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Broadband Plasma Relativistic Microwave Source with a Short Pulse Duration

Tue, 10/01/2019 - 00:00
Abstract

This study continues a series of works on creating a broadband plasma microwave sources on the basis of a smooth waveguide. Various methods for creating microwave sources and various methods for feedback uncoupling are considered with the purpose of generating continuous-spectrum microwave radiation. It is shown by numerical simulations that the plasma relativistic oscillator can be transformed into a noise amplifier by changing the parameters of the system. The numerical simulations allow one to determine the operating parameters of the experimental devices that are planned to be created and investigated in the future. It is shown that, when the time of the wave passage along the microwave oscillator exceeds the duration of the relativistic electron beam (REB) pulse, the microwave spectrum is continuous. However, if the wave passage time is much shorter than the REB pulse duration, then a line microwave spectrum will be observed at frequencies at which the oscillator length is a multiple of the half-wave, because microwave generation occurs on the longitudinal types of plasma oscillator modes. The tuning of the average microwave frequency from 3 to 9 GHz is achieved at a power of about 40 MW.

Stimulated Ignition and Quenching of the Cathode Spot in a Low-Power Discharge with Plasma Injection

Tue, 10/01/2019 - 00:00
Abstract

Experimental results are presented on the current commutation in a low-power gas discharge with external plasma injection. Correlation between current commutation and cathode spot ignition is established. It is shown that the cathode spot is ignited synchronously with the filling of the discharge gap with dense plasma and quenched synchronously with the release (gas-dynamic unloading) of the gap from the plasma. It is found that the spot is ignited over 1–30 ns due to the local explosive heating of the cathode surface (at a heating rate of higher than 1012 K/s) by the field emission current. It is also shown that the spot is quenched over a time of less than 100 ps and that the processes leading to the quenching develop simultaneously in all cells of the cathode spot.

Location of the Front of a Subthreshold Microwave Discharge and Some Specificities of Its Propagation

Tue, 10/01/2019 - 00:00
Abstract

It is proposed to determine the propagation velocity of a self-sustained subthreshold microwave (SSM) discharge by measuring the beatings between the reflected radiation and the reference signal. It is shown that the velocity of the head part of the SSM discharge determined from the fundamental frequency harmonic of the beating signal coincides with the velocity of the front of UV discharge radiation. The proposed method allows one to measure the velocity of the head part of the discharge along the microwave beam path, variations in this velocity caused by changes in the microwave beam power, the reduction in the gas density along the beam path, and the time delay in the initiation of the burning wave in methane–air mixtures.

Electron Motion in a Stationary Plasma Thruster

Tue, 10/01/2019 - 00:00
Abstract

Electron motion in a classical three-block model of the stationary plasma thruster (SPT) invented by A.I. Morozov is analyzed. It is demonstrated that the motion of electrons produced with a zero initial velocity due to working gas ionization critically depends on the position of the starting point along the accelerating channel axis. While the electron trajectories in the cathode region are regular, they become ergodic closer to the anode, which is accompanied by enhanced particle transport along the thruster channel. This transition is associated with the presence of a boundary at which erosion of the discharge chamber wall takes place in the experiment. The discovered ergodization of electron trajectories can be responsible for the anomalous electron conductivity in the near-anode region of the SPT.

Ion-beam Investigation in a 1.5-kJ Dense Plasma Focus Device

Tue, 10/01/2019 - 00:00
Abstract

An investigation of energetic ion beam emission from a low-energy (1.5 kJ) Mather-type plasma focus (PF) device operating with argon gas is studied with an operating pressure ranging from 2 × 10−2 to 1 Torr. The device has been energized by 30.84-μF capacitors charged up to 10 kV, giving a peak discharge current up to 120 kA with a rise time of 3 μs. A Faraday cup (FC) and its operation has been studied. This device is utilized to perform time-of-flight (TOF) measurements in order to characterize the beams of charged particles. The FC is placed at 20 cm from the tip of anode electrode at 0° with respect to anode axis to detect the ions. The FC operating in bias ion collector mode has been developed for measuring and characterizing pulsed ion beam. Energy and velocity of ions are determined with the TOF method, taking into account distance from the center electrode to the detection plane. The surface morphology of the pristine and irradiated ZnOCo0.04 samples is investigated as imaged by scanning electron microscope (SEM). The untreated sample showed rather relatively smooth surface; meanwhile, the SEM micrographs of bombarded samples display numerous small voids on the surface of the ZnOCo0.04 samples as a result of ion beam emission from PF device. X-ray fluorescence analysis showed that there are some new elements appeared on the surface of the sample after exposed to the ion beam; these elements and their concentrations are recorded. The surface roughness values for the pristine and the irradiated ZnOCo0.04 sample with Ar ion beam at different plasma shots are measured. A Vickers microhardness tester is used to measure the surface hardness, and the results demonstrate a significant increase in the hardness depending on the ion dose and decreases by increasing the applied load at 0° angular position.

