Microwave processes in the SPD-ATON stationary plasma thruster

Results of experimental studies of microwave processes accompanying plasma acceleration in the SPD-ATON stationary plasma thruster are presented. Specific features of the generation of microwave oscillations in both the acceleration channel and the plasma flow outgoing from the thruster are analyzed on the basis of local measurements of the spectra of the plasma wave fields. Mechanisms for generation of microwave oscillations are considered with allowance for the inhomogeneity of the electron density and magnetic field behind the edge of the acceleration channel. The effect of microwave oscillations on the electron transport and the formation of the discharge current in the acceleration channel is discussed.     

Physicomathematical model of plasma acceleration in an ablative pulsed plasma thruster

A new comparatively simple quasi-one-dimensional physicomathematical model of plasma acceleration in an ablative pulsed plasma thruster with a capacitive energy storage is proposed. In spite of its simplicity, the model adequately reflects the main physical processes occurring in the thruster channel in the course of plasma blob acceleration: the blob dynamics, plasma radiation, absorption of radiation by the Teflon channel walls, ablation of the wall material, and plasma ionization. The results of computer simulations agree well with the experimental results.     

Formation of charge-exchange ion flows near the exit from the stationary plasma thruster acceleration channel

Ion currents onto the exit plane of the acceleration channel of a stationary plasma thruster model were measured using electrostatic probes the collecting surfaces of which could be oriented either upstream or downstream with respect to the thruster plume. Using the results of measurements, the so-called “back” flows of charge-exchange ions onto the exit plane are estimated. It is shown that the back ion flows are the most intense in the close vicinity of the thruster, but do not exceed 0.6% of the total ion flow from the thruster. The formation of steady-state ion flows near the exit from the acceleration channel of a stationary plasma thruster is simulated numerically by using a three-dimensional kinetic model that describes the dynamics of ions and neutral atoms exhausting from the acceleration channel and produced in the thruster plume and takes into account resonance charge exchange of ions with neutral atoms. The distribution of the back ion current density in the exit plane is determined. The effect of the flow rate of the working gas through the cathode on the distributions of the neutral atom density and charge-exchange ion flows is demonstrated. The obtained results can be used to analyze the effect of the thruster plume on the charge state of the surfaces located in the vicinity of the thruster.     

One-Dimensional hybrid model of a stationary plasma thruster

A one-dimensional hybrid model of the dynamics of atoms, ions, and electrons in the channel of a stationary plasma thruster is developed. The relevant set of integrodifferential equations is studied numerically. The results obtained are compared with the results of previous calculations based on a hydrodynamic model. It is shown that, with the use of one fitting parameter (the channel resistance), the calculated integral characteris-tics agree well with the experimental ones. The current-voltage characteristic is obtained. The general features of low-frequency oscillations that have been revealed in numerical simulations using the model proposed are also in fairly good agreement with experimental results. The value of the electron thermal conductivity is estimated.     

Concerning the width of spark channels with different polarities in submicrosecond sliding discharges in noble gases

Previously, the parameters of submicrosecond (with a duration of <200 ns) multichannel high-current discharges sliding along a ceramic surface in Ne, Ar, and Xe were studied only for the negative polarity of the applied voltage. The experimental data indicate that the channels expand in the transverse direction mainly due to electron drift from the channel surface layer under the action of the electric field perpendicular to the channel axis and subsequent gas ionization by these electrons. To investigate mechanisms for the channel development in a sliding discharge—in particular, to determine the contribution of electron drift—it is necessary to carry out experiments similar to those performed earlier for the opposite polarity of the applied voltage. Here, the results of measurements of the widths of the spark channels of negativeand positive-polarity sliding discharges excited in Ne, Ar, and Xe at pressures of 30 and 100 kPa are presented and discussed. It is shown that, depending on the pressure and sort of gas, the averaged optical width of positive-polarity channels is smaller by a factor of 1.27–1.60 than that of negative-polarity channels. The experimental data are analyzed using the theory of propagation of ionization waves with different polarities in gases. Analysis has shown that electron diffusion contributes insignificantly to channel expansion and that, for both polarities, the channel expansion rate exceeds the electron drift velocity in the transverse electric field near the channel. In the framework of the so-called approximation of nonlocalized initial conditions, the measured ratio between of the widths of negativeand positive-polarity channels and their relation to the electron mobility are explained by the channel expansion governed by both electron drift and primary free electrons produced by a short-term source in a narrow region ahead of the front of the expansion wave. Numerical simulations show that the width of this region is comparable with that of the wave front and is more than one order of magnitude smaller than the observed channel radius. Gas photoionization by the channel radiation can serve as a source of primary electrons.     

