Quasi-stationary Plasma Accelerator Research Articles

Experimental investigation of the characteristics of the plasma flow generated in quasi-stationary plasma accelerator using optical methods

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.

Structure and Phase Composition of WNb Alloy Formed by the Impact of Compression Plasma Flows.

The results of a tungsten-niobium alloy synthesis by the impact of pulsed compression plasma flows are presented. Tungsten plates with a 2 μm thin niobium coating were treated with dense compression plasma flows generated by a quasi-stationary plasma accelerator. The plasma flow with an absorbed energy density of 35-70 J/cm2 and pulse duration of 100 μs melted the niobium coating and a part of the tungsten substrate, which caused liquid-phase mixing and WNb alloy synthesis. Simulation of the temperature distribution in the top layer of the tungsten after the plasma treatment proved the formation of the melted state. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) were used to determine the structure and phase composition. The thickness of the WNb alloy was 10-20 μm and a W(Nb) bcc solid solution was found.

STABILITY OF THIN QUASI-CRYSTALLINE Ti-Zr-Ni FILMS AND RELATED CRYSTALLINE PHASES UNDER LOW-ENERGY TRANSIENT PLASMA IRRADIATION

The properties of Ti41Zr38.3Ni20.7 thin films under radiation-thermal action of hydrogen plasma with a surface heat load of 0.2 MJ/m2 was studied at the QSPA Kh-50 quasi-stationary plasma accelerator (NSC KIPT).The phase composition, structural state, and surface morphology were studied using X-ray diffraction and scanning electron microscopy. It was found that the quasicrystalline phase and related crystalline phases, the Laves phase, the α-solid solution, and the 2/1 phase of the Ti-Zr-Ni approximant crystal were stable under irradiation with up to 20 hydrogen plasma pulses. The phase composition did not change. It is shown that the changes in the coatings mainly manifest themselves as changes in the substructure of the observed phases. With an increase in the plasma exposure dose, the structure of the quasicrystalline icosahedral phase improves, and the size of the coherence regions increases. In the films consisting of crystalline phases, a partial phase transformation is observed with a redistribution of components between the 2/1 phase of the approximant crystal and the α-solid solution phase. It was found that thin films of the TiZr-Ni system containing a quasicrystalline icosahedral phase, irradiated with radiation-thermal plasma pulses, are less prone to cracking than coatings with crystalline phases of the same system.

Flows of ionizing gas in the plasma accelerator channel with longitudinal magnetic field

Model and results of numerical experiments are presented for axisymmetric flows of ionizing gas in the presence of an additional longitudinal magnetic field in the channel of quasi-stationary plasma accelerator considered as perspective electric propulsion plasma engine. A two-dimensional flow model for a three-component medium consisting of atoms, ions, and electrons is based on magnetogasdynamic equations written in terms of the vector potential of magnetic field, taking into account electrical conductivity, thermal conductivity, and radiation transfer. A three-dimensional model of radiative transfer includes main mechanisms of emission and absorption for different parts of the spectrum. Numerical studies of the ionization process and radiation transfer were carried out in an approximation of local thermodynamic equilibrium. The stabilizing effect of flow rotation in the presence of a longitudinal magnetic field and thermal conductivity along the magnetic field on non-stationary unstable flows of ionizing gas, which arise at low discharge currents, is considered.

Спектры излучения в потоках ионизующихся газов для установки КСПУ-Т с продольным полем

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.

Contribution of leading edge shape to a damaging of castellated tungsten targets exposed to repetitive QSPA plasma loads

The castellated tungsten monoblock is considered one of the preferred reference designs of components faced to plasma in the divertor of a fusion reactor. The behavior of such components during the development of transient events (disruptions or/and of Edge Localized Modes (ELMs)) is still among the most critical issues for future thermonuclear devices. Experimental studies of castellated tungsten surface erosion have been performed within the powerful quasi-stationary plasma accelerator QSPA Kh-50 in conditions relevant to unmitigated ITER Type I ELMs. The surface energy load from the impacting hydrogen plasma streams was 0.9 MJ m−2 during 0.25 ms to achieve pronounced melting of the target surface. The studies of solid and liquid particle generation were performed at sequentially oblique plasma exposure of the different chosen edges of the castellated structure. It was determined that most of the W droplets are ejected from the sharp leading edge of the target due to the development of instabilities in the plasma-liquid metal interface. At the same time, the formation of solid dust particles is mainly attributed to the cracking during the surface cooling after the plasma pulse. Particle velocities typically achieve tens m s−1. Both the re-solidified droplets as well as dust particles have been collected on the special plate near the exposed surface. The maximal size of collected particles reached tens of micrometers. Such particles could either fly away with the next plasma pulses or mix with the material of the collecting plate in the course of repetitive plasma pulses. Thin (submicron) layers of re-deposited tungsten also formed on the surface of the collecting plate.