On the Modes of Diffuse Spreading of Ionized Meteor Trails

Tue, 10/01/2019 - 00:00
Abstract

A special regime of the lower ionosphere sounding at altitudes of about 100 km reveals radio reflections from fairly dense (with a line density of 1014–1016 m–1) ionized meteor trails with a characteristic lifetime from a few tens of seconds to several tens of minutes. During the first 250 s, radio reflections from ionized trails with a line density of (2–3) × 1015 m–1 exhibit a power-law time dependence of the frequency of the reflected radio signal, $$f \propto {{t}^{{ - \gamma }}}$$ , with γ = 0.5 (classical diffusion mode). Less dense trails decay more slowly (γ < 0.5, subdiffusion mode), while denser ones decay faster (γ > 0.5, superdiffusion mode). It is shown that different modes of diffuse spreading of meteor trails can be caused by the high inhomogeneity of the medium and depend on the scale of ionized trails arising upon destruction of meteoroids.

Formation of Coronal Mass Ejections in the Solar Corona and Propagation of the Resulting Plasma Streams in the Heliosphere

Tue, 10/01/2019 - 00:00
Abstract

A review of studies of coronal mass ejections (CMEs) and the propagation of the resulting interplanetary coronal mass ejections (ICMEs) in the heliosphere is presented. The main parameters of ICMEs, their differences from other types of solar wind (SW) streams, and correlation of the ICME occurrence rate with the state of solar activity are considered. Special attention is paid to the formation and simulation of the ion composition of the CME/ICME plasma, which is one of the key factors in identifying the types and sources of SW streams, especially in complex structures formed in the heliosphere during the interaction of streams. Models for predicting the parameters of SW streams from observational data are considered. The review also presents the lists of literature sources on coronal ejections and databases on the parameters of SW streams, as well as numerous references to the works on the phenomena under study.

Formation and Evolution of Dusty Plasma Structures in the Ionospheres of the Earth and Mars

Tue, 10/01/2019 - 00:00
Abstract

A self-consistent model of the formation and evolution of dusty plasma structures in the ionospheres of the Earth and Mars is presented. The model allows describing the formation of a stratified dust structure as a result of dust cloud evolution in the Earth’s ionosphere. The structure forms due to the splitting of the primary cloud and is characterized by the presence of a cluster of dust grains at altitudes corresponding to noctilucent clouds and polar mesosphere summer echoes. The characteristic formation time of polar mesospheric clouds in the Earth’s ionosphere obtained within this model agrees with observational data. The possibility of the formation of oversaturated carbon dioxide clouds in the Martian ionosphere, similar to noctilucent clouds in the Earth’s ionosphere, is shown. It is demonstrated that phenomena similar to polar mesosphere summer echoes on the Earth can also take place in the Martian ionosphere. The theoretically estimated dimensions and charges of dust grains in the Martian ionosphere agree with observational data.

