Recently, in a series of articles Mumtaz, Choi, and others [i.e., S. Mumtaz and E. Choi, IEEE Electronic Device Lett. 43, 1756 (2022)] studied a relativistic gridded vircator with one or more dielectric anodes, in which considerable increase in the efficiency of high-power microwave (HPM) generation compared to ordinary vircators was observed. This is very important since the main disadvantage of vircators is their low efficiency. It was suggested that the repelling of electrons by the initial impacting electron charge collected on the dielectric reflectors forms a multi-vircator, which increases HPM production. The present article reports the results of experiments and numerical simulations, which show that this mechanism does not result in high multi-vircator efficiency. Similar dielectric anodes were investigated 50 years ago in a configuration known as the Luce diode, in which collective acceleration of ions was studied. It was also known that the operation of the Luce diode is accompanied by a HPM burst, which was never measured. Results of experiments and simulations in various configurations based on a gridded vircator and dielectric reflectors confirm the existence of high-energy ion acceleration. Unfortunately, none of these configurations produce HPM of higher efficiency than a regular gridded vircator.

A relativistic magnetron (RM) with diffraction out-put, modified from the original Kovalev, FuksX-band magnetron(MDO) presented more than 50 years ago, has been designedand studied experimentally using pulse generators with voltageamplitudes <= 300 kV and pulse duration <= 200 ns.X-bandmagnetrons are naturally small devices, and for high voltages,microwave pulse shortening occurs because of cathode explosiveemission plasma expansion. A split cathode can potentiallysolve this problem, but in this research, the MDO with splitcathode experienced pulse shortening as well. It is suggestedbased on preliminary results of simulations using the MAGICparticle-in-cell (PIC) code that the reason for this is that thismagnetron needs optimization by simulations that were notavailable originally

In earlier research Phys. Plasmas 27, 103102 (2020), a split cathode was proposed to avoid pulse shortening of microwave generation in relativistic $\text{S}$ -band magnetrons. Experiments confirmed the generation of microwave pulses limited only by the power generator’s pulselength ( $\sim$ 200 ns) J. Appl. Phys. 131, 023301 (2022). In the current research, the results of experiments with the same magnetron but powered by a generator producing up to $\sim$ 500-ns-long high-voltage pulses are presented. It is shown that the power and duration of the microwaves depend strongly on the applied magnetic field, anode–cathode gap length, and the applied voltage amplitude. In the experiments, $\sim$ 400-ns, $\sim$ 100-MW microwave pulses were measured. It was also identified that impedance matching between the relativistic magnetron (RM) load and the high-voltage (HV) pulse generator is an important factor in the operation of these devices.

Results of experimental research and two-dimensional hydrodynamical simulations of close to critically damped microsecond timescale underwater electrical explosions of butterfly-shaped foils for six different materials are presented. Using current and voltage waveforms along with multi-frame shadow images of the shocks generated in water, the values of the specific action integral, h, were determined. It is shown that values of h can be calculated based on the average current density and that its value (within error bars) does not change in the range of current densities (0.5-1) x 10(8) A/cm(2). The values of h were found to be consistent with those obtained for sub-microsecond underwater electrical explosions of wires made of the same material but differ from those obtained in earlier research with explosion of wires in vacuum. (c) 2024 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

The current state of research on generating and controlling a radial electric field in a magnetized plasma in cylindrical geometry is reviewed. Such systems are widely used in many plasma-physics applications ranging from thermonuclear fusion and plasma mass separation to the development of plasma thrusters. Primarily experimental studies are analyzed with an emphasis on the installation parameters and experiments conditions that enable generating controlled distributions of electrical potential. Data reported by various research teams are summarized in tables, allowing predictions of the potential reproducible under specific experimental conditions.

This work is devoted to determining the azimuthal ion rotation velocity in a reflex discharge with a thermionic cathode. For the experimental determination of the ion velocity, a Mach probe with directional particle collection was used. The Mach probe rotation velocity measurements are compared with the drift speed in crossed Ex B fields, where the radial electric field is measured with an emissive probe. The rotation of the plasma was found to be predominantly due to this drift, corrected for centrifugal effects. One of the important results of the work is the determination of the ion temperature. The obtained value Ti=0.12 eV, agrees with the ion temperature estimates in works with similar experimental conditions. A general parameter has been obtained that makes it possible to estimate the necessity to take into account centrifugal effects under given conditions. Keywords: plasma, thermionic cathode, reflex discharge, ion rotation, crossed fields.