Vapour shielding of liquid-metal CPS-based targets under ELM-like and disruption transient loading

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 ∼ 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 ∼ 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.

MODIFICATION AND ALLOYING EFFECTS IN EUROFER STEEL UNDER POWERFUL PULSED PLASMA IMPACTS

Experimental studies of surface modification of Eurofer samples have been performed with a quasi-stationary plasma accelerator QSPA Kh-50. The heat load on the surface was near the tungsten melting threshold (i.e. about 0.6 MJ/m2). The plasma streams exposures result in modification of steel-based materials and formation of cellular submicron structures in re-solidified layers. Phase characterized by body-centered cubic lattice appeared due to recrystallization of affected material.

Numerical Model of Plasma Flow Injection in a Solenoid’s Magnetic Field

Injection of plasma flow generated by a quasi-stationary plasma accelerator into a region with a magnetic field created by a series of ring current-carrying conductors forming a solenoid is considered. Numerical studies are carried out based on the set of magnetogasdynamic (MGD) equations represented in terms of the vector potential of the magnetic field, taking into account electrical conductivity, thermal conductivity, and radiation transport, provided that the plasma accelerator and ring conductors are located on the same axis. It is shown that small currents in ring conductors create a magnetic field inside the solenoid, which does not significantly affect the main stream, while simultaneously isolating it from the current coils as a result of the formation of a rarefied plasma layer separating the high-speed flow of dense plasma. Calculations of the MGD problem also show that the plasma flow within the solenoid is not accompanied by an increase in the temperature of the medium.

Исследование пульсирующих течений ионизующегося водорода в плазменном ускорителе на основе двумерной модели

Numerical study of the hydrogen ionization process in the channel of the quasi-stationary plasma accelerator is presented. Calculations of pulsating and stationary flows of ionizing hydrogen were carried out within the framework of two-dimensional MHD model in the approximation of local thermodynamic equilibrium, taking into account the radiation transport and under the condition of uniform gas supply at the channel inlet. As a result of a series of calculations, the empirical condition for the stationarity of axisymmetric flows of ionizing hydrogen was determined.

Исследование устойчивости процесса ионизации гелия в канале плазменного ускорителя

Numerical study of the ionization process stability 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. Results of modeling of the ionizing gas flows and the integral characteristics of the radiation are presented. As a result of a series of calculations, the empirical condition for the stability of ionizing helium flows was obtained in terms of the experimentally measured parameters.

Influence of distance between coils with current on the effect of cascade plasma heating in flows injected from the QSPA

Research of the interaction of high-speed dense plasma flows generated by the quasi-stationary plasma accelerator with the magnetic field of the set of ring current-carrying conductors located at different distances from each other is presented. The study was based on the model of radiation magnetic gas dynamics, taking into account electrical conductivity, thermal conductivity and radiation transport. It was found that a decrease in the distance between the coils with a current leads to the disappearance of the effect of step-by-step or cascade transformation of kinetic energy into thermal energy of the plasma.

Mechanisms of crack generation in high-pure tungsten exposed to high power density plasma

The mechanisms of crack formation in the tungsten divertor plates irradiated by hydrogen plasma under ITER-type steady-state and transient events are investigated. The stresses and defects of samples‘ initial structure are annealed as a result of steady-state irradiation with Falcon ion source, which generates 2 keV hydrogen ion flux of ~ 1022 m-2s−1 and power density ~ 1.7 MW × m−2. Quasi-stationary plasma accelerator QSPA Kh-50 provides pulsed hydrogen plasma loads which operating mode simulates transient events (type I ELMs) in ITER: ion impact energy is about 400 eV, maximum plasma pressure is 0.32 MPa, pulse duration is 0.25 ms. The diffraction maxima profiles, positions, intensities, half-widths, and intensity distributions are analyzed. The origin of crack formation is found to be in the plastic deformation of surface layers by the twinning mechanism. The latter is reinforced by the interaction of twins with the hydrogen-filled micro-pores. The micro-pores appear as a result of hydrogen saturation of subtraction loops formed due to the coalescence of vacancy complexes.

