Discharge Oscillations in Morozov’s Stationary Plasma Thruster as a Manifestation of Large-Scale Modes of Gradient Drift Instability

Plasma Physics Reports - The phenomenon of large-scale discharge oscillations in Morozov’s stationary plasma thruster (SPT) is physically interpreted by analyzing global modes of gradient...     

Electron energy balance and ionization in the channel of a stationary plasma thruster

The paper presents results of numerical simulations of the electron dynamics in the field of the azimuthal and longitudinal waves excited in the channel of a stationary plasma thruster (SPT). The simulations are based on the experimentally determined wave characteristics. The simulation results show that the azimuthal wave displayed as ionization instability enhances electron transport along the thruster channel. It is established that the electron transport rate in the azimuthal wave increases as compared to the rate of diffusion caused by electron scattering from neutral atoms in proportion to the ratio between the times of electron? neutral collisions responsible for ionization and elastic electron scattering, respectively. An expression governing the plasma conductivity is derived with allowance for electron interaction with the azimuthal wave. The Hall parameter, the electron component of the discharge current, and the electron heating power in the thruster channel are calculated for two model SPTs operating with krypton and xenon. The simulation results agree well with the results of experimental studies of these two SPTs.     

Study of the plasma parameters in a high-current pulsed magnetron sputtering system

Results are presented from experimental studies of the current-voltage characteristics and spatial and temporal parameters of the plasma in a high-current pulsed magnetron sputtering system with a 10-cm-diameter plane disk cathode. It is shown that the plasma density in such a system is three orders of magnitude higher than that in conventional dc magnetron discharges and reaches 10¹³ cm⁻³ at a distance of 250 mm from the cathode at a peak discharge current of 500 A. The plasma propagates from the cathode region at a velocity of 1 cm/μs in the axial direction and 0.25 cm/μs in the radial direction. Optical emission spectroscopy shows that the degree of plasma ionization increases severalfold with increasing discharge current, mainly at the expense of the sputtered material.     

Study of the parameters of hydrogen-titanium plasma in a pulsed reflective discharge

The parameters of a dense (10¹³–10¹⁴ cm⁻³) plasma produced by ionization of a H₂ + Ti mixture in a moderate-power (W ≤ 10 MW) pulsed reflective discharge are investigated. The dynamics of the plasma density, the elemental composition of the generated plasma, the radial distribution of the electron density, the rotation velocity and frequency of the plasma layer with n _p ≥ n _cr, the radial electric field, the coefficient of plasma particle separation, and the coefficient of plasma recombination in the stage of plasma decay are determined.     

Experimental study of ATON stationary plasma thrusters

Results from experimental studies of integral characteristics of laboratory models of second-generation ATON stationary plasma thrusters are presented. Special attention is paid to high-voltage modes with a sufficiently high specific anode propulsive burn. Integral parameters of the thrusters were measured using a test bench with diffusion evacuation at the Moscow State Institute of Radioengineering, Electronics, and Automation and that with cryogenic evacuation at the Fakel Experimental and Design Bureau. The values of the thrust, specific propulsive burn, and efficiency measured in these test benches in the main operating mode coincide to within measurement errors. At a discharge power of 2 kW and voltage of about 700 V, the specific anode propulsive burn and anode thrust efficiency reach 3000 s and 60%, respectively. The experimental data show that the efficiency of the ATON stationary plasma thruster operating in a high-voltage mode is higher than that of other similar thrusters.     

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.     

Plasma decay in the afterglow of a high-voltage nanosecond discharge in air

The decay of air plasma produced by a high-voltage nanosecond discharge at room temperature and gas pressures in the range of 1–10 Torr was studied experimentally and theoretically. The time dependence of the electron density was measured with a microwave interferometer. The initial electron density was about 10¹² cm⁻³. The discharge homogeneity was monitored using optical methods. The dynamics of the charged particle densities in the discharge afterglow was simulated by numerically solving the balance equations for electron and ions and the equation for the electron temperature. It was shown that, under these experimental conditions, plasma electrons are mainly lost due to dissociative and three-body recombination with ions. Agreement between the measured and calculated electron densities was achieved only when the rate constant of the three-body electron-ion recombination was increased by one order of magnitude and the temperature dependence of this rate constant was modified. This indicates that the mechanism for three-body recombination of molecular ions differs from that of the well-studied mechanism of atomic ion recombination.     

