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The application of kinetic models, describing one-particle distribution function, to constructing continuum mechanic models suitable for computer systems with extra-massive parallelism is considered. The construction of such models utilizes the existence of space-time scales below which there is no physical sense to further refine the solution. The proposed approach has been used to solve some three-dimensional problems involving approximations by grids with more than billion space nodes [ABSTRACT FROM AUTHOR]

A critical analysis is given of the well-known expression for the electron-impact ionization rate constant α of neutral atoms and ions, derived by linearization of the ionization cross section σ(ε) as a function of the electron energy near the threshold I and containing the characteristic factor ( I + 2 kT). Using the classical Thomson expression for the ionization cross section, it is shown that in addition to the linear slope of σ(ε), it is also necessary to take into account the large negative curvature of this function near the threshold. In this case, the second term in parentheses changes its sign, which means that the commonly used expression for α (∼4 kT/I) already at moderate values of the temperature ( kT/I ∼ 0.1). The source of this error lies in a mathematical mistake in the original approach and is related to the incorrect choice of the sequential orders of terms small in the parameter kT/I. On the basis of a large amount of experimental data and considerations similar to the Gryzinski theory, a universal two-parameter modification of the Thomson formula (as well as the Bethe-Born formula) is proposed and a new simple expression for the ionization rate constant for arbitrary values of kT/I is derived. [ABSTRACT FROM AUTHOR]

An experiment with exploding foils was carried out at a current density of 0.7×108 A/cm² through the foil with a current density rise rate of about 1015 A/cm² s. To record the strata arising during the foil explosions, a two-frame radiographic system was used that allowed tracing the dynamics of strata formation within one shot. The original striation wavelength was 20-26 μm. It was observed that as the energy deposition to a foil stopped, the striation wavelength increased at a rate of ~(5-9)×10³ cm/s. It is supposed that the most probable reason for the stratification is the thermal instability that develops due to an increase in the resistivity of the metal with temperature. [ABSTRACT FROM AUTHOR]

Involving the idea for the Biermann effect known from space physics as well as recent discussions on non-ambipolarity of the electron and ion fluxes in low-pressure discharges, the study builds the discharge pattern in a source with localized RF power deposition outside the region of high electron density. A vortex dc current flowing in an RF discharge and a steady-state magnetic field induced by this current govern the discharge behavior. Owing to a shift in the positions of the electrondensity and plasma-potential maxima, the dc current is driven with the purpose of keeping the conservativity of the dc field in the discharge. The results present the spatial structure of a discharge in a regime of non-ambipolarity of the electron and ion fluxes, including its modifications by the magnetic field. [ABSTRACT FROM AUTHOR]

We present the results of our numerical simulations of the stable cathode-flare expansion stage in the vacuum-breakdown switching period. We used the model of two-fluid hydrodynamics with corrections that allowed it to be applied to a consistent description of the electron and ion transport in plasma and the electron transport in a plasma-anode vacuum gap without imposing any joining conditions at the expanding plasma boundary. We have analyzed the dynamics of the density, directed velocity, and temperature profiles for both components and the influence of various factors on the plasma-cloud expansion velocity. [ABSTRACT FROM AUTHOR]

Research data for drag currents in the Globus-M spherical tokamak are presented. The currents are generated by injecting atomic beams of hydrogen and deuterium. Experiments were carried out in the hydrogen and deuterium plasma of the tokamak. It has a divertor configuration with a lower X-point, a displacement along the larger radius from-1.0 to-2.5 cm, and a toroidal field of 0.4 T at a plasma current of 0.17-0.23 MA. The beam is injected into the tokamak in the equatorial plane tangentially to the magnetic axis of the plasma filament with an impact diameter of 32 cm. To provide a 28-keV 0.5-MW atomic beam with geometrical sizes of 4 × 20 cm (at a power level of 1/ e), an IPM-2 ion source is used. The generation of noninductive currents is detected from a rise in the loop current and a simultaneous dip of the loop voltage. The injection of the hydrogen and deuterium atomic beams into the deuterium plasma results in a noticeable and reproducible dip of the loop voltage (up to 0.5 V). Using the ASTRA transport code, a model is constructed that allows rapid calculation of noninductive currents. Calculations performed for a specific discharge confirm that the model adequately describes the effect of drag current generation. [ABSTRACT FROM AUTHOR]

