Table of contents

Closed drift thrusters are reviewed. The publications on these thrusters constitute a large body of information. This article can therefore include only the most prominent theoretical and experimental features of closed drift thrusters. In some regards, this article is also an attempted synthesis of the differing views of these thrusters found in literature, as well as in our own work.

In a closed drift thruster, the electric field that accelerates the ions is established by an electron current that passes through and is impeded by a magnetic field. The precessing electrons in this magnetic field follow a closed drift path giving this thruster its name. Closed drift thrusters are divided into magnetic layer and anode layer types, based both on the geometrical and material differences in the discharge channels of the two types, and on the different physical processes that take place within the discharge plasma.

Considered as a whole, the publications on closed drift thrusters constitute an impressive body of information that, for the most part, was generated in Russia independently of US research on electric thrusters.

Closed drift thrusters are reviewed. The publications on these thrusters constitute a large body of information. This article can therefore include only the most prominent theoretical and experimental features of closed drift thrusters. In some regards, this article is also an attempted synthesis of the differing views of these thrusters found in literature, as well as in our own work.

In a closed drift thruster, the electric field that accelerates the ions is established by an electron current that passes through and is impeded by a magnetic field. The precessing electrons in this magnetic field follow a closed drift path giving this thruster its name. Closed drift thrusters are divided into magnetic layer and anode layer types, based both on the geometrical and material differences in the discharge channels of the two types, and on the different physical processes that take place within the discharge plasma.

Considered as a whole, the publications on closed drift thrusters constitute an impressive body of information that, for the most part, was generated in Russia independently of US research on electric thrusters.

A non-LTE argon cascaded arc plasma is studied and modelled with the general plasma simulation program PLASIMO. The structure of PLASIMO is flexible and transparent, so that apart from the study given in the present paper several other multicomponent stationary plasmas in a wide pressure range ( to 1 bar), from local thermal equilibrium (LTE) to non-LTE, and with different energy coupling mechanisms can be simulated as well. The modular structure is divided into three main parts: the transport part which forms the heart of the model, the plasma configuration part, and the composition part. The latter two parts define the input parameters for the transport part and are controlled by the PLASIMO user. The three parts are again divided into separate modules. The strong modularity makes PLASIMO easy to handle and easy to adjust or expand. Results of PLASIMO applied on the cascaded arc are compared with experimental data and show reasonable agreement. The influence of the boundary conditions on the simulation results is discussed.

We present a plasma source which works on the principle of the arc torch discharge. The powered electrode of the arc torch discharge was made from a thin pipe that simultaneously acts as the nozzle through which the working gas flows to the discharge region. The flow of the working gas stabilizes the arc torch discharge and a well defined plasma channel is created. The advantage of this system is that it is able to work at high pressure of working gas up to atmospheric pressure inside the plasma-chemical reactor and also in free space.

A Langmuir probe was used to determine the electron density, electron temperature, plasma potential and the electron energy probability function (EEPF) in plasma in a planar inductive discharge for a pressure range of 2-30 mTorr. The electron density increases with increased argon content and increases more steeply with increased argon content after the fractional argon flowrate has reached 50%. The measured dc plasma potential and the average electron energy gradually decrease with increased argon content. Mass spectroscopy indicates that the ratio increases with increased pressure and decreases with increased fractional argon flowrate. The measured values are compared to global model (volume averaged) calculations assuming a Maxwellian electron energy distribution.

This paper extends the kinetic and gas dynamic model for low-pressure surface wave discharges in flowing oxygen presented in a previous paper to investigate the mechanisms of power dissipation and of gas heating in the discharge. To this end, the gas thermal balance equation is taken into account and supplements the basic set of equations previously used. A detailed discussion of the various mechanisms contributing to gas heating is provided. From a comparison of the predicted gas temperature distribution along the plasma column with measurements, it is concluded that exothermic reactions at the wall such as reassociation of oxygen atoms and quenching of molecules may contribute appreciably to gas heating. Such heating mechanisms are shown to be essentially nonlocal due to the long residence time of the involved species under the gas flow rates considered here.

