Coaxial Accelerator Research Articles

Quadruple Langmuir probe characterization of different fuel gases in a plasma deflagration accelerator

Astrophysical flows may be studied by reproducing similar conditions using a coaxial plasma accelerator operating in the deflagration regime (or plasma deflagration accelerator). This allows for the recreation and investigation of dynamics present in complex highly coupled plasma systems at the laboratory scale. We report on measurements of the plasma density, temperature, plasma potential and velocity found using a quadruple Langmuir probe (QLP) on such a deflagration accelerator in the form of the Stanford Coaxial High ENerGy (CHENG) device operating with multiple gases – specifically argon, nitrogen and hydrogen. Experiments show a general decrease in bulk plasma velocity with gas atomic mass from upwards of $120\ {\rm km}\ {\rm s}^{-1}$ with hydrogen to less than $30\ {\rm km}\ {\rm s}^{-1}$ with argon. There was an accompanying increase in peak plasma density with increasing atomic mass from ${\sim }3\times 10^{20}\ {\rm m}^{-3}$ with hydrogen to ${\sim }1.5 \times 10^{21}\ {\rm m}^{-3}$ with argon. It was found that the momentum flux and internal energy density also generally increase with atomic mass while the particle flux is constant between shots. Further investigation is needed to understand these correlations and the underlying physics. Lastly, comparisons with scaling laws show that while the CHENG device may be operated in such a way as to simulate the effects of bulk solar wind movement, it may not properly capture the thermal effects.

INFLUENCE OF CONFIGURATION OF THE ELECTRODE SYSTEM OF A COAXIAL MAGNETO PLASMA ACCELERATOR ON ARC DISCHARGE FORMATION AND DEVELOPMENT

Link for citation: Tsimmerman A.I., Shanenkov I.I., Nassyrbayev A.R., Nikitin D.S., Sivkov A.A. Influence of configuration of the electrode system of a coaxial magneto plasma accelerator on arc discharge formation and development. Bulletin of the Tomsk Polytechnic University. Geo Аssets Engineering, 2023, vol. 334, no. 8, рр. 39-50. In Rus. The relevance of the research is explained by a wide range of practical areas, where various plasma generators can be used, including analytical devices, technological installations, tokamaks, ion and plasma engines, satellites, laser technology, as well as by the possibility of applying them for both producing coatings on various surfaces and synthesizing fine and nanopowders. The main aim of the research is to determine the best way to initiate an arc discharge in the acceleration channel of the coaxial magnetoplasma accelerator, to estimate the effect of the plasma formation zone configuration on the electrodynamic loads of the accelerator, and to calculate the efficiency of conversion of stored energy to supplied one. Objects: coaxial magnetoplasma accelerator, arc discharge initiation, central electrode, electrode-barrel, plasma formation zone. Methods: experiment, plasma dynamic synthesis, measurement and registration of pulsed currents and voltages, electrical erosion measurement by means of weighing the eroding electrode-bore, high-speed photoregistration of the plasma jet. Results. Two methods of arc discharge initiation using metal wires and graphitization were considered. It is established that they do not affect the energy parameters of the plasma dynamic synthesis, but the graphitization looks preferable due to the greater processability of the procedure that provides easy accelerator preparation and reliability of its operation. The possibility of reducing the arc discharge current amplitude by 29 % with increasing the plasma formation zone length from 5,5 to 11,5 mm was established that results in reducing electrodynamic loads on all system nodes. The optimal plasma formation zone length of 9,5 mm was determined to provide obtaining a high efficiency of conversion of stored energy into supplied energy and reliability of the system operation.

Simplified Plasmadynamics Model of a Coaxial Self-Field Plasma Accelerator

A simplified plasmadynamics model is presented that examines radial and axial dependence of the current sheet structure of a coaxial quasi-steady self-field plasma accelerator, dominated by electromagnetic forces. Two radial electric field distributions are considered as limiting cases. In the first case, constant current flows between constant-radius electrodes; in the second case, constant current flows between electrodes and also extends downstream of the electrodes. Calculations for Teflon () plasma show the distribution of plasma parameters inside the current sheet, as well as terminal values for kinetic efficiency, voltage, and impedance for both distributions. Evaluation of the validity of simplifying assumptions is presented, as is the behavior of previously developed accelerators as interpreted by the model.