Conditions for gas ionization in the field of a slow surface wave excited by a microwave source are considered. The gas ionization rate and the plasma density distribution over the radius of the discharge tube were studied by the optical method. The experiments were conducted in a dielectric tube with a radius much smaller than the tube length, the gas pressure being ~1-3 Torr. It is shown that the stationary distribution of the plasma density is determined by diffusion processes. [ABSTRACT FROM AUTHOR]

The results of experimental investigations of electron emission of molybdenum cathodes with two-layer diamond films are presented. For two-layer coating, the first layer of the two was a high-conductivity nanocrystalline diamond film heavily doped with nitrogen, and the second layer was a thin microcrystalline high-quality diamond film. The emission characteristics of the cathodes were determined on the basis on analyzing the output parameters of a high-power rf pulse compressor, in which these cathodes were used in the high-current (>100 A) regime. In the experiment, the current increasing for the cathode with two-layer coating in comparison with cathodes without and with one layer was observed. This effect of the current increasing can be explained by amplification of the electron beam in the process of the secondary electron emission as the beam passes through the second diamond film. [ABSTRACT FROM AUTHOR]

Results of plasma excess conductivity investigations of gas discharge device under conditions of plasma decay have been presented. This case involves the need of taking into account the recombination of charged particles both on the device electrodes and in its volume. It was established that the volume recombination of charged particles at the stage of plasma excess conductivity materially affects the duration of plasma excess conductivity and modifies the pattern of its decay. [ABSTRACT FROM AUTHOR]

A global enhanced vibrational kinetic model (GEVKM) is developed for multitemperature, chemically reacting hydrogen plasmas in inductively coupled cylindrical discharges for low- to high-pressure regimes. The species in a GEVKM are ground-state hydrogen atoms H and molecules H2, 14 vibrationally excited hydrogen molecules \textrm H2 (v) , v=1-14 , electronically excited hydrogen atoms H(2) and H(3), ground-state positive ions H+, H2+, and H3+, ground-state negative ions H− and electrons e. The GEVKM involves volume-averaged steady-state continuity equations for the plasma species, an electron energy equation, a total energy equation, a heat transfer equation to the chamber walls, and a comprehensive set of surface and volumetric chemical processes governing vibrational and ionization kinetics of hydrogen plasmas. The GEVKM is verified and validated by comparisons with previous numerical simulations and experimental measurements of a negative hydrogen ion source in the low-pressure (20–100 mtorr), low-absorbed-power-density (0.053–0.32 W/cm ^{3}) regime and of a microwave plasma reactor in the intermediate- to high-pressure (1–100 torr), high-absorbed-power-density (8.26–22 W/cm ^{3})$ regime. The GEVKM is applied to the simulation of a high-current negative hydrogen ion source (HCNHIS). The HCNHIS consists of a high-pressure (20–65 torr) radio-frequency discharge chamber in which the main production of high-lying vibrational states of the hydrogen molecules occurs, a bypass system, and a low-pressure (0.1–0.4 torr) negative hydrogen ion production region where negative ions are generated by the dissociative attachment of low-energy electrons to rovibrationally excited hydrogen molecules. The discharge pressure and negative hydrogen ion current predicted by the GEVKM compare well with the measurements in the HCNHIS. [ABSTRACT FROM PUBLISHER]

Excitation of microwave discharges by pulsed microwave radiation (≤2 MW, 1.95 GHz, 1–10 \mu \texts ) on dielectric surfaces in vacuum ( 10^-6 torr) was studied experimentally. Different stages of a surface microwave discharge were observed: secondary-electron-emission microwave discharge (multipactor), surface microwave breakdown (filamentary microwave discharge), and plasma-flare microwave discharge. It is found that, in the stage of microwave breakdown (which lasts for \sim 0.1~\mu \texts ), ≥70% of the incident microwave power is absorbed by a dense plasma filament with a diameter of \sim 100~\mu \textm , an electron density of up to 2 \times 10^18 cm ^-3 , and an electron temperature of about 2 eV. Strong interaction of the dense plasma of the filamentary microwave discharge with the dielectric leads to local destruction of the dielectric surface in the form of a long thin erosion track ( $l \approx 6$ cm, d \sim 100~\mu \text{m} ). The coefficient of microwave power absorption by a plasma filament in a rectangular waveguide is calculated using an electrodynamic model that does not involve expansion in the waveguide modes and takes into account reflections of the scattered wave from the waveguide walls by introducing an array of filament mirror images. It is shown that the coefficient of microwave power absorption by the filamentary discharge plasma can reach 70% and more, which agrees well with the experimental data. [ABSTRACT FROM PUBLISHER]