High-Current Pulsed ECR Ion Sources

Tue, 10/01/2019 - 00:00
Abstract

At the present time, some ECR ion sources use a high-frequency powerful microwave radiation of modern gyrotrons for plasma heating. Due to high radiation power, such systems mainly operate in a pulsed mode. This type of ECR ion sources was developed at the Institute of Applied Physics of the Russian Academy of Sciences (IAP RAS), and most experimental research was performed at the SMIS 37 facility, at which 37 gyrotrons with 37.5- and 75-GHz frequencies and 100- and 200-kW maximum powers, respectively, were used for plasma production. Such heating microwaves allow creating plasma with unique parameters: electron density >1013 cm–3, electron temperature of 50–300 eV, and ion temperature of about 1 eV. The principal difference between these systems and conventional ECR sources is a so-called quasi-gas-dynamic regime of plasma confinement. In accordance with the confinement regime, such sources have been called “gas-dynamic ECR sources.” Typically, plasma lifetime in such systems is about several tens of microseconds, which, in combination with the high plasma density, leads to the formation of plasma fluxes from a trap with a density of up to 1–10 A/cm2. The possibility of production of multiply charged ion beams (nitrogen, argon) and proton (or deuterium) beams with currents of up to a few hundred mA and normalized rms emittance of about 0.1π mm mrad was demonstrated. The next step in the research is a transition to continuous wave operation. For this purpose, a new experimental facility is under construction at the IAP RAS. A future source will utilize 28- and 37.5-GHz gyrotron radiation for plasma heating. An overview of the obtained results and the status of the new source development is presented.

Estimation of the Variability of the Combined Electron and Positron Flux of Cosmic Rays

Tue, 10/01/2019 - 00:00
Abstract

The variability of the combined electron and positron flux (e+ + e–) of cosmic rays (CRs) is numerically estimated using the results obtained in the Fermi-LAT,AMS-02, and DAMPE satellite experiments. The validity of the variability of the (e+ + e–) CR spectra in the energy range of >30 GeV is demonstrated. Correlation between the reliability of the variability for each pair of spectra and the difference of the average monthly sunspot numbers averaged over the time interval of observation is established. The existence of the observed variable component of the (e+ + e–) CR flux on a time scale from ~1 month to ~1 year can be explained using the model of surfatron acceleration of CRs in the close neighborhood of the Solar System (at the periphery of the heliosphere and in the nearby interstellar clouds).

Study of the Compression of a Condensed Deuterated Target Installed on the Wire Array Axis

Sun, 09/01/2019 - 00:00
Abstract

The implosion of combined loads consisting of an outer wire (fiber) array and inner cylindrical target was studied experimentally at the Angara-5-1 facility (3.5 MA, 100 ns) at currents of up to 3.5 MA. The experiments were carried out with 12- and 20-mm-diameter outer arrays made of 15-μm-diameter aluminum wires, composite arrays made of aluminum wires and 25-μm-diameter kapron fibers, and arrays made of kapron fibers with a 1-μm-thick aluminum coating. The number of wires varied from 10 to 40. The targets were made of agar-agar or low-density deuterated polyethylene. The parameters of the Z-pinch plasma were determined using the Angara-5-1 diagnostic complex, which included optical streak cameras, X-ray frame cameras, X-ray detectors, X-ray pinhole cameras, neutron detectors, and a mica-crystal X-ray spectrograph. It is established that the plasma compression dynamics and the formation of local plasma structures generating neutrons depend on the load configuration: the array diameter, the number of wires (fibers), and the diameter and density of the target. The most efficient compression and the highest plasma parameters (the compression ratio and plasma temperature), as well as the highest neutron yield, were achieved in experiments with 12-mm-diameter aluminum wire arrays inside which a 1-mm-diameter deuterated target with a mass density of 0.3 g/cm3 was installed As a result of collision of the bulk of the array mass with the inner target, a compact pinch with a diameter of ≈0.5 mm forms. The pinch formation is accompanied by the generation of a soft X-ray pulse. The development of MHD instabilities in the pinch plasma results in the formation of multiple hot spots (HSs) on the pinch axis with a typical size of 200–300 μm and an electron temperature of 0.4–0.7 keV. The HS formation is accompanied by emission of neutrons with a mean energy of 2.7 ± 0.2 MeV. The maximum neutron yield achieved in these experiments was 2.6 × 1010 neutrons/shot.