Effect of external longitudinal magnetic field on the dynamics of pulsed plasma stream

A spectroscopic study is carried out to investigate pulsed plasma stream produced in a coaxial plasma accelerator under the influence of an external longitudinal magnetic field. The dynamics of Argon plasma stream that includes the excitations and emissions of plasma species and its different parameter variations on the application of an external longitudinal magnetic field are reported in this work. The behavior of the ionized and neutral species of argon plasma is studied from their intensity distribution profiles which indicate the occurrence of faster recombination as compared to the field-free case. The variation of pressure reveals a significant influence on the recombination and diffusion process of the plasma species. Moreover, the variation of cathode voltage leads to variation in input energy to the system which influences the ionization and excitation processes of the plasma species. A plasma density (Ne) jump is achieved from 1021–1022 m−3 at a relatively higher pressure ∼8 bar plenum pressure in presence of the magnetic field. Saturation of the emissions and excitations of plasma species are revealed from the intensity profile and electron excitation temperature (Texc) profile which occur at different pressure and voltage both for field and field-free cases. The electron temperature (Te) is estimated using the FLYCHK code by comparing the simulated charge state distributions with the experimental findings. Using known parameters in the code, the different estimated Te shows a linear and an exponential decrement respectively in the absence and presence of the magnetic field. Maximum Te of 2.2 eV is reported which decreases to 1.51 eV at a significant increase of plasma density of 1022 m−3.

Experimental study on plasma generated by a tapered coaxial accelerator and its damage effects on a tungsten target at different angles

Plasma wall interaction inevitably occurs during the operation of tokamaks. The coaxial gun device has low operation cost and the parameters of plasma produced by the gun are close to those of type I edge localized mode (ELM); therefore, the coaxial gun is suitable in simulation experiments as a heat flux source of transient events such as type I ELM under the condition of H-mode in the International Thermonuclear Experimental Reactor. In this paper, the plasma generated by the discharge of a tapered coaxial accelerator thermal shock on a tungsten target is used to simulate the damage effect of the divertor. The plasma parameters are measured in the experiment. The velocity of the plasma is 41.7 km s−1, and the kinetic energy of a single hydrogen ion is 9.2 eV. The energy density at the center of the plasma can reach 1.5 MJ m−2, and the density can reach about 2.78 × 1015 cm−3. The reflection of plasma in the process of exposure at different angles is observed. It is observed that droplets of millimeter size splash from the target. Traces of liquid flow are observed on the surface of the target, which shows that there is a melting process on the surface of the target. The mass loss of the target is of the order of milligrams after 20 pulses. The ablation and residual stress of the target surface both decrease with a decrease in the angle. This is because the accumulated energy per unit area of the target surface decreases with a decrease in the angle. The results of the simulation experiment help us to understand the working state around the divertor target in tokamak devices.

Investigation of dust formation during changes in the structural and surface properties of plasma-irradiated materials

