Within the framework of the federal project “Development of controlled fusion technologies and innovative plasma technologies”, SRC RF TRINITI JSC is working on the creation of a prototype plasma rocket thruster based on a two-stage quasi-stationary high-current plasma accelerator where the processes of propellant preliminary ionization and final high-speed flow formation are separated. The discharge characteristics of the input ionization chamber were determined when using hydrogen and helium as a propellant in the relevant flow range of 1.5–3 mg per pulse, on the basis of which recommended values of specific energy input in the range of 2.2–2.6 kJ/mg for H2 and 1.2–1.6 kJ/mg for He were established. The plasma temperature estimation for hydrogen at 0.8 eV is consistent with the measured plasma flow velocity of 16 ± 3 km/s. The conversion coefficient of the input electrical energy into the plasma flow energy was 65

The equipment and method for measuring the plasma flow velocity of a quasi-stationary high-current plasma accelerator (QSPA) based on high-speed Doppler shift spectroscopy are described. Time sampling of measurements can reach 100 kHz, which makes it possible to study in detail processes lasting about 1 ms or more. The correspondence between the flow velocity values obtained by Doppler shift spectroscopy and the time-of-flight method has been demonstrated. The results of measurements are presented, showing that the velocities of the QSPA plasma flow lie in the range of 30–160 km/s, depending on the energy input into the discharge and the composition of the working gas.

The spatial and temporal dependencies of the characteristics of the hydrogen plasma flow generated in quasi-stationary plasma accelerator were investigated. The spatiotemporal structure of discharge radiation in the interelectrode gap was studied. The range of changes in the length of the plasma glowing region in the interelectrode gap during the discharge pulse was determined. The region with bright plasma radiation located in the output face of the accelerator electrode system was observed. The presence of impurities and increased electron concentration values were observed in this region. Fluctuations in the radiation intensity of the plasma flow were detected along the entire length of its propagation. The spatial and temporal characteristics of these fluctuations were determined. The electron concentration values near the output face of the electrode system were obtained by measuring the Stark broadening of the Hβ line. For the first time, the time dependence of the electron concentration of free plasma flow was obtained using two methods simultaneously. The measurements were conducted at a distance, which significantly exceeds the characteristic size of the electrode system and where the influence of interelectrode processes of plasma flow generation is reduced. The first is based on measuring the Stark broadening of Hβ. As a second method, heterodyne interferometry was used.

An experimental setup for studying pulse gas flows within short periods (up to 1 ms) was developed and an experimental data processing method is presented. Based on high-speed frame interferometry data and the results of dynamic pressure measurements, the spatial and temporal distributions of the helium flow density and velocity are determined. The optimal method for reconstructing the spatial density distributions with consideration for the experimental errors is described. The presented method allows characterization of the gas flows with a density of more than 0.0001 kg/m 3 and a velocity of more than 400 m/s.

In order to simulate ITER transient events with surface heat load parameters relevant to edge-localized-mode (ELM) impacts, tungsten samples were exposed to pulsed heat loads using pure deuterium (D) and with 10% helium (He) seeding plasmas in quasi-stationary high-current plasma gun QSPA-T. The pulse duration was 1 ms that is relevant to ELMs and number of pulses was varied from one to thirty. The power load was 0.7 MJ/m2 that is below the tungsten melting temperature (Tm). Two tungsten samples were used, namely, polycrystalline tungsten without (W) and with pre-existing He-induced W ‘fuzz’ (Wf). Similar to the steady state plasma, the presence of He in tungsten leads to a reduction of the D retention during transient events at temperature below Tm. We explained it as interruption of the D diffusion towards to the bulk of tungsten by (i) the strain field induced by He bubbles and (ii) the formation of interconnected He bubbles at high temperature which leads to an open porosity for accelerated D desorption, thus, decreasing the D influx into the tungsten bulk. But as the He retention in Wf decreases below 1019 He/m2, the effect of He on the D retention after the plasma gun irradiation disappears: the D retention in W and Wf is the same after 30 pulses of the exposure to pure D plasma. In both cases of pure D and He seeded D plasma gun exposures, the D retention is higher compared to the steady state plasma exposure at sample temperature above 600 K.

The results of calculations of the spectral and integral characteristics of radiation in flows of ionizing gases in the channel of the quasi-stationary plasma accelerator in the presence of an additional longitudinal magnetic field are presented. The numerical model of the radiative magnetogasdynamics is based on the modified transport equations for the multicomponent medium consisting of atoms, ions, and electrons. The MHD equations for two-dimensional axisymmetric flows are presented in terms of the azimuthal components of magnetic field and vector potential of magnetic field, taking into account the electrical conductivity, thermal conductivity, and radiation transport. The spectral and integral characteristics of radiation were determined within the framework of the 3D model using the method of long characteristics in the multigroup approximation.

Numerical study of the ionization process and radiation transport in a flow of ionizing helium in the channel of the quasi-stationary plasma accelerator is presented. The model of two-dimensional axisymmetric flows is based on the modified MHD equations for the multicomponent medium consisting of atoms, electrons, and multiply charged ions with different ionization multiplicity. The numerical model takes into account the electrical conductivity and thermal conductivity, and the 3D model of radiation transport includes the main mechanisms of radiation and absorption for different parts of the spectrum. The spectral field of radiation and its integral characteristics in the forming transonic stream of helium plasma are determined.

