Search

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 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.

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 work presents experimental studies of plasma surface interactions (PSIs) during QSPA plasma impacts on the Sn Capillary Porous Structures (CPSs) in conditions, simulating disruption and ELM-like loads. The 3D tungsten target filled by Sn was exposed to the plasma streams with an energy density of up to 3 MJ/m2 which resulted in tungsten melting and evaporation. The initial temperature of a sample between plasma pulses had two options: remained at a room value for one sample, and reached 300 C which is close to Sn melting point for another one. The number of plasma irradiations reached 100 pulses. It is shown that the particle ejection was observed during PSI for pre-heated as well as cold CPS targets. The difference in values of the number of particles ejected from the targets, their velocities, and the start-time during exposition is discussed. Particles leave the target surface during all expositional pulses for both CPS targets. The threshold character of the ejection is more pronounced for the cold target. After the 100 pulses reduced Sn pool is observed. Moreover, areas without Sn occur on the exposed targets. Nevertheless, the severe damage of the W base of the CPS targets is not revealed during high-cycle powerful plasma loads.

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.

This paper is focused on the analysis of surface tension contribution to the erosion features of tungsten resolidified surfaces and resulting material response to a large number of repetitive transient hydrogen plasma impacts. Experimental investigations of erosion processes on castellated tungsten surfaces in conditions relevant to uncontrolled ITER ELMs have been performed within powerful quasi-stationary plasma accelerator QSPA Kh-50. The surface energy load measured with a calorimeter was varied between melting (0.6 MJ/m2) and evaporation (1.1 MJ/m2) thresholds, the plasma pulse duration was 0.25 ms. Observations of plasma interactions with exposed W surfaces, analysis of dust particle dynamics and the droplets monitoring have been performed with a high-speed digital camera. Repetitive plasma loads above the melting threshold led to the formation of melted and resolidified surface layers. Networks both macro and intergranular cracks appeared on exposed surfaces. Cracks propagate to the bulk mainly transversely to the irradiated surface. The splashing of dust/liquid particles has been analyzed in the course of repetitive plasma pulses. It was revealed that mountains of displaced material at the edges of castellated units are the primary source of the splashed droplets due to the development of instabilities in a melted layer. The solid dust ejection dominates by cracking processes after the end of pulse and surface resolidification. Due to the continuously growing crack width (from value of sub-μm till tens μm) with the increasing number of pulses the initially uniform melt pool on the castellated units became disintegrated into a set of melt structures separated by cracks. After a large number of exposures the progressive corrugation of the surface occurred due to the capillary effects on exposed W surfaces. Results of simulation experiments for castellated targets and developed surface structures are compared with repetitive plasma exposures of flat tungsten surfaces. Keywords: Plasma-Materials Interaction, Tungsten, Surface Effects, Divertor Materials

The paper describes the design features of a gas valve for the axial (parallel to the axis of the accelerator) working gas supply into the accelerator channel. The results of gas-dynamic studies of the injectors are presented. The amount of the working gas inlet was investigated as a function of gas pressure under the blocking valve element. A strong dependence of the amount of the injected working gas on the electric current value in the control coil was found.

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.

An impulsive gas valve with an electrodynamic drive of the locking element is described. The valve is designed to inject working gases into a discharge channel of plasma devices of the compression and acceleration types. The key gas-dynamic and electrical characteristics of the impulsive gas valve are presented. The valve is capable of forming gas pulses with a wide range of gas-dynamic parameters: the supplied volume of the working gas is 30–540 cm3 per pulse (at atmospheric pressure) and the gas pressure in the flow is as high as 760 Torr.

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 is devoted to plasma–surface interaction issues at high heat-loads which are typical for fusion reactors. For the International Thermonuclear Experimental Reactor (ITER), which is now under construction, the knowledge of erosion processes and the behaviour of various constructional materials under extreme conditions is a very critical issue, which will determine a successful realization of the project. The most important plasma–surface interaction (PSI) effects in 3D geometry have been studied using a QSPA Kh-50 powerful quasi-stationary plasma accelerator. Mechanisms of the droplet and dust generation have been investigated in detail. It was found that the droplets emission from castellated surfaces has a threshold character and a cyclic nature. It begins only after a certain number of the irradiating plasma pulses when molten and shifted material is accumulated at the edges of the castellated structure. This new erosion mechanism, connected with the edge effects, results in an increase in the size of the emitted droplets (as compared with those emitted from a flat surface). This mechanism can even induce the ejection of sub-mm particles. A concept of a new-generation QSPA facility, the current status of this device maintenance, and prospects for further experiments are also presented.

A full-scale high-power two-stage quasistationary plasma accelerator with active anode and cathode transformers (QSPA Kh-50) is described. Plasma streams with the velocity (4 to 4.2) × 107 cm/s, the energy flux density above 1 kJ/cm2 in the vicinity of the plasma stream axis and the total energy content in plasma of approximately 250 to 270 kJ were produced. Some integral functional relations obtained under QSPA Kh-50 operation (e.g., the current-voltage characteristic, Ud ≃ 1 2.8 d ; the discharge current dependence of the plasma velocity v ≃ 1 β d, 1.5 < β < 2.5; the time dependence of the velocity of certain plasma stream layers v(t)≃ 21/2 CA 0 = H0 /(2πρ)1/2; the effect of the screening magnetic fields in the cathode and anode transformers on the plasma stream velocity value, etc.) support, for the most part, the concept of a QSPA.

This paper is devoted to the investigation of plasma stream parameters and the intensity of extreme ultraviolet (EUV) radiation from the compression zone in various modes of operation of a magnetoplasma compressor (MPC). Two gases of different masses, either helium or argon, were used for the ignition of MPC discharge under the residual pressure. The plasma stream density along the axis and the EUV radiation energy were measured. It was shown that the compression zone position depends on the initial density of the residual gas. The EUV radiation energy was measured with a calibrated AXUV in the wavelength range of 12.2–15.8 nm. It was revealed that the radiation energy increased by 30–50% with decreasing initial gas pressure.

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.