Plasma electron-emitting source in a low-pressure beam-plasma discharge in a stationary plasma thruster

A new type of plasma electron-emitting source capable of increasing the temperature of plasma electrons behind the edge of a stationary plasma thruster (SPT) to 7–15 eV has been developed and investigated experimentally. For the same parameters of the main discharge, the thrust, the thrust efficiency, the mass use factor, and the lifetime of the “SPT anode unit-plasma electron-emitting source” assembly are found to increase substantially as compared to a thruster equipped with a conventional cathode compensator. Simultaneously, the neutral particle pressure required for the existence of self-consistent distributions of the electric field and charged particle density in the drift space of the neutralized ion beam decreases appreciably. It is shown that the volume of the region in which primary slow ions are produced increases with increasing ionization frequency. Three additional channels for discharge control are implemented. The ranges in which the discharge parameters can be controlled are extended.     

Dynamics of the electron thermal diffusivity at improved energy confinement during lower hybrid plasma heating in the FT-2 tokamak

The dynamics of electron heat transport at improved energy confinement during lower hybrid plasma heating in the FT-2 tokamak was studied experimentally. Evolution of the profiles of the electron temperature and density was thoroughly investigated under conditions of fast variation in the plasma parameters. The energy balance in the electron channel is calculated with the help of the ASTRA code by using the measured plasma parameters. Correlation is revealed between the dynamics of electron heat transport and the behavior of small-scale drift turbulence measured using the enhanced scattering correlation diagnostics. The suppression of heat transfer and turbulence agrees well with the increase in the shear of poloidal plasma rotation calculated from experimental data in the neoclassical approximation.     

Nonlinear ineraction of an annular electron beam with azimuthal surface waves

A nonlinear theory is developed that describes the interaction between an annular electron beam and an electromagnetic surface wave propagating strictly transverse to a constant external axial magnetic field in a cylindrical metal waveguide partially filled with a cold plasma. It is shown theoretically that surface waves with positive azimuthal mode numbers can be efficiently excited by an electron beam moving in the gap between the plasma column and the metal waveguide wall. Numerical simulations prove that, by applying a constant external electric field oriented along the waveguide radius, it is possible to increase the amplitude at which the surface waves saturate during the beam instability. The full set of equations consisting of the waveenvelope equation, the equation for the wave phase, and the equations of motion for the beam electrons is solved numerically in order to construct the phase diagrams of the beam electrons in momentum space and to determine their positions in coordinate space (in the radial variable-azimuthal angle plane).     

Self-consistent modeling of radio-frequency plasma generation in stellarators

A self-consistent model of radio-frequency (RF) plasma generation in stellarators in the ion cyclotron frequency range is described. The model includes equations for the particle and energy balance and boundary conditions for Maxwell’s equations. The equation of charged particle balance takes into account the influx of particles due to ionization and their loss via diffusion and convection. The equation of electron energy balance takes into account the RF heating power source, as well as energy losses due to the excitation and electron-impact ionization of gas atoms, energy exchange via Coulomb collisions, and plasma heat conduction. The deposited RF power is calculated by solving the boundary problem for Maxwell’s equations. When describing the dissipation of the energy of the RF field, collisional absorption and Landau damping are taken into account. At each time step, Maxwell’s equations are solved for the current profiles of the plasma density and plasma temperature. The calculations are performed for a cylindrical plasma. The plasma is assumed to be axisymmetric and homogeneous along the plasma column. The system of balance equations is solved using the Crank-Nicholson scheme. Maxwell’s equations are solved in a one-dimensional approximation by using the Fourier transformation along the azimuthal and longitudinal coordinates. Results of simulations of RF plasma generation in the Uragan-2M stellarator by using a frame antenna operating at frequencies lower than the ion cyclotron frequency are presented. The calculations show that the slow wave generated by the antenna is efficiently absorbed at the periphery of the plasma column, due to which only a small fraction of the input power reaches the confinement region. As a result, the temperature on the axis of the plasma column remains low, whereas at the periphery it is substantially higher. This leads to strong absorption of the RF field at the periphery via the Landau mechanism.     

