Quasistationary plasma accelerators (QSPAs) can be used for diverse applications, including plasmasurface interaction experiments in the conditions close to the fusion reactor transient events (e.g., current disruptions, VDE, and giant ELMs), modifying material surfaces, and creating new surface alloys [1]. A magnetoplasma compressor (MPC), a QSPA-type device capable of generating pinching plasma streams with densities up to $10^{19}-10^{20} \mathrm{~cm}^{-3}$, can be further enhanced by applying an additional magnetic field, opening the possibility for improved discharge characteristics and plasma parameters [2]. The influence of an external magnetic field in the discharge channel of the MPC on the plasma stream dynamics was studied. The complex structure of the plasma stream undergoes significant changes when exposed to an external magnetic field: the magnitude of the electric potential and current increases, fewer current vortices tend to form. The external magnetic field improves the compression characteristics of the plasma flow, in particular, it increases the compression zone size and radial component of the $\mathrm{J} \times \mathrm{B}$ force. Introducing a magnetic field causes the current-sheet-like structure found in our recent research [2] to form earlier. These results significantly expand the range of possible applications of the device.

The paper presents experimental studies of a shielding plasma layer formation in front of a tungsten surface exposed with hydrogen plasma in the QSPA-M test-bed facility under the conditions of additional seeding of argon (Ar) along the target surface into the zone of plasma-surface interaction. A pulsed gas injector on the base of a fast electromagnetic valve has been developed for the local injection of Ar. The injector is capable of generating a homogeneous argon gas flow with a maximum concentration above n _Ar = 6 × 10 ^23 m ^−3 and a pulse duration of 0.5 ms. It is shown that the increase in the argon gas density in front of the surface leads to an essential decrease (in 1.5–2 times) in the energy load delivered to the target surface. In the presence of a strong magnetic field (up to 1 T), both the thickness of the shielding layer and the fraction of energy dissipated by the shield increase further. Even for moderate energy densities of the QSPA plasma streams in the experiments with Ar gas injection, less than 40% of the impacting plasma load is absorbed by the tungsten surface. The results demonstrate that this additional shielding attributed to the formation of a dense Ar plasma layer in front of the exposed W surface would be favourable for the divertor armour performance, causing the decreasing erosion of plasma-facing components in the course of transient events in a fusion reactor.

The synergistic effects of tungsten exposure to combined hydrogen and helium particle fluxes as well as transient thermal loads need to be extensively studied for implementation of fusion reactor project. The mixture of hydrogen and helium was used as the working gas for plasma stream generation within the QSPA-M accelerator. The parameters of the mixed hydrogen and helium plasma were similar to those of pure hydrogen plasma generated in QSPA-M. It was shown that the small addition of helium (5 %) to hydrogen does not strongly influence plasma surface interaction. The influence of the external magnetic field on plasma surface interaction is also discussed.

The influence of the external axial magnetic field on pinching plasma flows generated by a magnetoplasma compressor (MPC) has been studied using magnetic and electric probes. In the presence of an external magnetic field, temperature measurements show two groups of electrons with different temperatures near the plasma stream core. The external magnetic field leads to a noticeable increase in the electric current in the plasma stream, electron temperature, and the formation of the current-sheet-like structure observed in the MPC for the first time.

The formation of the current-sheet-like structure under the influence of the external magnetic field in pinching plasma flows generated by the magnetoplasma compressor has been studied. A set of magnetic and electric probes were used to measure the self-generated magnetic field, electron temperature, and electric field locally with sufficiently high temporal and spatial resolution. The data obtained from the probe measurements were used to plot the spatial distributions of electric current and drift velocity in the plasma stream to identify the patterns of plasma flow. In the presence of external magnetic field, the current density, electron temperature, and the ion saturation current reach a peak in close proximity to the sheet. Measurements indicate the outflow of two electron jets of different temperatures from the layer.

Pinching plasma flows in an additional axial magnetic are an interesting phenomenon that has many applications in various fields of science and technology. The influence of the external axial magnetic field of 0.24 T on helium plasma flows generated by a plasma accelerator of a magnetoplasma compressor type has been studied. Measurements of the self-generated magnetic field were conducted using different magnetic probes inserted in the plasma flow outside the accelerating channel during the discharge. The data were obtained with and without an external axial magnetic field at different distances from the outlet of the accelerator, taking into account the axial symmetry of the plasma flow. Two-dimensional distributions of the electric current flowing outside the accelerating channel were plotted using the data from the magnetic probe measurements and analyzed. The external axial magnetic field leads to an increase in the magnitude of the self-generated magnetic field in plasma flows and, therefore, changes the spatial distribution of the electric current.

The local electron temperature measurements with the double electric probe in the compression zone are presented. Electric probes make it possible to measure the electron temperature with a reasonably good spatial resolution. Double electric probe application for electron temperature measurements in the dense self-compressed plasma stream is discussed. We have shown experimentally that the electric probe operates in a diffusion regime.