• The contribution of surface defects (cracks, craters, bubbles) and structural changes (recrystallization) exposed by the plasma beam candidate materials to dust formation is studied. • The scanning electron microscopy (SEM), and elemental and X-ray diffraction (XRD) analysis were used to study the surface morphology, chemical composition, and crystal structure of plasma beam irradiated materials. • On the surface of tungsten target the well-developed micro-cracks are formed, and defects such as melting and flaking of the surface layer, drop-shaped structures formation are also observed. The increasing of the pulse dose led to an increase in the width and depth of these microcracks. • Tungsten target erosion (mass loss) occurs due to the ejection of particles from the tungsten surfaces, both as droplets and as solid dust during crack development. • The measured peak shift in carbon was 0.03 degrees after x5 plasma load. Thereby, carbon is generally more resistant to plasma beam impact than tungsten. This paper presents the results of studying the impact of a plasma beam during the interaction of divertor candidate materials in a coaxial plasma accelerator, where graphite and tungsten plates were used as candidate materials. The authors consider the contribution of surface defects (cracks, craters, bubbles) and structural changes (recrystallization) in candidate materials to dust formation under the action of a plasma beam. The other aspect studied in the paper is the erosion of copper substrates (nearby components) under the action of a hot target dust cloud with account for the kinetic influence of hot dust particles on nearby components. It should be noted that these particles are formed by the erosion of the material by the plasma beam. Scanning electron microscopy (SEM), elemental and X-ray diffraction analysis (XRD) were used to study the material surface morphology, chemical composition, and crystal structure. The authors also consider some undesirable effects revealed after irradiation of divertor candidate materials. The obtained experimental results will be useful for evaluating the prospects for further application of these materials in existing and future fusion reactors (e.g., in the international fusion reactor ITER).

Pulsed plasma flow diagnostics

This paper presents the results of optical and probe diagnostics of hydrogen plasma flow in a coaxial plasma accelerator. The triple probe and the Optosky ATP2000P linear spectrometer with a spectral range of 200-1100 nm were used for measuring the electron current and obtaining the hydrogen plasma emission spectrum. The individual spectral lines were observed using an M833 monochromator spectrometer with a spectral resolution of 0.024 nm. From the measured electron currents and emission spectrums, the electron densities in the flux of a hydrogen pulsed plasma were calculated. We used the Stark method of Hβ hydrogen line broadening for calculations of electron densities. The obtained experimental results from the probe and spectroscopic measurements of electron density correspond well. In this work, we also obtained the dependence of the electron density on the voltages applied to the capacitor bank. The electron density increases with increasing voltage, because of the increased energy applied to the discharge. The average electron densities in the plasma flux showed  1.13 ∙ 1021 m-3,  4.14 ∙ 1021 m-3, and  5.57 ∙ 1021 m-3 at three values of voltages of 3 kV, 4 kV, and 5 kV, respectively.

Investigation of self-generated magnetic field and dynamics of a pulsed plasma flow

Due to the growing interest in studying the compression and disruption of the plasma filament in magnetic fusion devices and Z-pinches, this work may be important for new developments in the field of controlled thermonuclear fusion. Recently, on a coaxial plasma accelerator, we managed to obtain the relatively long-lived (∼300 μs) plasma filaments with its self-magnetic field. This was achieved after modification of the experimental setup by using high-capacitive and low-inductive energy storage capacitor banks, as well as electrical cables with low reactive impedance. Furthermore, we were able to avoid the reverse reflection of the plasma flux from the end of the plasma accelerator by installing a special plasma-absorbing target. Thus, these constructive changes of the experimental setup allowed us to investigate the physical properties of the plasma filament by using the comprehensive diagnostics including Rogowski coil, magnetic probes, and Faraday cup. As a result, such important plasma parameters as density of ions and temperature of electrons in plasma flux, time dependent plasma filament’s azimuthal magnetic field were measured in discharge gap and at a distance of 23.5 cm from the tip of the cathode. In addition, the current oscillograms and I–V characteristics of the plasma accelerator were obtained. In the experiments, we also observed the charge separation during the acceleration of plasma flow via oscillograms of electron and ion beam currents.

Плазмодинамический синтез оксидов металлов в среде углекислого газа на примере систем Ti – O и Fe – O