A self-consistent model of capacitively coupled low-pressure rf discharge is formulated. The non-local mechanism of electron heating is considered under simple assumption about the plasma density profile. The transition of electron heating from the plasma body to electrode sheaths is observed. The criterion of a heating mode transition is formulated and expressed in terms of external discharge parameters. The asymptotic solutions for low- and high-current regimes are obtained. The comparison of calculation results with experimental data demonstrates the validity of the proposed model for a wide range of discharge conditions.

When a plasma is sustained in the open air, nitrogen will diffuse into the plasma. Especially for plasmas sustained by the `Torche à Injection Axiale' (TIA) this appears to be the case, since this turbulent jet draws gases from the surroundings. In the argon plasma the entrained nitrogen is probably converted into (via charge transfer with argon ions), which is consequently destroyed by dissociative recombination (DR). This mechanism affects the plasma in two ways: (1) it offers an important loss channel for the free electrons and (2) the gas is heated by the kinetic energy of the nitrogen atoms produced in the DR reaction.

The ion energy distribution in H₂ and H₂/Ar plasma in a planar inductive discharge was measured using a quadrupole mass spectrometer. The mean ion energy increases with decreasing pressure from 15 to 40 eV with corresponding ion energy distribution width increasing from roughly 6 to about 14 eV as the discharge pressure decreased from 24 to 1 mTorr. Furthermore the ion energy distribution becomes narrower and the mean ion energy decreases as the fractional argon content is increased. The sheath is found to be slightly collisional. The dominant ions in H₂ plasma in the pressure range investigated (10-40 mTorr) are found to be H₂⁺ and H₃⁺ of roughly equal densities.

This paper investigates the possibility of optimizing the performance of plasma centrifuges by an appropriate choice of operating gas mixture. Plasma centrifuges with partial ionization are known to be adversely affected by plasma nonuniformities. Using the concept of a fully ionized seed may overcome the ionization instability problem. Theoretical calculations show that at a temperature as low as 3800 K, with a 0.01% mole fraction of caesium seeding in a hydrogen plasma centrifuge, the figure of merit for separation A can reach about 20, which means the separative power should be improved by approximately a factor of 2500 compared with a conventional mechanical centrifuge.

Using a time-resolving, floating double probe, the transition to an electron-free, or ion-ion, plasma state has been observed for a pulsed inductive radio-frequency SF₆ discharge at low pressure (10 mTorr). The probe current is dominated by the interaction between positive ions and electrons when electrons are present and by positive and negative ions when the electron density is small. When the radio-frequency excitation ceases the plasma electrons cool and rapidly attach to neutrals. At approximately five microseconds after turn-off, the current-voltage characteristics of the probe change to those consistent with orbital-limited and drift/diffusion ion current to each electrode. Microwave interferometry also reveals a decrease in the electron density below detectable levels at this time. Theoretical probe characteristics are used to verify the validity of the data fitting algorithm for various plasma conditions. In Ar/SF₆ mixtures the transition moves out in time, and the electron temperature decreases more slowly due to the increase in elastic over inelastic collisions between electrons and neutrals.

The axial wavelengths and azimuthal mode structures of a cylindrical helicon plasma source have been investigated over a broad range of input conditions (pressure 1-16microbar, field 0-450 G, rf power 0-2500 W @ 13.56 MHz) in an argon plasma. Several distinct modes of operation, separated by discontinuous jumps, have been identified: an electrostatic mode, an m = 0 wave mode and three m = 1 helicon wave modes. Two different types of mode jump have also been identified. A cavity mode transition has been identified in which a mode jump occurs when the wavelength of standing waves governed by the length of the plasma vessel leads to a node in the wave field at the centre of the antenna. A radial mode transition has also been identified in which the favourable condition that the axial wavelength should be equal to twice the length of the antenna for efficient coupling of the rf power is maintained at high powers by the plasma making a transition from the first to the second radial mode. A procedure based on these observations is suggested for predicting where mode transitions are likely to occur within the parameter space of a helicon source.

In this paper we report on our investigations of the effects of a magnetic cusp plasma homogenizer on thin film deposition profiles produced by cathodic vacuum arcs as a function of distance between the homogenizer and the plasma beam source. Both filtered and unfiltered arc plasma sources were examined. The thickness profiles of titanium films deposited onto transparent media were mapped using a quantitative optical technique based on depositing the films onto transparence foils and analysing the resulting image. The thickness homogeneity of the films was found to improve as the distance between the plasma source and homogenizer increased; however the deposition rate decreased with distance. The optimal distance that produced good homogeneity and maintained the highest rate of deposition was found to vary depending on the divergence of the plasma beam.