A simple model to determine the interrelation between the integral characteristics of hall thrusters

A simple model to determine the interrelation between the integral characteristics of Hall thrusters with an anode layer is proposed. The model includes the equation describing the balance of forces acting on the Hall current region, as well as the relationship between the current of accelerated ions and the rate of working gas consumption. For fixed geometrical characteristics of a specific Hall thruster, this model allows one to interrelate the main integral characteristics of the thruster, such as the accelerating voltage, the magnetic field, the propellant flow rate, and the current of accelerated ions. The results of calculations for TAL-WSF/D-55—one of the most widely used Hall thrusters with an anode layer—are presented. The domain of existence of the discharge in the channel of the Hall thruster is analyzed. It is shown that taking into account kinetic effects of neutral particle expansion and working gas preheating lead to a decrease in the current of accelerated ions and the engine thrust.     

Simulation of electron beam formation and transport in a gas-filled electron-optical system with a plasma emitter

The results of computer simulations of the electron-optical system of an electron gun with a plasma emitter are presented. The simulations are performed using the KOBRA3-INP, XOOPIC, and ANSYS codes. The results describe the electron beam formation and transport. The electron trajectories are analyzed. The mechanisms of gas influence on the energy inhomogeneity of the beam and its current in the regions of beam primary formation, acceleration, and transport are described. Recommendations for optimizing the electron-optical system with a plasma emitter are presented.     

Stimulated ionization scattering of a wave beam forming a discharge channel in a magnetic mirror trap

The stimulated scattering of a whistler wave beam forming an extended discharge channel in a magnetic mirror trap is discovered and investigated experimentally. It is shown that the beam is scattered by relaxaction oscillations of the lattice of plasma inhomogeneities excited by the beam field. The spectrum of the pump field in the RF discharge plasma is found to broaden considerably and to contain individual modulation peaks corresponding to lattice oscillations. The peaks are observed at working gas pressures at which the electron mean free path is close to the wavelength of the standing wave forming the discharge channel. A physical model describing the phenomena observed is developed.     

Experimental study on the structure of high-frequency perturbations near the exit plane of a stationary plasma thruster with a high specific impulse

Results from experimental studies of the structure of high-frequency waves near the exit plane of a stationary plasma thruster are presented. The role played by such waves in the electron transport across the magnetic field is discussed on the basis of the obtained experimental data.     

Dynamics of ion heating in a gas-dynamic trap during neutral beam injection

The dynamics of the ion temperature of the target plasma in a gas-dynamic trap during high-power neutral beam injection is measured by using the Rutherford scattering technique. A comparison of the experimental results with the results of simulations by a model based on the theory of pair Coulomb collisions indicates no significant anomalous losses from the ion plasma component.     

Emission spectroscopy diagnostics of rare gases in the PNX-U facility

Results are presented from measurements of the electron temperature and neutral atom density in a low-temperature microwave plasma by the method of emission spectroscopy. The measurements were conducted in the PNX-U facility—a magnetic confinement system with a “magnetic wall.” Multichord measurements of plasma radiation at a wavelength of 750.37 nm were performed with the help of an absolutely calibrated monochromator. The neutral atom density was calculated using the collisional-radiative model. The degree of plasma ionization near the axis of the facility was found to be close to unity. The electron temperature of the argon plasma was measured from the relative intensities of the spectral lines of neutral helium injected in small amounts into the plasma (the so-called helium thermometer method). At a low microwave heating power, the results of these measurements agree well with the results of probe measurements.     