Wave Processes in the Dusty Plasma at Phobos and Deimos

Sun, 09/01/2019 - 00:00
Abstract

Linear and nonlinear waves in the near-surface plasma at Phobos and Deimos are considered. It is shown that the motion of the solar wind relative to photoelectrons and charged dust grains violates the isotropy of the electron distribution function in the near-surface plasma at the Martian satellites, which leads to the development of instability and excitation of high-frequency waves with frequencies in the range of Langmuir and electromagnetic waves. Moreover, the propagation of dust acoustic waves, which can be excited, e.g., in the terminator regions of the Martian satellites, is possible. Solutions corresponding to the parameters of the plasma–dust systems over the illuminated parts of the Phobos and Deimos surfaces are found in the form of dust acoustic solitons. The ranges of possible Mach numbers and soliton amplitudes are determined.

Influence of Trapped Electrons on the Plasma Potential in the Expander of an Open Trap

Sun, 09/01/2019 - 00:00
Abstract

The influence of electrons trapped between the magnetic mirror and the Debye sheath on the potential profile in the expander of an open trap is considered. A numerical model is developed to calculate the electron distribution function in the self-consistent electrostatic potential. Passing electrons are assumed to be collisionless, while the distribution function of trapped electrons is calculated by solving the bounce-averaged kinetic equation with the Landau collision integral. The self-consistent profile of the potential is obtained. The solution is shown to agree well with Ryutov’s semi-analytical model.

Fast Ignition by a Proton Beam and Burning of a DT Cylindrical Shell Target

Sun, 09/01/2019 - 00:00
Abstract

Edge ignition of a cylindrical shell target by a proton beam with a small mass heating depth of about 0.5 g/cm2 is analyzed for two values of the initial mass density of the DT fuel, $${{\rho }_{0}} \approx 110$$ and 22 g/cm3, and given beam parameters (the intensity $$J \propto {{\rho }_{0}}$$ and impact time $$\Delta {{t}_{{{\text{pr}}}}} \propto \rho _{0}^{{ - 1}}$$ ). By comparing results obtained using different models of heat transfer through the fuel–shell interface, it is shown that application of a strong magnetic field that suppresses heat transfer but does not affect the trajectories of the α-particles produced in the DT reaction reduces the ignition energy by only about 10%. The unsteady detonation wave generated in the course of ignition transforms into a steady-state fast shockless burning wave, in which the cold fuel is heated by α-particles. The wave parameters depend on the deposited energy. As the wave propagates through the fuel, the α-particles escaping from the fuel volume carry away about one-half of their initial power. For one of the simulation versions, the target length H is determined ( $${{\rho }_{0}}H \approx 10$$ g/cm2) at which the gain reaches a value of $$G = 1250$$ . An approximate formula is derived that relates the slope of the pressure profile in a steady-state wave to the wave velocity and the heating power per unit mass of the fuel near the wave front. The applicability of the formulas relating the pressure and velocity at the Chapman–Jouguet point to the propagation velocity of a strong detonation wave is demonstrated.

Plasma Current Sheath Shape and Trapping Efficiency in the 2.2-kJ EAEA-PF1 Plasma Focus Device

Sun, 09/01/2019 - 00:00
Abstract

The plasma current sheath (PCS) shape and trapping efficiency η are investigated experimentally. The experiments are carried out at the 2.2-kJ Egyptian Atomic Energy Authority Plasma Focus device, EAEA-PF1, of Mather-type geometry. Three versions of the inner electrode (IE) lengths, z0 = 9.5, 10.5, and 11.5 cm, are used. The investigations are performed with argon gas at a pressure ranging between P = 0.2 to 1.8 Torr. The PCS shape in term of its inclination angle of curvature θ and thickness λ are detected at an axial distance approaching the coaxial electrode muzzle and at three different radial distances through the annular space within the coaxial electrode assembly. Results on the trapping efficiency η are obtained from the magnetic force and PCS acceleration data under these discharge conditions. Diagnostic tools applied are a Rogowski coil and magnetic probes. Results on the PCS profile show that it has the best profile at an argon gas pressure of 0.8 Torr and IE length of 10.5 cm. The trapping efficiency η has the maximum value at gas pressures within the range of 0.6−1.8 Torr, where the maximum axial force is detected for most experimental data.

Collective Ion Drag Force

Sun, 09/01/2019 - 00:00
Abstract

The forces acting on the ensemble of dust grains in a plasma flow are analyzed. It is shown that the nonreciprocal character of the forces results in the appearance of an ion drag force, which depends on the intergrain distance. Results of calculations for two grains and an unbounded hexagonal lattice are presented. Estimates show that the collective ion drag force under typical dusty plasma conditions can be comparable with the weight of an individual grain.