Исследование переноса излучения в потоке ионизующегося гелия в плазменном ускорителе

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.

Damaging of inclined/misaligned castellated tungsten surfces exposed to a large number of repetitive QSPA plasma loads

In fusion devices like ITER and DEMO, transient events could result in substantial damage in the form of erosion of exposed surfaces due to the extreme energy loads. Castellated tungsten mock-ups were chosen as reference design for the plasma-facing components (PFC) of fusion devices. The material response of these mock-ups is crucial for the lifetime of the PFC and needs to be evaluated for a large number of repetitive plasma impacts. Therefore, an experimental investigation of erosion processes on castellated tungsten surfaces has been performed within the powerful quasi-stationary plasma accelerator QSPA Kh-50 with conditions that simulated a failure of the edge localized mode control in ITER. The surface energy load measured with a calorimeter was 0.9 MJ m−2, i.e. above the melting (0.6 MJ m−2) and below the evaporation (1.1 MJ m−2) thresholds of tungsten. The plasma pulse duration was 0.25 ms. Intense overheating of the castellated edge, which interacted with the plasma first, was observed alongside the formation of an excrescence of shifted material during the first few tens of plasma pulses. The maximum number of ejected particles was registered after the plasma pulses, i.e. during the cooling process. Nevertheless, the amount of particles ejected during the plasma exposure increased with the plasma pulse number. The splashing of droplets and ejection of dust was suppressed on flat surfaces of castellated monoblocks, whereas the edges were identified as the main source of ejected particles.

Surface Structure Modification and Deuterium Retention in Tungsten under Pulsed Plasma Loads

Modification of the surface layer and deuterium accumulation in tungsten targets under plasma irradiation in a quasi-stationary plasma accelerator with an intrinsic magnetic field QSPA-T, which reproduces the conditions (plasma thermal load of 0.2-5 MJ/m2, pulse duration of 0.1-1.2 ms) typical of ELM events in ITER, are studied. Using a scanning electron microscope, structure modifications at the surface and in the bulk after deuterium plasma irradiation are analyzed. The observed changes in the near-surface layer are compared with the calculated data on the change in the internal structure of tungsten under intense thermal action obtained as a result of the numerical solution of the heat conduction problem. The total deuterium retention in the samples was measured using thermal desorption spectroscopy, and it was in the range of (3-4) × 1016 particles/cm2 for the samples melted during plasma exposure. These numbers exceed by an order of magnitude the values obtained for samples without traces of melting.

Study of the ionizing gas flow in the channel of plasma accelerator with different ways of gas inflow at the inlet

Numerical studies of the ionization process are presented for various ways of the gas supplying at the inlet of the coaxial channel of the quasi-stationary plasma accelerator. Simulation of two-dimensional axisymmetric flows of ionizing gas was carried out on the basis of the equations of radiation magnetic gas dynamics taking into account radiation transport. The approximation of local thermodynamic equilibrium in three-component medium consisting of atoms, ions, and electrons was used in the MHD model. It was determined that non-uniform gas inflow at the channel inlet leads to even greater instability of the pulsating unsteady flows of the ionizing gas in the plasma accelerator. The method of long characteristics was used in the 3D model of radiation transport, including the basic mechanisms of emission and absorption for different portions of the spectrum. The radiation field in the ionizing gas flow was determined, including the integral radiation characteristics and the spectral radiation intensities in the narrow ranges of frequencies or photon energies.

Plasma Injectors for Quasi-Stationary High-Power Plasmodynamic Systems

Pulsed plasma injectors, that is, an “input ionization chamber” (IIC), an “anode ionization chamber” (AIC), and a high-pressure plasma injector, are described. Plasma injectors are designed to generate primary plasma in quasi-stationary high-power plasmodynamic systems, in particular, in the KSPU Kh-50 quasi-stationary high-current plasma accelerator. The results of studies of the main characteristics of plasma pulsed injectors are presented. The following charge currents in plasma were obtained: 23.5 kA for the AIC injector at a voltage of 3 kV, 90 kA for the IIC injector at a voltage of 0.65 kV, and 96 kA for the high-pressure plasma injector at a voltage of 3 kV.

Influence of a magnetic field on plasma energy transfer to material surfaces in edge-localized mode simulation experiments with QSPA-M

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.

Influence of surface tension on macroscopic erosion of castellated tungsten surfaces during repetitive transient plasma loads

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.