Optical studies of plasma inhomogeneities in a high-current pulsed magnetron discharge

Results are presented for experimental studies of the plasma glow in a high-current pulsed magnetron discharge by using a high-speed optical frame camera. It is found that the discharge plasma is inhomogeneous in the azimuthal direction. The plasma bunches rotate with a linear velocity of ∼1 cm/μs in the direction of electron Hall drift, and their number is proportional to the discharge current. Plasma inhomogeneities in the form of plasma jets propagate in the form of plasma jets from the cathode region toward the anode. It is shown analytically that the formation of inhomogeneities is caused by the necessity to transfer high-density electron current across the magnetic field.     

Simulations of a surface glow discharge in a supersonic gas flow in the presence of external ionization

Results of two-dimensional hydrodynamic simulations of a surface glow discharge operating at pressures of 0.2–0.5 Torr in a nitrogen flow propagating with a velocity of 1000 m/s in the presence of external ionization are presented. The effect of the external ionization rate on discharge operation is analyzed. The current-voltage characteristics of the discharge are calculated for different intensities of external ionization in both the presence and absence of secondary electron emission from the cathode. The discharge structure and plasma parameters in the vicinity of the loaded electrode are considered. It is shown that, when the discharge operates at the expense of secondary emission from the cathode, the discharge current and cathode sheath configuration are insensitive to external ionization. It is also demonstrated that, even at a high rate of external ionization, the discharge operates due to secondary emission from the cathode.     

A Highlights from MBoC Selection: Cega: a single particle segmentation algorithm to identify moving particles in a noisy system

Improvements to particle tracking algorithms are required to effectively analyze the motility of biological molecules in complex or noisy systems. A typical single particle tracking (SPT) algorithm detects particle coordinates for trajectory assembly. However, particle detection filters fail for data sets with low signal-to-noise levels. When tracking molecular motors in complex systems, standard techniques often fail to separate the fluorescent signatures of moving particles from background signal. We developed an approach to analyze the motility of kinesin motor proteins moving along the microtubule cytoskeleton of extracted neurons using the Kullback-Leibler divergence to identify regions where there are significant differences between models of moving particles and background signal. We tested our software on both simulated and experimental data and found a noticeable improvement in SPT capability and a higher identification rate of motors as compared with current methods. This algorithm, called Cega, for “find the object,” produces data amenable to conventional blob detection techniques that can then be used to obtain coordinates for downstream SPT processing. We anticipate that this algorithm will be useful for those interested in tracking moving particles in complex in vitro or in vivo environments.     

Effect of Movable Anode on Plasma Parameters in Multi-Dipole Discharge

Plasma Physics Reports - We are reporting about the effect of a movable anode on plasma parameters in a multi-dipole magnetic cage. We have used electron-emitting filaments as the cathode and a...     

Double layer in a cylindrical hollow-cathode discharge

A dc cylindrical coaxial glow discharge with an inner grid anode has been studied. The region between the two electrodes is seen dark, while a brightly glowing region forms inside the grid anode up to the center. The current-voltage characteristic of a dc cylindrical glow discharge in nitrogen is similar to that of a normal glow discharge, while the normal glow discharge voltage decreases with increasing pressure. The minimum plasma potentials are observed in the hollow cathode region due to the accumulation of electrons at the back of the grid anode. At the center, some of the passed electrons are converged, so their potential is decreased. These electrons have a sufficient time to be redistributed to form one group with a Maxwellian electron energy distribution function. The electron temperature measured by electric probes varies from 1.6 to 3.6 eV, while the plasma density varies from 3.9 × 10¹⁶ to 7 × 10¹³ m⁻³, depending on the discharge current and probe position. The plasma density increases as the electrons move radially from the grid toward the central region, while their temperature decreases.     

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.     