Reviewing results of experimental research of picosecond pulses of runaway electrons (REs) generated by discharges in dense gases at multiple overvoltages, including, along with routine measurements of voltage pulses and RE current, direct measurements of RE energy distributions, pressure dependence of RE numbers and experiment with retarding voltage similar to the accelerating voltage, a reality of the effect of 'abnormal energy' REs is being substantiated. With this goal we emphasize non-conventional qualitative RE characteristics rather than quantitative. [ABSTRACT FROM AUTHOR]

This paper presents the experimental results of magneto hydrodynamics (MHD) instability suppression in the HT-7 Tokamak by using electrode biasing. The biased voltage is applied between the electrode and the shell of the vacuum chamber. Three kinds of electrode biasing are used in the experiments and the effect on the MHD instability suppression is proportional to the current of the biased electrode. When positive voltage was applied to the electrode, we observed significant increases in the plasma electron density in the core area, extreme-ultraviolet bolometer emission and soft X-ray emission. Meanwhile the edge density is decreased. The plasma stored energy and the energy confinement time are increased with the biased voltage. Experimental results show that the biased voltage could help to suppress the MHD instability and to improve the gross global particle confinement in the HT-7 Tokamak. [ABSTRACT FROM AUTHOR]

Results are presented from experiments on the inflammation of a stoichiometric methane-oxygen mixture by a high-current multielectrode spark-gap in a closed cylindrical chamber. It is shown that, in both the preflame and well-developed flame stages, the gas medium is characterized by a high degree of ionization ( n ≈ 10 cm) due to chemoionization processes and a high electron-neutral collision frequency (ν ≈ 10 s). [ABSTRACT FROM AUTHOR]

The spherical torus or spherical tokamak (ST) is a member of the tokamak family with its aspect ratio (A=R0/a) reduced to A~1.5, well below the normal tokamak operating range of A≥2.5. As the aspect ratio is reduced, the ideal tokamak beta β (radio of plasma to magnetic pressure) stability limit increases rapidly, approximately as β~1/A. The plasma current it can sustain for a given edge safety factor q-95 also increases rapidly. Because of the above, as well as the natural elongation κ, which makes its plasma shape appear spherical, the ST configuration can yield exceptionally high tokamak performance in a compact geometry. Due to its compactness and high performance, the ST configuration has various near term applications, including a compact fusion neutron source with low tritium consumption, in addition to its longer term goal of an attractive fusion energy power source. Since the start of the two mega-ampere class ST facilities in 2000, the National Spherical Torus Experiment in the United States and Mega Ampere Spherical Tokamak in UK, active ST research has been conducted worldwide. More than 16 ST research facilities operating during this period have achieved remarkable advances in all fusion science areas, involving fundamental fusion energy science as well as innovation. These results suggest exciting future prospects for ST research both near term and longer term. The present paper reviews the scientific progress made by the worldwide ST research community during this new mega-ampere-ST era. [ABSTRACT FROM AUTHOR]

The development of the two-dimensional fluid-plasma model of a low-pressure hydrogen discharge, presented in the study, is regarding description of the plasma maintenance in a discharge vessel with the configuration of the SPIDER source. The SPIDER source, planned for the neutralbeam-injection plasma-heating system of ITER, is with localized high RF power deposition to its eight drivers (cylindrical-coil inductive discharges) and a large-area second chamber, common for all the drivers. The continuity equations for the charged particles (electrons and the three types of positive ions) and for the neutral species (atoms and molecules), their momentum equations, the energy balance equations for electrons, atoms and molecules and the Poisson equations are involved in the discharge description. In addition to the local processes in the plasma volume, the surface processes of particle reflection and conversion on the walls as well as for a heat exchange with the walls are included in the model. The analysis of the results stresses on the role of the fluxes (particle and energy fluxes) in the formation of the discharge structure. The conclusion is that the discharge behavior is completely obeyed to non-locality. The latter is displayed by: (i) maximum values of plasma parameters (charged particle densities and temperatures of the neutral species) outside the region of the RF power deposition, (ii) shifted maxima of the electron density and temperature, of the plasma potential and of the electron production, (iii) an electron flux, with a vortex structure, strongly exceeding the total ion flux which gives evidence of a discharge regime of nonambipolarity and (iv) a spatial distribution of the densities of the neutral species resulting from their fluxes. [ABSTRACT FROM AUTHOR]