MHD stability of a hot rotating plasma: A brief review of PSP-2 experiments

The paper presents a review of theoretical and experimental studies of plasma stability in systems with a rotating plasma (devices with crossed E×B fields) carried out in 1975–1985 at the Institute of Nuclear Physics, Siberian Branch of the Russian Academy of Sciences. The first group of studies involves numerical simulations based on the classical work by M. Rosenbluth and A. Simon. The conditions in which the centrifugal flute instability in the plasma of such devices is absent were investigated. It is shown that, in order for this instability to be suppressed, it is necessary that the following two requirements be satisfied simultaneously: azimuthal plasma rotation should be sheared and the longitudinal electric current should be strong enough (the so-called line-tying effect). The second group of studies involves experiments in the PSP-02 and PSP-2 devices. The main experiments were carried out in PSP-2 at proton energies of 15–20 keV (30–40 keV in the laboratory frame) and electron energies of a few kiloelectronvolts, the plasma potential being up to 0.4 MV. The possibility of completely suppressing centrifugal flute instability was demonstrated experimentally for the first time. In the stable (classical) regime, the only processes are ionization and proton charge exchange. When centrifugal instability is suppressed, no other types of MHD plasma activity are observed.     

Simulations of tokamak discharges with fast L-H transitions

Results are presented from the simulations of discharges with fast L-H transitions in the JET tokamak. During a transition, electron temperature perturbations propagate into the plasma core over a time much shorter than the transport time characteristic of this device. It is shown that the experimentally observed variations in the electron temperature may be caused by the change in the particle source intensity in the plasma when the atomic flux decreases, which is detected from the drop in the intensity of the D_α hydrogen spectral line. Hence, the experiments under consideration can be explained without the assumption about the nonlocal character of transport processes in tokamaks, which was made in some papers devoted to JET experiments. The plasma component responsible for the apparent nonlocal character of transport processes is the neutral component, whose propagation time across the plasma column is sufficiently short (t<100 μs). __________ Translated from Fizika Plazmy, Vol. 28, No. 1, 2002, pp. 3–8. Original Russian Text Copyright ? 2002 by Leonov.     

Possible mechanism for radial ion acceleration in a beam-plasma discharge

A mechanism is proposed that can lead to radial ion acceleration in a plasma discharge excited by an electron beam in a relatively weak longitudinal magnetic field. The mechanism operates as follows. The beam generates an azimuthally asymmetric slow potential wave, which traps electrons. Trapped magnetized electrons drift radially with a fairly high velocity under the combined action of the azimuthal wave field (which is constant for them) and a relatively weak external longitudinal magnetic field. The radial electron flux generates a radial charge-separation electric field, which accelerates unmagnetized plasma ions in the radial direction. The ion flux densities and energies achievable in experiments with kiloelectronvolt electron beams in magnetic fields of up to 100 G are estimated.     

Analysis of Data on the Cross Sections for Electron-Impact Ionization and Excitation of Electronic States of Atomic Hydrogen (Review)

A review is given of experimental and theoretical data on the cross sections for ionization, excitation, and deexcitation of atomic hydrogen. The set of the cross sections required to calculate the electron energy distribution function and find the level-to-level rate coefficients needed to solve balance equations for the densities of neutral and charged particles in hydrogen plasma is determined.     

Simulation study of HL-2A-like plasma using integrated predictive modeling code

Self-consistent simulations of HL-2A-like plasma are carried out using 1.5D BALDUR integrated predictive modeling code. In these simulations, the core transport is predicted using the combination of Multi-mode (MMM95) anomalous core transport model and NCLASS neoclassical transport model. The evolution of plasma current, temperature and density is carried out. Consequently, the plasma current, temperature and density profiles, as well as other plasma parameters, are obtained as the predictions in each simulation. It is found that temperature and density profiles in these simulations are peak near the plasma center. In addition, the sawtooth period is studied using the Porcilli model and is found that before, during, and after the electron cyclotron resonance heating (ECRH) operation the sawtooth period are approximately the same. It is also observed that the mixing radius of sawtooth crashes is reduced during the ECRH operation.