The issues of decarbonization, which have recently received increased attention from the world community, mainly due to global warming, require the search for new solutions related to the utilization of carbon dioxide. The main problem of carbon dioxide conversion is the need to overcome the high stability of its molecules that requires the supply of a significant amount of energy. This paper presents the experimental results on the high-energy plasma dynamic synthesis of metal oxides in a carbon dioxide medium using the Ti – O and Fe – O systems as an example. By applying a sectioned capacitive energy storage device and a coaxial magnetoplasma accelerator with titanium and steel electrodes, which are the main elements of the plasma dynamic synthesis system, nano- and micropowders of iron and titanium oxides were obtained by utilizing CO2. Synthesis of powders was carried out as a result of plasma-chemical reactions occurring during high-speed sputtering of carbon electric discharge plasma into the atmosphere of a reactor chamber filled with carbon dioxide. The possibility of controlling the volume of CO2 utilized in the synthesis process by changing the type of electrodes and the number of successive power supply pulses is shown. It has been established that when using titanium electrodes and a multi-pulse operating mode in the considered system, it is possible to utilize up to 15 vol. % of CO2 to obtain up to ~ 9 g of dispersed products. It should be noted that when titanium electric discharge plasma is sputtered in a CO2 medium, the formation of “classical” rutile and anatase phases, as well as Magneli phases and titanium carbide without traces of pure metal is observed. This indicates that when implementing the plasma dynamic synthesis, it is possible not only to utilize carbon dioxide, but also to obtain useful powder products, which can later be used as feedstock for various applications.

Modification and optical degradation of thin multilayers under VUV/UV radiation from compressed plasma flows

Coaxial plasma accelerators are under consideration for generation of compressed plasma flows which are suitable for emitting of powerful broadband radiation (including the VUV/UV ranges). The using of different gases in a chamber allows to control the spectrum. For inert gases the upper value of energy is limited by its first ionization potential (for neon ≈ 21.55 eV). For air the maximum energy is limited by ≈ 6 eV. Such technical systems are suitable for studying of optical properties stability for thin multilayers and the other coatings. Such tests were fulfilled for bilayers based on HfO2/SiO2 pair on silica substrates which is stable for laser radiation in the visible and IR ranges. It was found that a single exposure of the radiation (for neon and air) caused a relative decline of the radiation durability in ≈ 1.03…1.14 times. Spectral measuring demonstrated that the maximum decline of transmission (up to of ≈ 3…4%) was detected for exposures in neon in the range of 320…450 nm.

ВЛИЯНИЕ ИНДУКТИВНОСТИ НА ПАРАМЕТРЫ ПЛАЗМЕННОГО ФОКУСА В ИМПУЛЬСНОМ КОАКСИАЛЬНОМ УСКОРИТЕЛЕ

Based on the physical analysis of the processes occurring in pulsed plasma accelerators, the pos­sibility of their use for the creation and study of a plasma focus is justified. In particular, the influence of inductance on the parameters of the plasma focus in kilojoule­range «Plasma focus» (PF) installations created on the basis of a pulsed coaxial accelerator is studied. An equivalent installation scheme is proposed and justified, based on the analysis of which the influence of the inductance of the circuit and the capacitance of the capacitor bank on the value of the maximum current and neutron output is analyzed, without taking into account the parameters of the spark gap and the conducting wires. Based on the theoretical estimate of the inductance of the installation, its most probable value is calculated. . It turned out that in the installations of the considered energy range, the inductance is approximately 7.5•10­7 Gn and depends on the capacitance of the capacitor bank. On the contrary, in installations with megajoule energy, the inductance does not depend on the number and capacity of capacitors, so that an increase in the latter does not affect the increase in current strength. Experimental and theoretical dependences of the discharge current on the applied voltage at different capacitances of the capacitor bank are obtained. A comparative analysis of theoretical and experimental current waveforms is pre­sented. The greatest coincidence of theoretical and experimental results was found for the duration of the discharge pulse T = 30 microseconds.

Plasma Jet Synthesis of Cubic Tungsten Carbide of Variable Stoichiometry

This paper reports plasma jet synthesis of nanocrystalline cubic tungsten carbide. Using a coaxial magnetoplasma accelerator with graphite electrodes, we have implemented an experimental process that allows one to obtain up to 95% crystalline WC1 – x. We have demonstrated the feasibility of tuning the parameter 1 – x (stoichiometric ratio) in this compound by varying the energetics of the process and the composition of the starting mixture. Varying the released energy from 8 to 23 kJ and the ν(C)/ν(W) atomic ratio of the starting reagents from 0.6 to 2.5 allows the composition of cubic tungsten carbide to be tuned in the range WC0.64 to WC0.78.