A method for the study of a thin plasma channel (dipole or plasmoid) is based on the analysis of the scattered radiation. The method is experimentally tested using channels that are formed due to the development of the microwave streamers in air in the vicinity of the breakdown threshold in the pressure interval 1 × 10-1.4 × 10 Pa. The effective scattering surface, amplitude of total charge, and the current that is averaged over the channel length are estimated using the results on the plasmoid dipole moment. The lower-bound limits of volume-averaged electron conductivity and concentration are also estimated. The experimental data are compared with the results of calculations using the approximate electrodynamic integral model of the microwave streamer. [ABSTRACT FROM AUTHOR]

The electrons in capacitively coupled plasmas (CCPs) absorb energy via ohmic heating due to electron-neutral collisions and stochastic heating due to momentum transfer from high voltage moving sheaths. We use Particle-in-Cell (PIC) simulations to explore these heating mechanisms and to compare the PIC results with available theories on ohmic and stochastic heating. The PIC results for ohmic heating show good agreement with the ohmic heating calculation of Lafleur et al. [Phys. Plasmas 20, 124503 (2013)]. The PIC results for stochastic heating in low pressure CCPs with collisionless sheaths show good agreement with the stochastic heating model of Kaganovich et al. [IEEE Trans. Plasma Sci. 34, 696 (2006)], which revises the hard wall asymptotic model of Lieberman [IEEE Trans. Plasma Sci. 16, 638 (1988)] by taking current continuity and bulk oscillation into account. [ABSTRACT FROM AUTHOR]

The heating of plasma electrons during discharges driven by thermionic emission is studied using one-dimensional particle-in-cell Monte Carlo collisions modeling that self-consistently takes the dependence of the thermionic current on the plasma parameters into account. It is found that at a gas pressure of 10²Pa the electron two-stream instability is excited. As a consequence, the electro-static plasma wave propagates from the cathode to the anode. The trapping of electrons by this wave contributes noticeably to the heating of the plasma. At a larger gas pressure, this instability is not excited. As a consequence, plasma electrons are heated only because of the generation of energetic electrons in ionization events and the scattering of emitted electrons [ABSTRACT FROM AUTHOR]

We describe a hard sphere model of direct three-body recombination of the Cs and Br ions in the presence of neutral atoms Hg, Xe, Kr, or Ar as the third bodies. Calculations are carried out for the ion approach energy and the third body energy in the range from 1 to 10 eV under the assumption of non-central approach of the ions. The calculation results include the dependences of the total recombination probability on these energies as well as the opacity functions for two impact parameters and the dependences of the recombination probability on the angles determining the mutual orientation of the velocities of the reagents. The classification of the three-body collisions according to the sequences of pairwise encounters of the particles is considered. The most widespread mechanism of energy removal from the ionic pair is a single impact of the third body with the Br ion. [ABSTRACT FROM AUTHOR]

Asymptotic analysis of the continuity equations for the positive charge plasma of a low- and moderate-pressure dc electronegative discharge is performed under the simple assumption of independence of the rates of plasma-chemical processes from coordinates. As a novelty, the ion diffusion is taken into account for the case when the ion-ion recombination dominates among the perishing mechanism of negative ions. The resultant simple analytic expressions make it possible to estimate (with an error not exceeding 15-25%) the geometrical parameters of spatial distributions of charged particle concentrations in this plasma, as well as the energy parameters, without resorting to complicated numerical simulation. [ABSTRACT FROM AUTHOR]