The electron distribution in the argon plasma of a capacitively coupled parallel plate reactor (13.56 MHz) is measured with spatial resolution. The line-of-sight integrated electron density is obtained with a quasi-optical heterodyne interferometer operating at the wavelength lambda = 1 mm. The electron density distribution is measured in a sequence of planes between the electrodes whereby the radial distribution is determined in each plane by Abel inversion. This allows us to generate an (r, z) mapping of the electron density. The lateral resolution is 6.5 mm in the focal region; the phase resolution amounts to 7 × 10^-5 of a period so that a line density of 2 × 10¹⁴ m^-2 represents the lower limit of applicability. The spatial distributions are compared with electron density measurements at similar discharges by other groups and with probe measurements in the midplane of our discharge.

The theory of dynamic similarity has been used to construct a generalized current-voltage characteristic (CVC) of a non-transferred, straight polarity, solid electrode arc-plasma torch. The enthalpy number obtained from the energy equation has been found to significantly influence the characteristics of the electric arc. The generalized CVC developed has been found to match with experimental parameters of other torches having the same basic principle of design and used elsewhere. The electrothermal efficiency of the plasma torch has been determined and put in the form of a generalized correlation using dimensionless numbers. The use of the theory of dynamic similarity to obtain a generalized correlation has been therefore found to be more appropriate.

A consistent theoretical analysis of a low-pressure (p = 1 Torr) microwave discharge ( MHz and 2450 MHz) in O₂-H₂ is presented. The model is based on the coupled solutions to the homogeneous electron Boltzmann equation and a system of rate balance equations for the dominant neutral and charged particles in the discharge. The sustaining electric field is self-consistently derived from the set of continuity and momentum transfer equations for electrons and ions O₂⁺, O⁺, NO⁺, N₄⁺, and O^-. A deviation from the classical ambipolar diffusion is observed due to the presence of O^- ions. The recombination of O(³P) atoms on the tube walls is taken into account through a Monte-Carlo-like simulation of a sequence of elementary surface processes. This formulation provides results for the mean input power absorbed from the field per electron, , and for the concentrations of and O(³P) in satisfactory agreement with measured data. Other important calculated data such as the concentrations of the ionic species and the percentage contributions of the various ionization mechanisms are also reported.

The characteristics of a sputtering plasma source using electron cyclotron resonance, operating on the ERIC facility dedicated to isotope separation by the plasma process, are described. A simple model based upon the particle conservation equation allows us to derive the ionization probability and the reflux fraction that govern the source operating conditions, both with and without carrier gas. Other data such as sputtering yields of Ar⁺ and Kr⁺ ions on Ni, Cu, Pd and Gd targets as well as their self-sputtering yields have been also determined for the first time in the case of gadolinium. The axial deposit profile of metal atoms on the lateral screen of the source is in accordance with the Yamamura semi-empirical differential sputtering yield, and the ionization coefficient of GdI is deduced.

Spatial structures of electronegative plasma were described in an inductively coupled discharge driven by an internal helical antenna in a uniform magnetic field. This new type of plasma source enables production of a high density (10¹¹ cm ^-3) electronegative plasma at a low pressure (2 mTorr). Good uniformity was measured for the plasma density and the electron temperature along the central axis. These same parameters formed respectively hill and valley profiles in the radial direction. Mode conversion of RF discharge from capacitively coupled to inductively coupled discharge that eventually might lead to a helicon discharge was observed at various gas pressures. The space potential was found to decline with introduction of the electronegative (SF₆) gas to the electropositive (Ar) gas, while the RF oscillating amplitude and the RF excited magnetic field increased. The electron energy distribution functions were found to be non-Maxwellian in electronegative plasma unlike in electropositive plasmas.

Investigations on the breakdown and the maintenance phase in a pulsed N₂ microwave plasma at 2.45 GHz under low pressure (1-10 mbar) have been carried out. The discharge chamber has a cylindrical geometry with a length of 80 mm and a radius of 45 mm. Microwave pulses with a duration of 50-200 microseconds and a repetition rate of 10-500 Hz were typical for the experiments. The behaviour of the electric field in the duct has been measured and calculated. The electron number density has been determined using an HCN laser interferometer and a Langmuir probe. The microwave power absorbed in the plasma was obtained measuring the forwardly directed and the reflected time resolved power flux. The vibrational excitation of the N₂ molecules in the plasma was studied with spatial and temporal resolution on the basis of the coherent anti-Stokes Raman scattering (CARS) technique.