This paper presents experimental studies of plasma-surface interactions during powerful plasma impacts of a quasi-stationary plasma accelerator (QSPA) on the Sn capillary porous systems (CPSs) in conditions simulating disruption and edge localized modes (ELM) like loads. Experiments were carried out using two QSPA devices. ELM-like plasma exposures were performed with QSPA-M test-bed facility. A large-scale QSPA Kh-50 device was used to simulate plasma disruptions and giant ELMs. Variation of the plasma stream energy density has been performed to study the onset of vapour shield. It is shown that during plasma exposures of a Sn-CPS target with the QSPA plasma load <1 MJ m(-2), single dust particles traces have been registered. A further increase in the heat load leads to the splashing of the eroded material. For ELM-like impacts, a rather weak melt motion was observed on the target surface. A post-mortem analysis has shown that the CPS structure was not destroyed in the course of many repetitive ELM-like pulses. Surface morphology has changed from a smooth surface to corrugation structures with the formation of some cavities in mesh cells due to the influence of the surface tension and capillary effects. Spectral lines of Sn I and Sn II have been identified by optical emission spectroscopy in the near-surface plasma. A plasma shield, that consists mostly of Sn neutrals appears at Q similar to 0.1 MJ m(-2). An increase in the surface heat load resulted in the intensive emission of Sn II lines, which started to be observed at Q similar to 0.3 MJ m(-2). The plasma electron density near the surface increases significantly at Q > 0.5 MJ m(-2), which corresponds to the strong vapour shielding of the exposed surface. A comparison between the obtained results on the vapour shielding of Sn CPS and available numerical simulation using the TOKES code has been performed.

Present experimental studies are aimed at analysis of hydrogen plasma stream parameters in various working regimes of QSPA-M operation. Temporal distributions of plasma electron density are reconstructed with optical emission spectroscopy. The magnetic field influence on plasma streams parameters is analyzed. It is shown that in regimes with additional magnetic field the plasma electron density increases by an order of magnitude in comparison with a density value without magnetic field. The plasma velocity and energy density parameters as well as their temporal behaviors were estimatedin different operating regimes of QSPA-M facility. Features of plasma visible radiation were analyzed. This information is important for QSPA-M applications in experiments on interaction of powerful plasma streams with material surfaces.

This paper reports the findings of our recent experiments on the evaluation of the external axial magnetic field effect on the discharge parameters of a magnetoplasma compressor (MPC). The discharge-voltage characteristics were obtained for varied magnitudes of an external axial magnetic field produced by a solenoid installed on the accelerating channel. Present experiments were carried out with helium (at the initial pressures of 2 and 10 Torr) and argon (at the initial pressure of 1 Torr) as working gases at different initial voltages. The external magnetic field varied up to 0.4 T. The discharge-voltage characteristics can be altered by the magnitude of the external magnetic field, as well as by the geometrical properties of an accelerating channel.

The features of plasma energy transfer to material surfaces during plasma-surface interactions (PSIs) in the presence of a strong magnetic field are investigated within the recently developed quasi-stationary plasma accelerator, QSPA-M. This novel PSI test-bed facility can reproduce edge localized mode (ELM) impacts, both in terms of heat load and particle flux to the surface, and provide plasma transportation in an external magnetic field, which mimics the divertor conditions. Investigations of energy transfer to the material surface have been performed for varied plasma heat load and external magnetic field values. Calorimetry, optical emission spectroscopy and high-speed imaging were applied for PSI characterization. For perpendicular plasma incidence, it has been shown that the transient plasma layer is formed in front of the surface by the stopped head of the plasma stream even for rather small plasma heat loads, which do not result in surface melting. The plasma density in this near-surface layer is much higher than in the impacting stream. It leads to the arisen screening effect for energy transfer to the surface. For B = 0, the thickness of the screening layer is less than 3 cm, but it increases to 15 cm when B = 0.8 T. The shielding effect due to the formation of a dense plasma layer in front of the exposed surface should be favorable for material performance, being important for decreasing the overall erosion of plasma-facing components during a large number of repetitive ELMs.

This paper reports the outcomes of recent experiments on the assessment of the external longitudinal magnetic field effect on the discharge characteristics and the main plasma stream parameters of the magnetoplasma compressor (MPC). The MPC device has been upgraded with the solenoid installed on the accelerating channel. Present experiments were carried out with helium as a working gas (P = 2 Torr) at a voltage up to 20 kV. The magnetic field varied up to 0.3 T. The additional longitudinal magnetic field exerts significant influence on the potential difference, the radial component of the electric field in the vicinity of the MPC electrodes and the electric currents that flow outside the MPC accelerating channel. PACS: 52.40.Hf; 52.70

The experiments have been recently carried out in the MPC facility. Three gases were used at different initial concentrations. Spatial distributions of the Ampere force have been plotted by using the data retrieved from the magnetic probe measurements. The results have clearly shown that the peculiarities of the compressive structures where the force is directed mainly to the near-axis region or/and opposite to the plasma flow are intrinsically related to specific operating modes. Plasma velocity and density measurements conducted earlier, which illustrate dynamics of the plasma stream deceleration and formation of the compression zone with the average electron density above 10 cm, are in agreement with obtained distributions of the local Ampere force.