Simulations of ITER plasma during an L-H transition triggered by pellet injection are carried out using the 1.5D BALDUR integrated predictive modeling code. In these simulations, plasma core transport is predicted using a combination of the Multimode (MMM95) turbulent transport model and neoclassical transport (NCLASS) model. The pellet ablation behavior is described using the neutral gas shielding pellet model with the grad- B drift effect also included. Because of the increase of the plasma density and the reduction of the temperature, the plasma self-heating powers (ohmic heating and alpha heating) increase, whereas the plasma energy loss (through radiation) decreases. As a result, the total power across the separatrix increases. On the other hand, the L-H power threshold decreases. With all the changes of heating, an L-H transition can be induced even though the auxiliary heating is not enough for the L-H transition before pellet injection. It is also found that the L-H transition triggered by pellet injection depends sensitively on the pellet radius, but only moderately on the pellet velocity. [ABSTRACT FROM AUTHOR]

In the present work, results of investigations into the dynamics of secondary electrons with helium atoms in the presence of the reverse electric field arising in the flare of a high-voltage pulsed beamtype discharge and leading to degradation of the primary electron beam are presented. The electric field in the discharge of this type at moderate pressures can reach several hundred V/cm and leads to considerable changes in the kinetics of secondary electrons created in the process of propagation of the electron beam generated in the accelerating gap with a grid anode. Moving in the accelerating electric field toward the anode, secondary electrons create the so-called compensating current to the anode. The character of electron motion and the compensating current itself are determined by the ratio of the field strength to the concentration of atoms (E/n). The energy and angular spectra of secondary electrons are calculated by the Monte Carlo method for different ratios E/n of the electric field strength to the helium atom concentration. The motion of secondary electrons with threshold energy is studied for inelastic collisions of helium atoms and differential analysis is carried out of the collisional processes causing energy losses of electrons in helium for different E/n values. The mechanism of creation and accumulation of slow electrons as a result of inelastic collisions of secondary electrons with helium atoms and selective population of metastable states of helium atoms is considered. It is demonstrated that in a wide range of E/n values the motion of secondary electrons in the beam-type discharge flare has the character of drift. At E/n values characteristic for the discharge of the given type, the drift velocity of these electrons is calculated and compared with the available experimental data. [ABSTRACT FROM AUTHOR]

Silicon dioxide (SiO2) films formed by chemical vapor deposition (CVD) have been treated with Ar plasma excited by microwave. The changes of the mass densities, carrier trap densities, and thicknesses of the CVD-SiO2 films with the Ar plasma treatments were investigated. The mass density depth profiles were estimated with X-Ray Reflectivity (XRR) analysis using synchrotron radiation. The densities of carrier trap centers due to defects of Si-O bond network were estimated with X-ray Photoelectron Spectroscopy (XPS) time-dependent measurement. The changes of the thicknesses due to the oxidation of Si substrates were estimated with the XRR and XPS. The mass densities of the CVD-SiO2 films are increased by the Ar plasma treatments. The carrier trap densities of the films are decreased by the treatments. The thicknesses of the films are not changed by the treatments. It has been clarified that the mass densification and defect restoration in the CVD-SiO2 films are caused by the Ar plasma treatments without the oxidation of the Si substrates. [ABSTRACT FROM AUTHOR]

A generalized diagnostics principle of microwave cut-off probe is presented with a full analytical solution. In previous studies on the microwave cut-off measurement of weakly ionized plasmas, the cut-off frequency rnc of a given electron density is assumed to be equal to the plasma frequency rnp and is predicted using electromagnetic simulation or electric circuit model analysis. However, for specific plasma conditions such as highly collisional plasma and a very narrow probe tip gap, it has been found that rnc and rnp are not equal. To resolve this problem, a generalized diagnostics principle is proposed by analytically solving the microwave cut-off condition Re[εr,eff (ω= ωc)] = 0. In addition, characteristics of the microwave cut-off condition are theoretically tested for correct measurement of the absolute electron density. [ABSTRACT FROM AUTHOR]

The generation of an ion-ion plasma where only few electrons are present in the discharge could be appropriated in the context of ion plasma source applications. We present in this paper results obtained with a one-dimensional fluid model in the context of low pressure electronegative SF6 plasma. Without magnetic field, results show that the electron density is still large in the discharge. With a localized magnetic filter, where the magnetic field strength is such that the transport of the electrons is affected while the transport of ion species remains unmagnetized, we show that a region with a negative-positive ion plasma is found downstream the magnetic filter. The negative ions are produced in the filter due to the decrease of electron temperature. We also find conditions when the plasma sheath near the biased electrode collapses and the negative ion extraction from the plasma becomes possible. In addition, the influence of E x B electron transport on the one-dimensional model results is discussed. [ABSTRACT FROM AUTHOR]