In low pressure capacitive rf plasma discharges, stochastic sheath heating, combined with potentials that exclude low energy electrons from reaching the sheath, produce an electron energy probability function (EEPF) which approximates a two-temperature Maxwellian, as seen in both experiments and numerical simulations. We have used the fundamental kinetic equation to derive a space- and time-averaged kinetic equation to analytically calculate this EEPF. In this paper, we improve over our model for stochastic heating by allowing the stochastic heating to be gradually turned on over a range of electron energy, instead of abruptly. A complete set of equilibrium conditions are used so that all the unknown parameters can be solved self-consistently. The theory is applied to both argon and oxygen discharges, and the results agree well with particle-in-cell (PIC) simulations and experiments. For electronegative oxygen discharges, the equilibrium equations for the two ion species are coupled with the electron kinetic equation. The spatial diffusion profiles and the EEPF both agree well with PIC simulations.

A self-consistent plasmachemical model describing the dynamics of the non-equilibrium microwave discharge in molecular nitrogen with consideration for kinetic, photochemical and electrodynamic phenomena is proposed. The photochemical block of the model accounts for the processes of photoexcitation and photoionization of nitrogen molecules in the ground and excited electronic states. Radiative emittance of the discharge plasma is conditioned by the processes of photorecombination of electrons and positive ions as well as by the processes of spontaneous radiation of electronically excited molecules. Solution of radiation transfer equations and calculation of photochemical constants were made with allowance for the vibrational-rotational structure of the corresponding radiative transitions. The calculations performed have shown that the velocities of the ionization front propagation in the microwave discharge that were observed experimentally may be explained without invoking considerations concerning the existence of easily ionizable admixtures in nitrogen. The phenomenon of electron generation in the photohalo region of the discharge is mainly conditioned by the processes of stepwise photoionization of nitrogen molecules. The mechanisms leading to the development of kinetic instabilities in the discharge region are analysed, and a satisfactory agreement between calculation and experimental results is noted.

The collection of positive ions by spherical and cylindrical probes is investigated for electronegative plasmas in collisionless, unmagnetized and non-flowing regimes as a function of the ratio of negative ion density to positive ion density. The pre-sheath is given by a solution assuming quasineutrality. The potential in the sheath and the current-voltage characteristics are obtained by solving Poisson's equation with the initial value given by the pre-sheath solution. The calculations are mostly based on the radial motion theory but the orbital motion theory is also considered to take the finite ion temperature effect into account. Particularly, the thin sheath model is considered for the latter case. A method for diagnostics in negative ion plasmas is suggested.

The collection of positive ions by spherical and cylindrical probes is investigated for electronegative plasmas in collisionless, unmagnetized and non-flowing regimes as a function of the ratio of negative ion density to positive ion density. The pre-sheath is given by a solution assuming quasineutrality. The potential in the sheath and the current-voltage characteristics are obtained by solving Poisson's equation with the initial value given by the pre-sheath solution. The calculations are mostly based on the radial motion theory but the orbital motion theory is also considered to take the finite ion temperature effect into account. Particularly, the thin sheath model is considered for the latter case. A method for diagnostics in negative ion plasmas is suggested.

In this paper we report on our investigations of the effects of a magnetic cusp plasma homogenizer on thin film deposition profiles produced by cathodic vacuum arcs as a function of distance between the homogenizer and the plasma beam source. Both filtered and unfiltered arc plasma sources were examined. The thickness profiles of titanium films deposited onto transparent media were mapped using a quantitative optical technique based on depositing the films onto transparence foils and analysing the resulting image. The thickness homogeneity of the films was found to improve as the distance between the plasma source and homogenizer increased; however the deposition rate decreased with distance. The optimal distance that produced good homogeneity and maintained the highest rate of deposition was found to vary depending on the divergence of the plasma beam.