This paper is devoted to the investigation of magnetohydrodynamic characteristics of plasma streams generated by a magnetoplasma compressor (MPC) and control mechanisms of a compression zone position. Nitrogen, helium, and argon were used as working gases. The measurement results of electric currents spatial distributions in the plasma streams identified that for helium (P = 10 Ton) both toroidal vortices and magnetic field displacement from the near-axis region are observed, then, the electric current direction reverses. Similar spatial structure of the electric currents was observed for helium with the initial pressure of 2 Torr. However, in this case, the electric current direction changes much earlier. The electric currents flow from 20 cm to 30 cm from the central electrode of MPC accelerating channel in the modes with nitrogen (P = 0.6 and P = 0.3 Ton). There are current vortices and a sizable magnetic field displacement at a distance of a 6 cm to 18 cm from the MPC output. The duration of a plasma stream generation is about two times less for helium than for the modes of operation with other gases.

In this report a concept of a new generation QSPA with external B-field up to 2 T has been discussed. A novel test-bed facility, which was recently constructed in Kharkov IPP NSC KIPT, has been described. It allows for a new level of plasma stream parameters and its wide variation in new QSPA-M device, as well as possible combination of steady-state and pulsed plasma loads to the materials during the exposures. First plasma is recently obtained. Careful optimization of the operational regimes of the plasma accelerator’s functional components and plasma dynamics in the magnetic system of QSPA-M device has started approaching step by step the necessary level of plasma parameters and their effective variation. The relevant results on plasma stream characterization are presented. Energy density distributions in plasma stream have been measured with calorimetry. Spectroscopy and probe technique have also been applied for plasma parameters measurements. The obtained results demonstrate the ability of QSPA-M to reproduce the ELM impacts in fusion reactor, both in terms of heat load and particle flux to the surface.

The analysis of fundamental properties of the compression zone in the self-compressed plasma streams generated by a magnetoplasma compressor (MPC) is carried out. The main attention is attended to the research of the dependences of basic plasma parameters in a compressed plasma stream depending on the initial conditions. It has been shown experimentally that the reduction of the initial concentration of a working gas leads to an increase of the plasma density in the compression zone. The detailed studies of the spatial distributions of currents in the plasma flows are fulfilled for different initial concentrations of a substance in the accelerating channel of MPC. Under the experiment conditions, it is found that a decrease of the initial concentration of a working gas leads to the displacement of the currents from the compression zone.

The results of experimental investigations on self-compressed plasma streams and compression zone formation are presented for varied mass flow rate and initial concentrations of particles of working gas that depend on initial pressure. Experiments were carried out in the Magnetoplasma Compressor (MPC) facility. Space-time distributions of the electric current and electron density in the plasma stream compression region were measured under different experimental conditions. High-speed images of plasma stream dynamics in the MPC accelerating channel with a high temporal resolution were also obtained for different initial pressures. The experimental results show a strong dependence of plasma stream parameters and compression zone location on the initial gas concentration. The maximum electron density is obtained in the range of Ne = (1 divided by 5) x 10(18) cm(-3). Plasma streams have a good radial symmetry under all experimental conditions. The distributions of plasma parameters along the plasma stream flows are discussed.

The spectroscopic technique used to measure the parameters of the plasma jets generated in the plasma focus discharge and those of the plasma of the immobile gas through which these jets propagate is described. The time evolution of the intensities and shapes of spectral lines in experiments carried out with helium at the PF-3 facility was studied by means of electron-optical streak cameras. The plasma electron temperature, T ≈ 4–5 eV, was determined from the intensity ratio of two spectral lines, one of which (λ1 = 5876 Å) belongs to neutral helium, while the other (λ2 = 4686 Å), to hydrogen-like helium ions. The plasma density at different time instants was determined from the Stark broadening of these lines in the electric fields of different nature. The plasma density is found to vary from 4 × 1014 to 2 × 1017 cm−3.

Studies of main characteristics of the plasma streams generated by magneto-plasma compressor (MPC) and quasi-stationary plasma accelerator QSPA Kh-50 have been performed. Main features of Plasma Surface Interaction (PSI) have been researched in dependence on plasma heat loads, plasma density and pulses duration. QSPA Kh-50 creates long pulse plasma streams (pulse length of 0.25 ms) with heat load on exposed surfaces of (0.45...0.75) MJ/m(2). The MPC generates short (duration of 10...20 ms) compressed plasma streams with plasma density up to 1018 cm(-3), and plasma energy density of (0.05...0.5) MJ/m(2). Performed studies of plasma-surface interaction include measurements of plasma energy deposited to the material surface as a function of the impacting energy and kind of targets. Temporal and spatial dependencies of electron density and temperature have been found. Special attention was paid to the dynamics of the spectral lines near surfaces of exposed targets.