In the ℓ = 3 Uragan-3M torsatron, hydrogen plasma is produced and heated by RF fields in the Alfvén range of frequencies (ω ≲ ω). To this end, a frame antenna with a broad spectrum of generated parallel wavenumbers is used. The RF discharge evolution is studied experimentally at different values of the RF power fed to the antenna (the anode voltage of the oscillator and the antenna current) and the initial pressure of the fueling gas. It is shown that, depending on the antenna current and hydrogen pressure, the discharge can operate in two regimes differing in the plasma density, temperature, and particle loss. The change in the discharge regime with increasing anode voltage is steplike in character. The particular values of the anode voltage and pressure at which the change occurs are affected by RF preionization or breakdown stabilization by a microwave discharge. The obtained results will be used in future experiments to choose the optimal regimes of the frame-antenna-produced RF discharge as a target for the production and heating of a denser plasma by another, shorter wavelength three-half-turn antenna. [ABSTRACT FROM AUTHOR]

We present an extended numerical model of the thermal force based on the Fokker-Planck collision method. Our model is designed for the use of the test particle transport simulation in a fluid-like magnetized background plasma. By a series of systematic test simulations performed in this study, we have confirmed that our model is able to correctly simulate the thermal force which is caused not only by parallel, but also by perpendicular temperature gradient with respect to the direction of magnetic field. Effective length of collision time step for numerical calculations has also been investigated. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

Significant progress on both L- and H-mode long-pulse discharges has been made recently in Experimental Advanced Superconducting Tokamak (EAST) with lower hybrid current drive (LHCD) [J. Li et al., Nature Phys. 9, 817 (2013) And B. N. Wan et al., Nucl. Fusion 53, 104006(2013).]. In this paper, LHCD experiments at high density in L-mode plasmas have been investigated in order to explore possible methods of improving current drive (CD) efficiency, thus to extend the operational space in long-pulse and high performance plasma regime. It is observed that the normalized bremsstrahlung emission falls much more steeply than 1/ne_av (line-averaged density) above ne_av=2.2x1019m-3 indicating anomalous loss of CD efficiency. A large broadening of the operating line frequency (f=2.45GHz), measured by a radio frequency (RF) probe located outside the EAST vacuum vessel, is generally observed during high density cases, which is found to be one of the physical mechanisms resulting in the unfavorable CD efficiency. Collisional absorption of lower hybrid wave in the scrape off layer (SOL) may be another cause, but this assertion needs more experimental evidence and numerical analysis. It is found that plasmas with strong lithiation can improve CD efficiency largely, which should be benefited from the changes of edge parameters. In addition, several possible methods are proposed to recover good efficiency in future experiments for EAST [ABSTRACT FROM AUTHOR]

Gyrotrons form a specific group of devices in the class of fast-wave vacuum electronic sources of coherent electromagnetic wave radiation known as electron cyclotron masers (ECMs) or cyclotron resonance masers (CRMs). The operation of CRMs is based on the cyclotron maser instability which originates from the relativistic dependence of the electron cyclotron frequency on the electron energy. This relativistic effect can be pronounced even at low voltages when the electron kinetic energy is small in comparison with the rest energy. The free energy for generation of electromagnetic (EM) waves is the energy of electron gyration in an external magnetic field. As in any fast-wave device, the EM field in a gyrotron interaction space is not localized near a circuit wall (like in slow-wave devices), but can occupy large volumes. Due to possibilities of using various methods of mode selection (electrodynamical and electronic ones), gyrotrons can operate in very high order modes. Since the use of large, oversized cavities and waveguides reduces the role of ohmic wall losses and breakdown limitations, gyrotrons are capable of producing very high power radiation at millimeter and submillimeter wavelengths. The present review is restricted primarily by the description of the development and the present state-of-the-art of gyrotrons for controlled thermonuclear fusion plasma applications. The first gyrotron was invented, designed and tested in Gorky, USSR (now Nizhny Novgorod, Russia), in 1964. [ABSTRACT FROM AUTHOR]