The characteristics of a sputtering plasma source using electron cyclotron resonance, operating on the ERIC facility dedicated to isotope separation by the plasma process, are described. A simple model based upon the particle conservation equation allows us to derive the ionization probability and the reflux fraction that govern the source operating conditions, both with and without carrier gas. Other data such as sputtering yields of Ar⁺ and Kr⁺ ions on Ni, Cu, Pd and Gd targets as well as their self-sputtering yields have been also determined for the first time in the case of gadolinium. The axial deposit profile of metal atoms on the lateral screen of the source is in accordance with the Yamamura semi-empirical differential sputtering yield, and the ionization coefficient of GdI is deduced.

The axial wavelengths and azimuthal mode structures of a cylindrical helicon plasma source have been investigated over a broad range of input conditions (pressure 1-16microbar, field 0-450 G, rf power 0-2500 W @ 13.56 MHz) in an argon plasma. Several distinct modes of operation, separated by discontinuous jumps, have been identified: an electrostatic mode, an m = 0 wave mode and three m = 1 helicon wave modes. Two different types of mode jump have also been identified. A cavity mode transition has been identified in which a mode jump occurs when the wavelength of standing waves governed by the length of the plasma vessel leads to a node in the wave field at the centre of the antenna. A radial mode transition has also been identified in which the favourable condition that the axial wavelength should be equal to twice the length of the antenna for efficient coupling of the rf power is maintained at high powers by the plasma making a transition from the first to the second radial mode. A procedure based on these observations is suggested for predicting where mode transitions are likely to occur within the parameter space of a helicon source.

Spatial structures of electronegative plasma were described in an inductively coupled discharge driven by an internal helical antenna in a uniform magnetic field. This new type of plasma source enables production of a high density (10¹¹ cm ^-3) electronegative plasma at a low pressure (2 mTorr). Good uniformity was measured for the plasma density and the electron temperature along the central axis. These same parameters formed respectively hill and valley profiles in the radial direction. Mode conversion of RF discharge from capacitively coupled to inductively coupled discharge that eventually might lead to a helicon discharge was observed at various gas pressures. The space potential was found to decline with introduction of the electronegative (SF₆) gas to the electropositive (Ar) gas, while the RF oscillating amplitude and the RF excited magnetic field increased. The electron energy distribution functions were found to be non-Maxwellian in electronegative plasma unlike in electropositive plasmas.

A Langmuir probe was used to determine the electron density, electron temperature, plasma potential and the electron energy probability function (EEPF) in plasma in a planar inductive discharge for a pressure range of 2-30 mTorr. The electron density increases with increased argon content and increases more steeply with increased argon content after the fractional argon flowrate has reached 50%. The measured dc plasma potential and the average electron energy gradually decrease with increased argon content. Mass spectroscopy indicates that the ratio increases with increased pressure and decreases with increased fractional argon flowrate. The measured values are compared to global model (volume averaged) calculations assuming a Maxwellian electron energy distribution.

The ion energy distribution in H₂ and H₂/Ar plasma in a planar inductive discharge was measured using a quadrupole mass spectrometer. The mean ion energy increases with decreasing pressure from 15 to 40 eV with corresponding ion energy distribution width increasing from roughly 6 to about 14 eV as the discharge pressure decreased from 24 to 1 mTorr. Furthermore the ion energy distribution becomes narrower and the mean ion energy decreases as the fractional argon content is increased. The sheath is found to be slightly collisional. The dominant ions in H₂ plasma in the pressure range investigated (10-40 mTorr) are found to be H₂⁺ and H₃⁺ of roughly equal densities.

In low pressure capacitive rf plasma discharges, stochastic sheath heating, combined with potentials that exclude low energy electrons from reaching the sheath, produce an electron energy probability function (EEPF) which approximates a two-temperature Maxwellian, as seen in both experiments and numerical simulations. We have used the fundamental kinetic equation to derive a space- and time-averaged kinetic equation to analytically calculate this EEPF. In this paper, we improve over our model for stochastic heating by allowing the stochastic heating to be gradually turned on over a range of electron energy, instead of abruptly. A complete set of equilibrium conditions are used so that all the unknown parameters can be solved self-consistently. The theory is applied to both argon and oxygen discharges, and the results agree well with particle-in-cell (PIC) simulations and experiments. For electronegative oxygen discharges, the equilibrium equations for the two ion species are coupled with the electron kinetic equation. The spatial diffusion profiles and the EEPF both agree well with PIC simulations.