Underwater electrical explosion is a subject which is related to Warm Dense Plasma and High Energy Density Matter. This explosion is characterized by specific action integral which determines explosion time, i.e. when magnetic pressure of the current becomes smaller than the thermal pressure and materials experience fast expansion. We present the measurements’ results of value of for underwater electrical explosions of foils having butterfly-tie shape and made of different metals, namely Cu, Al, Ta, Zn, Ti and stainless steel. The experiments were carried out in a wide range of the current densities using microsecond [1] and sub-microsecond [2] pulse power generators. Time resolved shadow images obtained by fast frame intensified camera were used to observe the positions of the strong shock wave generated in water for each material at different times and foil cross-sections. These positions allow to determine the time of the explosion using linear extrapolation of shock wave position to its origin. Using waveforms of the discharge current and resistive voltage, as well as COMSOL simulation, we define the value of the current density and current action integral value for the considering foil cross-section. As result, the dependence of the specific current action integral on the explosion current density was obtained. These data are useful for verification of two-dimensional numerical hydrodynamic simulations coupled with equations of state of the metals.

We present results exploring various methods of aluminum flyer acceleration. One method uses the shock wave generated by underwater electrical explosions of thin foils supplied by a pulse generator with stored energy of ∼4.7 kJ. Utilizing the shock created by an exploding foil, a maximal free flyer velocity of ∼2000 m/s is obtained. This acceleration method is compared to results exploiting only magnetic pushing to accelerate flyers using a common strip-line configuration, resulting in much lower velocities of ∼300 m/s. We also present a modified strip-line configuration, for which a significant increase in the flyer velocity to ∼1200 m/s is measured. Finally, a hybrid strip configuration, incorporating both the effects of magnetic pushing and acceleration by exploding foil and its subsequent shock wave, results in ∼1400 m/s flyer velocity. These experimental results are analyzed by numerical simulations and analytical modeling of the conservation equations of mass and momentum.

We present results on underwater electrical explosions of thin aluminum and copper foils using a generator delivering ∼200 kA current amplitude, ∼0.9 μs rise time pulses. Time-resolved shadow imaging displays the generation of a strong planar shock wave in water in the vicinity of the exploding foil. Using time-resolved spectroscopy, aluminum oxide (AlO) absorption bands were observed in a Planckian-like spectrum, indicating that aluminum combustion starts when aluminum vaporizes. It is also shown that the strongest shock wave is obtained for the largest linear energy deposition rate to the foil.

Одной из актуальных задач атомной энергетики является переработка отработавшего ядерного топлива. Такая переработка подразумевает отделение актиноидов от продуктов деления урана. Одним из методов переработки может стать плазменная масс сепарация. В ОИВТ РАН в последние 10 лет активно велись исследования, направленные на развитие различных аспектов, связанных с плазменной масс-сепарацией. В статье приведен обзор основных результатов этих исследований по четырем направлениям: численные расчеты и анализ схем сепарации; генерация плазмы буферного газа и создание потенциала в ней; источник плазмы для инжекции смеси разделяемых веществ; сепарация модельных веществ.

This work is devoted to the modeling of a Penning discharge with a hot cathode, which is used to create a background plasma with a radial electric field in a plasma mass separator. We used a 2D3V electrostatic particle-in-cell plus Monte Carlo Collision (PIC/MCC) simulation model with a gyrokinetic approximation for magnetized electrons and a geometrical scaling scheme. The proposed model predicts the development of a rotating spoke structure, leading to fluctuations in the electric potential. The fluctuation amplitude and the averaged radial profile of the potential are in qualitative agreement with experiment. On the example of a model flow of silver and lead ions, we have shown that potential fluctuations can impair separation, leading to a partial overlap of ion deposition spots on a substrate. Each of the separated fractions contain about 11% impurities.