Numerical modeling and theoretical analyses of spatial distribution with detailed set of plasmachemical processes in elegas have been made for a wide range of pressures. Evolution of main charged and neutral particles in time and space has been obtained. It is shown that at low pressures, the Boltzmann distribution holds for both electrons and negative ions and separation of the plasma into two regions with an almost flat profile of the electron density. At high pressures, electronegativity of plasma is extremely high and the self-consistent electric field is determined not only by the electrons, as in the conventional plasma, but also by the ions of both signs. This leads to an abnormally small potential drop in the plasma and nonmonotonic ion profiles. [ABSTRACT FROM PUBLISHER]

Plasma parameters in the Globus-M spherical tokamak have been measured upon injection of a deuteron beam with energy about 30 keV and a power of up to 700 kW. Dependences of the ion temperature and neutron yield on the plasma current and density and the gap width between the wall and plasma are considered. Possible characteristics of fast-ion confinement in the modified Globus-M2 tokamak are discussed. [ABSTRACT FROM AUTHOR]

Results from a 3D self-consistent Particle-In-Cell Monte Carlo Collisions (PIC MCC) model of a high power fusion-type negative ion source are presented for the first time. The model is used to calculate the plasma characteristics of the ITER prototype BATMAN ion source developed in Garching. Special emphasis is put on the production of negative ions on the plasma grid surface. The question of the relative roles of the impact of neutral hydrogen atoms and positive ions on the cesiated grid surface has attracted much attention recently and the 3D PIC MCC model is used to address this question. The results show that the production of negative ions by positive ion impact on the plasma grid is small with respect to the production by atomic hydrogen or deuterium bombardment (less than 10%). [ABSTRACT FROM AUTHOR]

This paper presents research results on the ion charge state and energy distribution in the anode plasma generated at the spark stage of a high-current vacuum discharge. The cathode and the anode are both made of liquid metal Ga–In alloy. The use of a circuit with a relatively low impedance (6 \Omega) and a small interelectrode gap (\sim1~mm) made it possible to obtain a relatively high discharge current (\sim1~kA) at an initial voltage of 10–15 kV. Therefore, an electron beam consisting of a train of pulses with a duration of 5 ns and power density of \sim2.5\cdot 10^8~W/cm^2 is formed in \sim50~ns, giving rise to high-density anode ablation plasma. The parameters of the ion component of this plasma are investigated with a mass energy analyzer. It is found that ions with Z=1\--4 are present in the plasma, and the ion energy increases roughly in proportion with increasing ion charge. Increasing the initial voltage causes an increase in the fraction of highly charged ions and in their energy. The absolute values of the ion energy reach \sim1~keV. Two mechanisms of ion acceleration are discussed: adiabatic expansion of the anode plasma and ion acceleration by an ambipolar electric field. It is shown that the regularities found in this paper can be explained only with resort to both mechanisms. [ABSTRACT FROM AUTHOR]

With the increase of interruption frequency of high-current vacuum arc (HCVA), the relaxation time of arc plasma is close to characteristic time of arc current and transient characteristics of vacuum arc plasma become more obvious. In this paper, based on a 2-D transient magneto-hydrodynamic model, HCVA under different interruption processes is simulated and analyzed. The simulation results of power-frequency (50 Hz) current show that transient simulation results are the same as those of the steady-state model, as characteristic time of current is much longer than relaxation time of plasma. For fast current drop stage in dc interruption process, simulation results show that higher frequency makes current drops faster and ion number density larger; besides, residual plasma density between electrodes at current-zero moment is higher, and interruption becomes more difficult. Finally, transient characteristics of high-frequency HCVA is also simulated and analyzed. Simulation results show that change curve of ion number density lagged behind current curve; higher ion number density in the second 1/4 cycle can lead to reignition more easily. [ABSTRACT FROM AUTHOR]