Using a time-resolving, floating double probe, the transition to an electron-free, or ion-ion, plasma state has been observed for a pulsed inductive radio-frequency SF₆ discharge at low pressure (10 mTorr). The probe current is dominated by the interaction between positive ions and electrons when electrons are present and by positive and negative ions when the electron density is small. When the radio-frequency excitation ceases the plasma electrons cool and rapidly attach to neutrals. At approximately five microseconds after turn-off, the current-voltage characteristics of the probe change to those consistent with orbital-limited and drift/diffusion ion current to each electrode. Microwave interferometry also reveals a decrease in the electron density below detectable levels at this time. Theoretical probe characteristics are used to verify the validity of the data fitting algorithm for various plasma conditions. In Ar/SF₆ mixtures the transition moves out in time, and the electron temperature decreases more slowly due to the increase in elastic over inelastic collisions between electrons and neutrals.

A self-consistent model of capacitively coupled low-pressure rf discharge is formulated. The non-local mechanism of electron heating is considered under simple assumption about the plasma density profile. The transition of electron heating from the plasma body to electrode sheaths is observed. The criterion of a heating mode transition is formulated and expressed in terms of external discharge parameters. The asymptotic solutions for low- and high-current regimes are obtained. The comparison of calculation results with experimental data demonstrates the validity of the proposed model for a wide range of discharge conditions.

The electron distribution in the argon plasma of a capacitively coupled parallel plate reactor (13.56 MHz) is measured with spatial resolution. The line-of-sight integrated electron density is obtained with a quasi-optical heterodyne interferometer operating at the wavelength lambda = 1 mm. The electron density distribution is measured in a sequence of planes between the electrodes whereby the radial distribution is determined in each plane by Abel inversion. This allows us to generate an (r, z) mapping of the electron density. The lateral resolution is 6.5 mm in the focal region; the phase resolution amounts to 7 × 10^-5 of a period so that a line density of 2 × 10¹⁴ m^-2 represents the lower limit of applicability. The spatial distributions are compared with electron density measurements at similar discharges by other groups and with probe measurements in the midplane of our discharge.

This paper extends the kinetic and gas dynamic model for low-pressure surface wave discharges in flowing oxygen presented in a previous paper to investigate the mechanisms of power dissipation and of gas heating in the discharge. To this end, the gas thermal balance equation is taken into account and supplements the basic set of equations previously used. A detailed discussion of the various mechanisms contributing to gas heating is provided. From a comparison of the predicted gas temperature distribution along the plasma column with measurements, it is concluded that exothermic reactions at the wall such as reassociation of oxygen atoms and quenching of molecules may contribute appreciably to gas heating. Such heating mechanisms are shown to be essentially nonlocal due to the long residence time of the involved species under the gas flow rates considered here.

Investigations on the breakdown and the maintenance phase in a pulsed N₂ microwave plasma at 2.45 GHz under low pressure (1-10 mbar) have been carried out. The discharge chamber has a cylindrical geometry with a length of 80 mm and a radius of 45 mm. Microwave pulses with a duration of 50-200 microseconds and a repetition rate of 10-500 Hz were typical for the experiments. The behaviour of the electric field in the duct has been measured and calculated. The electron number density has been determined using an HCN laser interferometer and a Langmuir probe. The microwave power absorbed in the plasma was obtained measuring the forwardly directed and the reflected time resolved power flux. The vibrational excitation of the N₂ molecules in the plasma was studied with spatial and temporal resolution on the basis of the coherent anti-Stokes Raman scattering (CARS) technique.

A consistent theoretical analysis of a low-pressure (p = 1 Torr) microwave discharge ( MHz and 2450 MHz) in O₂-H₂ is presented. The model is based on the coupled solutions to the homogeneous electron Boltzmann equation and a system of rate balance equations for the dominant neutral and charged particles in the discharge. The sustaining electric field is self-consistently derived from the set of continuity and momentum transfer equations for electrons and ions O₂⁺, O⁺, NO⁺, N₄⁺, and O^-. A deviation from the classical ambipolar diffusion is observed due to the presence of O^- ions. The recombination of O(³P) atoms on the tube walls is taken into account through a Monte-Carlo-like simulation of a sequence of elementary surface processes. This formulation provides results for the mean input power absorbed from the field per electron, , and for the concentrations of and O(³P) in satisfactory agreement with measured data. Other important calculated data such as the concentrations of the ionic species and the percentage contributions of the various ionization mechanisms are also reported.