Giant Jets as Higher Transverse Modes of an Open Cavity

Sun, 09/01/2019 - 00:00
Abstract

The spatial structure of giant blue jets in the upper atmospheric layers is considered on the basis of a nonlinear plasma waveguide model of electric gas breakdown. Laser analogues for such waves are proposed, and the azimuthally equidistant conical structure formed by the rays of a giant jet is explained. The field parameters and the electron density required for this process are estimated. Using the model proposed, requirements to the diagnostics of waves in the upper atmosphere are formulated. Similar models for analyzing thunderstorm phenomena in the lower atmosphere are offered.

Head-on Collision of Ion-Acoustic Solitary Waves with Two Negative Ion Species in Electron–Positron–Ion Plasmas and Production of Rogue Waves

Sun, 09/01/2019 - 00:00
Abstract

The interactions between ion acoustic solitary waves (IASWs) are investigated considering completely ionized electron–positron–ion (epi) plasmas consisting of ions with positive and two negative species, nonthermal electrons, and positrons. Two-sided Korteweg–de Vries (KdV) and modified KdV (mKdV) equations are derived using extended Poincaré–Lighthill–Kuo (ePLK) reductive perturbation method. To investigate the production of ion-acoustic rogue waves (IARWs), the rational solution of nonlinear Schrödinger equation (NLSE) is derived from the mKdV equation. Two types of plasmas containing $${\text{A}}{{{\text{r}}}^{ + }}$$ , $${{{\text{F}}}^{ - }}$$ , and $${\text{SF}}_{{\text{5}}}^{ - }$$ species, as well as $${\text{SF}}_{{\text{5}}}^{ + }$$ , $${{{\text{F}}}^{ - }}$$ , and $${\text{SF}}_{{\text{5}}}^{ - }$$ species, are taken into account to study their effects on the amplitudes and phase shifts after collision, as well as the production and properties of rogue waves (RWs). It is observed during collision that a high-amplitude wave is produced in the interaction region depending on the type and parameters of plasmas. The nonthermality of electrons and positrons, electron-to-positron temperature ratio, and the density of negative ions modify the phase shift and amplitude of the waves produced during the collision of the two solitons. The amplitude of the RW for the $${\text{A}}{{{\text{r}}}^{ + }}$$ , $${{{\text{F}}}^{ - }}$$ , and $${\text{SF}}_{{\text{5}}}^{ - }$$ plasmas is found to be larger than that for the $${\text{SF}}_{{\text{5}}}^{ + }$$ , $${{{\text{F}}}^{ - }}$$ , and $${\text{SF}}_{{\text{5}}}^{ - }$$ plasmas.

Simulation of an Inductive Discharge in Argon with the Gas Flow and Inhomogeneous Gas Temperature

Thu, 08/01/2019 - 00:00
Abstract

With the purpose to create new methods for monitoring the parameters of low-temperature nonequilibrium plasma, a numerical drift-diffusion model of an inductive RF discharge in argon is developed and a study is made of ion transport onto the surface of the processed material. The model was tested against the available experimental and theoretical data. The calculations were performed for an inductive discharge in argon with parameters typical of modern plasmachemical reactors (a frequency of 13.56 MHz and a gas pressure in the chamber of 10 mTorr). The plasma density, electron temperature, and ion flux onto the processed surface are calculated; the gas temperature is found as a function of the input RF power; and the discharge parameters are determined as functions of the gas flow rate.

Parametric Excitation of Surface Electromagnetic Waves by a Pump Wave Incident Obliquely on a Semi-Infinite Plasma

Thu, 08/01/2019 - 00:00
Abstract

Parametric excitation of surface waves by an s-polarized electromagnetic wave incident obliquely on a semi-infinite supercritical plasma is considered. The growth rates and thresholds of the instability associated with the decay of the pump wave into two surface electromagnetic waves are calculated as functions of the incidence angle and plasma electron density. It is shown that the instability growth rate depends linearly on the electric field of the pump wave and is maximal at a plasma electron density slightly exceeding the critical density.

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