Surfaces facing the gap between W tiles of the ring limiter of tokamak T-10 were analyzed after T-10 decommissioning using LIBS, SEM/EDA, XRD, TDS, and NRA techniques. Gaps with the width of 5 mm and 0.1 mm were nearly completely covered to their full depths of 22 and 15 mm, respectively, by a deposited film. The film was formed mainly by deposition of lithium that came from Li limiter and transformed in air to Li2CO3 and Li2O. Carbon was deposited from volatile hydrocarbons sputtered from the tokamak walls. Besides, carbon appeared due to chemical reaction with lithium in air. Chemical interactions of W with C, O, and Li led to formation W2C, WC, WO2, and Li2WO4. Carbides formed in W over the entire surface to the full depth of the gaps. Trapping of deuterium and helium in tiles was demonstrated. Possible influence of auto-oscillating discharges on ionization and ion trapping of C,D, and He in gaps is discussed.

A method of ion saturation current measurement in a nonequilibrium plasma by a self-oscillating voltage sweep on the probe is described. The generation of voltage pulses is possible if a high secondary electron emission from the probe surface is ensured, significantly exceeding unity at a moderate negative potential. The theoretical basis of the self-oscillating probe method has been considered, which made it possible to describe the ways for controlling the repetition rate and pulse amplitude and the effect of plasma parameters on the shape of current and voltage signals. It is shown that the features of the phase trajectory of the signal from the probe on the (U, I) plane allow one to determine the reference points of instantaneous probe characteristic, in particular, the ion saturation current. The results of experiments on testing the self-oscillating probe technique, which were carried out using the PR-2 plasma-beam facility, are presented.

Positron annihilation lifetime spectroscopy (PALS) was applied to study the annealing of radiation-induced defects in polycrystalline tungsten (W) irradiated with 21.6 MeV protons at 100 degrees C up to a fluence of 5 x 10(15) p/cm(2). Three components were observed in the measured spectra: short-lifetime of 100 - 120 ps (positron annihilation in the defect-free W lattice), medium-lifetime of similar to 190-330 ps (annihilation at mono-vacancies and small vacancy cluster containing similar to 2-4 vacancies) and long-lifetime of similar to 500 ps (annihilation in large vacancy clusters containing more than 10 vacancies). The irradiation of W with protons at 100 degrees C, primary, led to the formation of mono-vacancies, self-interstitial defects were created as well but migrated towards sinks during the irradiation. Onset of vacancy diffusion in W starts already at 200 degrees C before defect recovery stage III. After annealing at similar to 400 degrees C, a sharp drop in the intensity of the positron medium-life component together with a simultaneous increase in positron lifetime from similar to 220 to similar to 280 ps is observed, and a long-life component appears. This indicates migration and annealing of vacancies and their agglomeration in large vacancy clusters. After annealing at 500-700C, the intensity of long-life component increases indicating the growth of large vacancy clusters but at 900 degrees C they anneal completely as the mean lifetime recovers nearly to the value measured in the un-irradiated material. (C) 2019 Elsevier B.V. All rights reserved.

In this paper we apply a time-dependent radiative-collision model for analysis of radiation of impurity atoms sputtered in uniform plasma. As an example we consider spectroscopy data of neutral molybdenum atoms sputtered in argon plasma at PR-2 linear plasma device. We use effective collision strengths for electron impact excitation/de-excitation and spontaneous emission obtained earlier by relativistic R-matrix calculations. The ionization rates resolved by excited levels are estimated by the approximate ECIP method. The main feature of the experimental spectra is the maximum of radiation intensity situated at a noticeable distance from the target surface, different for different wavelength. This feature is reproduced by our calculations proving that appearance of the maximum is due to slow excitation of some levels. In contrast to the previous works on the problem, we demonstrate that neither electron-impact ionization nor escape of atoms from the plasma column are necessary for appearance of the maximum. We also show that if the initial population of atoms in excited state exceeds a certain limit, 5% in our particular case, the maximum disappears completely, giving the upper estimate of the initial population.

Sputtering by ions with low near-threshold energies is investigated. Experiments and simulations are conducted for tungsten sputtering by low-energy, 85-200 eV Ar atoms. The angular distributions of sputtered particles are measured. A new method for molecular dynamics simulation of sputtering taking into account random crystallographic surface orientation is developed, and applied for the case under consideration. The simulations approximate experimental results well. At low energies the distributions acquire "butterfly-like" shape with lower sputtering yields for close to normal angles comparing to the cosine distribution. The energy distributions of sputtered particles were simulated. The Thompson distribution remains valid down to near-threshold 85 eV case. (C) 2017 Elsevier B. V. All rights reserved.

A new system of probe diagnostics at linear plasma simulator PR-2 is described, allowing us to measure the profiles of plasma temperature and density in different cross sections of the plasma column. The Langmuir probe fixed to the movable part of the two-coordinate positioning system built into the PR-2 passes the region of the discharge area during the process of measuring plasma parameters. The overall dimensions of the positioning system make it possible to mount electrical probes (magnetic probes, optical fibers, and other diagnostic equipment) covering almost the entire volume of the vacuum chamber between the magnetic mirrors of the device. We present the measurement results of local plasma parameters of the beam-plasma discharge (BPD) for different values of the input power and working gas pressure. The boundaries of appearance of discharge of three types were determined: the diffusive BPD mode, the BPD mode, and the arc mode. Dependences of the plasma concentration and temperature on the input power for different values of pressure have been also determined.

The features of linear simulators with beam-plasma discharge are examined. A description of five discharge modes is provided. They can be implemented in such devices, depending on working-gas pressure, operating parameters of the electron gun and collecting electrode: vacuum beam transport, diffuse discharge, beam-plasma discharge, vacuum arc and self-oscillating modes. We describe how to use these modes in combination with diagnostics methods and sample holders, which allow studies of thermal and plasma loading, thermal cycling, sputtering, redeposition, gas capture, cleaning of deposited layers, development of instabilities, and ion etching.