Peculiarities of the Discharge Formation in a Plasma Accelerator and Structure of a Jet Flowing into Vacuum

We report on results of investigation into the structure of a plasma jet generated by a coaxial plasma accelerator with conical insert in the discharge formation region. Using high-speed optoelectronic camera, we detected an inhomogeneous structure of glowing fractions of argon and deuterium jets. It was found that the jet radiation intensity oscillates with a characteristic frequency of 0.3–0.9 MHz. The argon plasma jet consisted of fragments diverging from the axis; plasma recombined 20 μs after the beginning of the discharge. The deuterium plasma jet resembled flakes. Its radiation intensity decreased significantly by 8 μs. The characteristic frequencies of radiation nonuniformities on the jet axis were about 0.09 cm–1 for argon and 0.3 cm–1 for deuterium. Analysis of the behavior of the discharge in the plasma accelerator revealed that a diffuse discharge was initiated at the base of the conical insert. After 0.5 μs, the discharge emerged at the outer boundary of the insert, was contracted, and occurred between central and external electrodes. The emergence of the discharge from the accelerator was observed 3 μs later and was accompanied with a glow of the end face of the central electrode. In the measurements of temperature of the lateral surface of external electrode using an infrared camera, it was found that heating is localized near the edge of conical insert and at the outlet of the accelerator. Results of investigations can be used for gas ionization, material irradiation, and simulation of interaction of plasma flows with the magnetic field in the outer space in laboratory conditions.

Characteristics of plasma stream evolution in a pulsed plasma accelerator

The time-integrated spectroscopic measurements of argon plasma are carried out to characterize the transient plasma stream produced in a coaxial pulsed plasma accelerator. Transient plasmas produced in these types of devices have a temporal change in density and temperature. The temporal evolution of the plasma stream and its characteristic variation are studied from a time of 50–300 μs in steps of 50 μs. Analysis of different spectral lines shows the existence of some metastable states of Ar+ having a long lifetime. The evolution of different Ar+ transitions to metastable and non-metastable lower levels is observed for different time frames. From the Stark broadening and Boltzmann plot method assuming the partial local thermodynamic equilibrium model, the plasma densities and excitation temperatures are estimated. The temporal evolution study shows a decrease in plasma densities from 1.96 × 1021 m−3 at 50 μs to 1.23 × 1020 m−3 at 300 μs after the initiation of plasma formation. A reduction in excitation temperature is observed from 0.86 to 0.77 eV. Based on the density profile, the estimated energy density of the plasma stream shows a time-dependent decreasing trend from 0.06 to 0.004 MJ/m2 during the observed period. The results may be useful for optimization of the plasma stream in pulsed plasma accelerators concerning its different parameters.

Simulation of the inner electrode geometry effect on the rundown phase characteristics of a coaxial plasma accelerator.

A 2D computational model, incorporating the Snowplow approximation in the mass balance, is used to simulate the acceleration of an annular current sheath along two coaxial electrodes, with the inner one having either cylindrical or conical shape. The circuit, mass and momentum equations are simultaneously solved in 2D (r, z) considering initial breakdown along the insulator surface, ideal gas mass accretion by the current sheath (snowplow model) and distributed inductance along a coaxial transmission line short-circuited by the current sheath. Plasma density and electron temperature in the current sheath are estimated using standard planar shock theory. Numerical integration of the model’s equations for a given electrode geometry yields the temporal evolution of the current sheath parameters during the axial acceleration phase. In order to see the effect of the inner electrode shape on sheath parameters (i.e. transit time, kinetic energy, total mass, shape, etc.) and/or circuit properties (i.e. circuit inductance, voltage and current evolution, etc.), the portion of the inner electrode beyond the insulator was given a conical shape. By changing the cone slant in a range between ±5°, it was found that the current driven on the plasma sheath varies nonlinearly with the angle. The divergent (positive angle) electrode gives the sheath the highest kinetic energy, being twice the value corresponding to that of the straight inner electrode case, and the transit time is reduced from 1.34 to 1.20 µs. The estimates of plasma density and electron temperature indicate that the achievable ion densities are on the order of 4x1022 m-3, which corresponds to 30 % ionization, and typical temperatures at the end of the rundown phase are on the order of 8 eV. These values are comparable with those measured in experimental devices. The development of this tool will enable us to benchmark its results against an experimental installation currently close to being operational, and a future follow-up paper will be devoted to the comparison between the prediction of the rundown phase behavior and experimental results utilizing conical electrodes.