One of the promising applications of low-temperature plasma in crossed electric and magnetic fields is plasma mass separation. To its implementation it is necessary to create a magnetized plasma with a given spatial distribution of the plasma potential. Plasma potential distribution determines the particle trajectories during separation. One of the difficulties that lie in the way of creating an efficient separator is the oscillations of the plasma potential resulting from the development of various types of instabilities. In the present work, fluctuations of the plasma potential in a reflex discharge with a thermionic cathode are studied. An analysis of the frequencies of plasma potential oscillations for magnetic fields in the range of 1–1.4 kG is presented. Measurements of the radial profiles of the root-mean-square deviation of the plasma potential are provided.

The concept of plasma mass separation of substances in a configuration with a potential well implies the creation of specialized plasma sources that comply with a number of requirements. The following can be distinguished as the main ones: a high degree of ionization of the plasma flow, single ionization, the possibility of working with a complex mixture of oxides and metals as a working substance, high productivity rate (up to 1 kg/h), kinetic energy at the level of several tens of electron-volts. One possible application of this concept is the separation of spent nuclear fuel. The conversion of condensed matter into a low-temperature plasma flow and its further injection into the separation chamber is the initial stage of the concept and it largely determines the efficiency of the technological process. This paper presents the results of studies of the energy distribution of lead ions in the plasma jet of a plasma source created on the basis of a non-self-sustained arc discharge with a hot cathode in a magnetic field. The influence of this distribution on the separation process was analyzed.

This work is devoted to determining the azimuthal ion rotation velocity in a reflex discharge with a thermionic cathode. For the experimental determination of the ion velocity, a Mach probe with directional particle collection was used. The Mach probe rotation velocity measurements are compared with the drift speed in crossed ExB fields, where the radial electric field is measured with an emissive probe. The rotation of the plasma was found to be predominantly due to this drift, corrected for centrifugal effects. One of the important results of the work is the determination of the ion temperature. The obtained value Ti=0.12 eV, agrees with the ion temperature estimates in works with similar experimental conditions. A general parameter has been obtained that makes it possible to estimate the necessity to take into account centrifugal effects under given conditions.

Plasma mass separation requires a lot of diagnostic techniques that not only demonstrate the separation effect but also show the efficiency of the process. During the test experiments, plasma flux to be separated may contain neutral particles that avoid the separation process due to their insensitivity to electromagnetic field. We present the diagnostics of the lost substance in experiments on plasma mass separation. The obtained data of the diagnostics helps determine the law of particle evaporation from the plasma source. We show that neutral flux is unable to distort the result of separation diagnostics. The presented approach can be used in experiments aimed at enhancing the separation effect and achieving target productivity for industry applications.

The mass separation of chemical element mixtures is a relevant task for numerous applications in the nuclear power industry. One of the promising approaches to solve this problem is plasma mass separation. In a recent study, the efficiency of plasma mass separation in a configuration with a potential well and a homogeneous magnetic field was experimentally demonstrated. This article examines the possibility of increasing the distance between the deposition regions of charged particles with different masses by varying the profile of the electric field potential. Such correlation can be considered as the control in a system of active particles. A cylindrical coordinate system is used. The electric field is axially symmetrical, and the magnetic field is directed along the axis of the symmetry. The corresponding mathematical problem was solved in a general way. The criteria for increasing the distance between the deposition areas of the “light” and “heavy” components of the mixture have been formulated. A high sensitivity of particle trajectories to the electric field potential in the region of the pericentres of the trajectories of charged particles was detected. Recommendations for the practical implementation of the optimal spatial separation of ion fluxes are proposed.

One of the relevant tasks of nuclear power industry is the reprocessing of spent nuclear fuel. Such processing implies the separation of actinides from uranium fission products. One of the processing methods can be plasma mass separation. In the last 10 years, research aimed at the development of various aspects related to plasma mass separation has been actively conducted at the Joint Institute for High Temperatures of the Russian Academy of Sciences. The article provides an overview of the main results of these studies in four areas: numerical calculations and analysis of separation schemes; generation of background plasma and formation of plasma potential distribution; plasma source for injection of a mixture of separated substances; and demonstration of the model substances separation.