper electrode for an extension broader than that of the plasma on the dielectric surface. The charge distribution strongly depends on the switching off phase and is heavily affected by the dielectric material. In order to evaluate the discharge structure and the extension of the plasma, images have been taken also.

This paper deals with the experimental investigation on the magneto fluid dynamic interaction around a test body immersed into a hypersonic unseeded air flow. The experiments have been carried out in the CIRA owned plasma wind tunnel SCIROCCO. Two test conditions have been utilized for the experiments, with a total pressure of 2.5 and 2.3 bar respectively and a total specific enthalpy of 16 and 12.1 MJ/kg, and the air flow accelerated in the nozzle up to Mach 10. The magnetic induction field is generated by an electromagnet enclosed in the test article, and can reach a 0.8 T maximum value in the interaction region. The work has been conjunctly funded by the European Space Agency and the Italian Space Agency.

This work proposes an experimental analysis on the magneto hydro dynamic (MHD) interaction induced by a magnetic test body immersed into a hypersonic argon flow. The characteristic plasma parameters are measured. They are related to the voltages arising in the Hall direction and to the variation of the fluid dynamic properties induced by the interaction. The tests have been performed in a hypersonic wind tunnel at Mach 6 and Mach 15. The plasma parameters are measured in the stagnation region in front of the nozzle of the wind tunnel and in the free stream region at the nozzle exit. The test body has a conical shape with the cone axis in the gas flow direction and the cone vertex against the flow. It is placed at the nozzle exit and is equipped with three permanent magnets. In the configuration adopted, the Faraday current flows in a closed loop completely immersed into the plasma of the shock layer. The electric field and the pressure variation due to MHD interaction have been measured on the test body w...

Summary form only given. Research which studies the possibility of realizing MHD (magnetohydrodynamic) conversion through a nonuniform plasma in a linear channel has been initiated. Based on 1-D time-dependent calculations for open-cycle MHD, operating with plasma vanes, enthalpy extractions larger than 0.40 have been predicted. The main project goal is to demonstrate the physical feasibility of the MHD conversion with plasma vanes. The conditions and the lifetime of a plasma vane inside the gas flow and the interaction between the hot plasma clot and the cold gas around are studied. The first experiments on the life of the plasma vane have been successfully carried out in the shock tube facility of Eindhoven University. The plasma vanes appear and to be created in a constricted form and to live flowing with the gas for a relevant portion of the channel.

A self-consistent model of a stationary discharge in an Ar-Cs mixture at atmospheric pressure is described. The model is particularly suitable for low values of the electron density (10 17 n e ≤ 10 21 m -3 ). The possibility of deviations from the Maxwellian electron energy distribution is taken into account by the use of a two-electron energy group model. The Cs atom is described by a three-level model. Excitation and de-excitation processes, two- and three-body recombination processes and ambipolar diffusion are taken into account. Deviations from the Maxwellian electron distribution occur for electron densities below a critical electron density which lies between 10 18 and 10 19 m -3 . Further the influence of ambipolar diffusion is discussed. Its effect results in an increase of the critical electron density.

A mathematical model which can be used to determine the forces acting on the modules of the toroidal winding and on the coils of the poloidal windings during nominal operation and in fault conditions is illustrated. The model is derived by means of network theory. The coefficients of self- and mutual induction and the unitary forces are calculated by means of magnetic energy associated with the coils.

An insulating sidewall diagonally connected MHD (magnetohydrodynamics) channel has been tested, and the experimental data have been analyzed. Power outputs of up to 330 kWe have been observed. The nearly linear shape of the I-V characteristic of the generator indicates a stable generation at any load. Through the experimental data the electrical parameters have been derived and an empirical model of the plasma has been set up. The experimental results are compared with the results obtained by means of a quasi-one-dimensional model when using the empirical plasma model mentioned above, and with a theoretical model which utilizes reduction formulas derived on the basis of the instability theory. Good agreement between the empirical model and the experiments is shown. The theoretical model tends to overestimate the electrical power. An analysis of the fluctuations shows a constricted discharge regime at any operating condition. The current density inside a discharge is constant and is approximately equal to 50 A/cm/sup 2/, irrespective of the operating conditions. >

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 the 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 a depletion of the high energy electrons. A decrease of the relaxation time for a recombining plasma is caused by an excess of high energy electrons. The relaxation time is strongly dependent on the seed ratio and the temperature step.

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.

The stationary discharge in argon and in an Ar-K mixture at atmospheric pressure, is investigated. The investigation is carried out by employing a self-consistent model which takes into account possible deviations from the Maxwellian electron energy distribution. A three-level model is assumed to describe the atom which can be excited or ionized. Deviations from the Maxwellian distribution occur for electron densities below a critical electron density n ec with n ec = 5 × 10 18 m -3 for the Ar-K plasma and n ec = 7 × 10 19 m -3 for the Ar plasma.

A method is proposed for the calculation of mutual inductances in axisymmetric iron-free current systems. It is based on a toroidal multipole expansion of the magnetic flux function. The mutual inductance is expressed by means of a series of toroidal multipole moments of the two currents. Each term of the series is the product of the internal moment of one of the two currents and the corresponding external moment of the other one. The method can be applied to any axisymmetric current system independently of the current distribution. An example application is presented. >

3In this paper is presented a numerical investigation on the results of a recent experiment on the MHD interaction. The experiments has been carried out on a sharp cone in an ionized argon flow. The investigation is performed utilizing a numerical code based on low magnetic Reynolds number approximation. A sensitivity analysis is performed on the impact of the electric transport properties of the plasma. The effect of the Hall current density on the efficience of the MHD interaction are analyzed as well.