A non-LTE argon cascaded arc plasma is studied and modelled with the general plasma simulation program PLASIMO. The structure of PLASIMO is flexible and transparent, so that apart from the study given in the present paper several other multicomponent stationary plasmas in a wide pressure range ( to 1 bar), from local thermal equilibrium (LTE) to non-LTE, and with different energy coupling mechanisms can be simulated as well. The modular structure is divided into three main parts: the transport part which forms the heart of the model, the plasma configuration part, and the composition part. The latter two parts define the input parameters for the transport part and are controlled by the PLASIMO user. The three parts are again divided into separate modules. The strong modularity makes PLASIMO easy to handle and easy to adjust or expand. Results of PLASIMO applied on the cascaded arc are compared with experimental data and show reasonable agreement. The influence of the boundary conditions on the simulation results is discussed.

We present a plasma source which works on the principle of the arc torch discharge. The powered electrode of the arc torch discharge was made from a thin pipe that simultaneously acts as the nozzle through which the working gas flows to the discharge region. The flow of the working gas stabilizes the arc torch discharge and a well defined plasma channel is created. The advantage of this system is that it is able to work at high pressure of working gas up to atmospheric pressure inside the plasma-chemical reactor and also in free space.

This paper investigates the possibility of optimizing the performance of plasma centrifuges by an appropriate choice of operating gas mixture. Plasma centrifuges with partial ionization are known to be adversely affected by plasma nonuniformities. Using the concept of a fully ionized seed may overcome the ionization instability problem. Theoretical calculations show that at a temperature as low as 3800 K, with a 0.01% mole fraction of caesium seeding in a hydrogen plasma centrifuge, the figure of merit for separation A can reach about 20, which means the separative power should be improved by approximately a factor of 2500 compared with a conventional mechanical centrifuge.

A self-consistent plasmachemical model describing the dynamics of the non-equilibrium microwave discharge in molecular nitrogen with consideration for kinetic, photochemical and electrodynamic phenomena is proposed. The photochemical block of the model accounts for the processes of photoexcitation and photoionization of nitrogen molecules in the ground and excited electronic states. Radiative emittance of the discharge plasma is conditioned by the processes of photorecombination of electrons and positive ions as well as by the processes of spontaneous radiation of electronically excited molecules. Solution of radiation transfer equations and calculation of photochemical constants were made with allowance for the vibrational-rotational structure of the corresponding radiative transitions. The calculations performed have shown that the velocities of the ionization front propagation in the microwave discharge that were observed experimentally may be explained without invoking considerations concerning the existence of easily ionizable admixtures in nitrogen. The phenomenon of electron generation in the photohalo region of the discharge is mainly conditioned by the processes of stepwise photoionization of nitrogen molecules. The mechanisms leading to the development of kinetic instabilities in the discharge region are analysed, and a satisfactory agreement between calculation and experimental results is noted.

The theory of dynamic similarity has been used to construct a generalized current-voltage characteristic (CVC) of a non-transferred, straight polarity, solid electrode arc-plasma torch. The enthalpy number obtained from the energy equation has been found to significantly influence the characteristics of the electric arc. The generalized CVC developed has been found to match with experimental parameters of other torches having the same basic principle of design and used elsewhere. The electrothermal efficiency of the plasma torch has been determined and put in the form of a generalized correlation using dimensionless numbers. The use of the theory of dynamic similarity to obtain a generalized correlation has been therefore found to be more appropriate.

When a plasma is sustained in the open air, nitrogen will diffuse into the plasma. Especially for plasmas sustained by the `Torche à Injection Axiale' (TIA) this appears to be the case, since this turbulent jet draws gases from the surroundings. In the argon plasma the entrained nitrogen is probably converted into (via charge transfer with argon ions), which is consequently destroyed by dissociative recombination (DR). This mechanism affects the plasma in two ways: (1) it offers an important loss channel for the free electrons and (2) the gas is heated by the kinetic energy of the nitrogen atoms produced in the DR reaction.