The influence of the method of an arc discharge initiation on the product of plasma dynamic synthesis in the W-C system

Cubic tungsten carbide WC1−x is distinguished by the difficulty of obtaining in a dispersed and bulk form. The article attempts to plasma dynamic synthesis of cubic tungsten carbide in powder form. The method is based on the use of a coaxial magnetoplasma accelerator for the generating a supersonic plasma jet, which is then rapidly cooled. The possibility of obtaining a powder with a predominant content of WC1−x (up to 95%) is shown. Studying the influence of the method of an arc discharge initiation allowed to reveal the efficiency of using graphitization in comparison with carbon fibers.

Beam optics in a co-axial cavity accelerator

This paper describes the basic design of electron accelerator using co-axial cavity. Theoretical calculations and simulation results are presented. Simulations are carried out using CST Particle studio, final energy ∼ 9 MeV is obtained. Nine Dipole magnets with magnetic field 0.07 T to 0.2 T are used to bend electron beam through 180 deg. Calculated values and simulation results are matching well.

The possibility of obtaining materials by plasma dynamic synthesis in the Ti-O system

The possibility of obtaining a dispersed material by the method of plasma dynamic synthesis in the Ti-O system is shown. The plasma dynamic synthesis is based on the operation of a pulsed high-current coaxial magnetoplasma accelerator with titanium electrodes. The obtained product was investigated by X-ray diffraction method and transmission electron microscopy. The results showed the presence of two main titanium oxide (IV) crystalline modifications: anatase with tetragonal syngony and rutile also with tetragonal syngony. Moreover, the dominant modification is anatase in synthesized material.

Plasma Dynamic Synthesis of Nanocrystalline WC1 – x and the Dependence of the Product Structure on the Ratio of Precursors

The results of plasma dynamic synthesis of nanocrystalline cubic tungsten carbide using a system based on a coaxial magnetoplasma accelerator are presented. It is found that an atomic ratio of C/W precursors of about 2.5 is necessary to achieve the maximum yield of the unique cubic phase of tungsten carbide WC1 – x (up to 95%). The synthesized materials are analyzed by transmission electron microscopy to assess the formation of the structure of nanocrystalline particles depending on the ratio of carbon and tungsten filling. It is found that particles with a core–shell structure are formed in the system, and the thickness of the shells increases with an increase in the amount of carbon in the mixture of initial components.

Control of tokamak discharge parameters using a plasma jet

Results of the experimental study of the ability of plasma jet of coaxial accelerator to breakdown and ionization the working gas at the initial discharge phase in Globus-M2 spherical tokamak are presented. Studies of the development of the initial stage of discharge initiated with a plasma jet showed that the plasma current in the discharge began to grow earlier by 2 ms than using the standard inductive breakdown. The breakdown voltage decreased from 6.1 to 3.1 V (-49%) at 4.7 kV accelerator electrode voltage. The emission intensity of line Dα decreased during formation of the discharge with the help of plasma jet without additional gas puffing. Radiation level CIII in the discharge generated by the plasma jet was the same as the radiation in discharge produced by induction breakdown of the gas. At the stage of increase in the plasma current the most intense visible radiation of the plasma was found near duct through which the jet was injected. Discharge radiation decreased and spread out along the cross section of the plasma column during plasma current plateau.