We report on model studies of stimulated L → H transitions [K. Miki et al., Phys. Rev. Lett. 110, 195002 (2013)]. These studies use a reduced mesoscale model. Model studies reveal that L → H transition can be triggered by particle injection into a subcritical state (i.e., PThresh). Particle injection changes edge mean flow shear via changes of density and temperature gradients. The change of edge mean flow shear is critical to turbulence collapse and the subsequent stimulated transition. For low ambient heating, strong injection is predicted to trigger a transient turbulence collapse. Repetitive injection at a period less than the lifetime of the collapsed state can thus maintain the turbulence collapse and so sustain a driven H-mode-like state. The total number of particles required to induce a transition by either injection or gas puffing is estimated. Results indicate that the total number of injected particles required is much smaller than that required for a transition by gas puffing. We thus show that internal injection is more efficient than gas puffing of comparable strength. We also observe that zonal flows do not play a critical role in stimulated transitions. For spontaneous transitions, the spike of the Reynolds work of turbulence on the zonal flow precedes the spike in the mean electric field shear. In contrast, we show that the two are coincident for stimulated transitions, suggesting that there is no causal link between zonal and mean flows for stimulated transitions. [ABSTRACT FROM AUTHOR]

A technique is presented for determining the parameters of a high-energy plasma jet during the irradiation of the first wall of a thermonuclear reactor on a plasma gun bench. The plasma density is measured with a Michelson interferometer mounted on a through flange. The plasma velocity is determined by observing the propagation of the ionization front using a K008 streak camera. The plasma composition at the output of the accelerator and near the irradiated target is found with an AvaSpec 3648 panoramic spectrometer. The kinetic energy of the jet is recorded calorimetrically. The action (pressure) of the plasma jet on the target is confirmed with piezoelectric and interferometric methods simultaneously in each radiation event. It is found that the plasma gun bench is able to generate a plasma with an energy flux density similar to that observed in the case of edge-localized modes in a tokamak and makes it possible to carry out experiments aimed at developing protective materials for the international thermonuclear experimental reactor. [ABSTRACT FROM AUTHOR]

The parallel Mach numbers are measured at three locations in the scrape-off layer (SOL) plasma of ADITYA tokamak by using Mach probes. The flow pattern is constructed from these measurements and the modification of flow pattern is observed by introducing a small puff of working gas. In the normal discharge, there is an indication of shell structure in the SOL plasma flows, which is removed during the gas puff. The plasma parameters, particle flux and Reynolds stress are also measured in the normal discharge and in the discharge with gas puff. It is observed that Reynolds stress and Mach number are coupled in the near SOL region and decoupled in the far SOL region. The coupling in the near SOL region gets washed away during the gas puff. [ABSTRACT FROM AUTHOR]

Through particle-in-cell simulations and analytics, a host of interesting and novel wave effects in nonstationary plasma are examined. In particular, Langmuir waves serve as a model system to explore wave dynamics in plasmas undergoing compression, expansion, and charge recombination. The entire wave life-cycle is explored, including wave excitation, adiabatic evolution and action conservation, nonadiabatic evolution and resonant wave-particle effects, collisional dissipation, and potential laboratory applications of the aforementioned phenomenology. [ABSTRACT FROM AUTHOR]

In this work we present new mathematical formulations for several classical models of a continuum media. The developed formulations take into account the physical constraints on the details of its description. As compared to classical approaches, the presented ones introduce additional terms, which enable the effective solution of these problems on high-performance computer systems. [ABSTRACT FROM AUTHOR]

We have experimentally studied the influence of toroidal magnetic field ( B) and plasma current ( I) on the capture and confinement of energetic ions (EIs) formed upon ionization of a neutral beam injected in a tokamak. Based on the results of measurements of the flux of 2.45-MeV fusion neutrons, it is concluded that the amount of EIs significantly grows with increasing B from 0.7 to 1.0 T and Ip from 140 to 180 kA. In addition to the classical Coulomb slowing down, a supplementary channel of EI losses is found that accounts for a 15% decrease in their confinement time. [ABSTRACT FROM AUTHOR]

Experimental results on the generation and maintenance of the toroidal current in the Globus-M spherical tokamak by using waves in the lower hybrid frequency range without applying an inductive vortex electric field are presented. For this purpose, the original ridge guide antennas forming a field distribution similar to that produced by multiwaveguide grills were used. The high-frequency field (900 MHz) was used for both plasma generation and current drive. The magnitude of the generated current reached 21 kA, and its direction depended on the direction of the vertical magnetic field. Analysis of the experimental results indicates that the major fraction of the current is carried by the suprathermal electron beam. [ABSTRACT FROM AUTHOR]