Spatial distribution of the RF power absorbed in a helicon plasma source

The spatial distributions of the RF power absorbed by plasma electrons in an ion source operating in the helicon mode (ω _ci < ω < ω _ce < ω _pe ) are studied numerically by using a simplified model of an RF plasma source in an external uniform magnetic field. The parameters of the source used in numerical simulations are determined by the necessity of the simultaneous excitation of two types of waves, helicons and Trivelpiece-Gould modes, for which the corresponding transparency diagrams are used. The numerical simulations are carried out for two values of the working gas (helium) pressure and two values of the discharge chamber length under the assumption that symmetric modes are excited. The parameters of the source correspond to those of the injector of the nuclear scanning microprobe operating at the Institute of Applied Physics, National Academy of Sciences of Ukraine. It is assumed that the mechanism of RF power absorption is based on the acceleration of plasma electrons in the field of a Trivelpiece-Gould mode, which is interrupted by pair collisions of plasma electrons with neutral atoms and ions of the working gas. The simulation results show that the total absorbed RF power at a fixed plasma density depends in a resonant manner on the magnetic field. The resonance is found to become smoother with increasing working gas pressure. The distributions of the absorbed RF power in the discharge chamber are presented. The achievable density of the extracted current is estimated using the Bohm criterion.     

Analysis of α3 GlyR single particle tracking in the cell membrane

Single particle tracking (SPT) of transmembrane receptors in the plasma membrane often reveals heterogeneous diffusion. A thorough interpretation of the displacements requires an extensive analysis suited for discrimination of different motion types present in the data. Here the diffusion pattern of the homomeric α3-containing glycine receptor (GlyR) is analyzed in the membrane of HEK 293 cells. More specifically, the influence of the α3 RNA splice variants α3K and α3L on lateral membrane diffusion of the receptor is revealed in detail. Using a combination of ensemble and local SPT analysis, free and anomalous diffusion parameters are determined. The GlyR α3 free diffusion coefficient is found to be 0.13 ± 0.01 μm²/s and both receptor variants display confined motion. The confinement probability level and residence time are significantly elevated for the α3L variant compared to the α3K variant. Furthermore, for the α3L GlyR, the presence of directed motion was also established, with a velocity matching that of saltatory vesicular transport. These findings reveal that α3 GlyRs are prone to different types of anomalous diffusion and reinforce the role of RNA splicing in determining lateral membrane trafficking.     

Nonstationary Model of a Low-Density Plasma Jet Ejected from a Stationary Plasma Thruster

A time-dependent problem on finding the parameters of a low-density plasma jet ejected from a stationary plasma thruster is analyzed. In contrast to earlier works in which a stationary problem was solved using a Crook-type kinetic model, a set of kinetic equations is derived at the accuracy level accepted in the kinetic theory with allowance for resonant charge-exchange interaction between ions and neutrals. A method to solve the obtained set of equations is developed and used to perform test simulations, some results of which are presented.     

Experimental Studies of the High-Frequency Oscillation Structure in the Discharge Channel of a Hall Thruster in Two Stable Discharge Modes

Plasma Physics Reports - The structure of high-frequency oscillations in the plasma of a Hall thruster (HT) is studied experimentally in two different stable discharge modes: the “jet”...     

Dynamics of the Electron Energy Distribution Function in the Gas-Discharge Channel of a Single-Stage Plasma Thruster with Anode Layer

Plasma Physics Reports - Results of experimental studies of local plasma parameters in the gas-discharge channel of a model single-stage plasma thruster with an anode layer (TAL) are presented. The...     

Global characteristics of an ATON stationary plasma thruster operating with krypton and xenon

The global characteristics of an ATON stationary plasma thruster operating on xenon and krypton are investigated. It is shown that, with krypton, the thrust at the same mass flow rate of the working gas is greater and the efficiency is somewhat lower than those with xenon. An efficiency of ～60% was achieved with krypton for the specific impulse attaining 3000 s. The jet divergence is ～±22° for krypton and ～±11° for xenon.     

Visualization of plasma-induced processes by a projection system with a Cu-laser-based brightness amplifier

A novel method for visualization of the process of interaction of high-power energy fluxes with various surfaces is proposed. The possibility of the dynamic visualization of a surface covered with a ∼3-cm-thick plasma layer with a linear density of ∼10¹⁶ cm⁻² is demonstrated experimentally. A scheme of intracavity shadowgraphy of phase objects with the use of a laser projection microscope is developed. Shadow images illustrating the development of the plasma torch of an erosion capillary discharge in air are presented.