Plasma Stream Research Articles

Macroparticle Reduction and Its Transport Mechanism through a Magnetic Filter during Cathodic Vacuum Arc Deposition with an HEA Target

By cathodic arc deposition, the effects of the magnetic field, working pressure, inner-wall structure, and cross-section area of the magnetic-filter duct on the macroparticle (MP) distribution were investigated with a high-entropy alloy target. The MP density increased with the density of the plasma beam transporting through the filter duct, which was increased by the magnetic field or working pressure. In order to reduce the MP density, equally spaced circumferential Cu-sheet baffle and lining of 304-stainless-steel wire mesh were used as the inner-wall structure, respectively, but the improvement was limited. However, inserting an Al foil disk with a round opening for the passage of the main plasma stream at the bend position of the duct remarkably reduced the area fraction of the MPs from 4.8% to 0.6%. These results demonstrate that the main transport mechanism of the MPs was the entrainment in the plasma beam through the duct. In addition, reducing the cross section of the filter duct was suggested to be an effective method to reduce MPs. This method could be utilized for high-MP generation targets such as high-entropy alloys.

First direct experimental evidence of the merging of two colliding field reversed configurations

The collisional merging of two Alfvénic-speed translated field reversed configurations (FRCs) via magnetic reconnection is presented with a two-dimensional magnetic probe array in the Keda Mirror with AXisymmetricity-FRC experiment. The collision is accompanied by axial compression and radial expansion, resulting in the increase in the FRC’s current density and poloidal flux. A reversed-current sheet is found to form during collision, indicating the occurrence of magnetic reconnection. After merging, the ion and electron temperature are increased. By comparing three different scenarios, i.e. the single-translated FRC, the FRC colliding solely with a stream plasma, and the collisional-merging FRC, we identify that axial compression and magnetic reconnection both contribute the electron heating.

The dynamic axial compression of FRC with high-speed translated θ-pinch plasma

A novel field reversed configuration (FRC) axial compression experiment with high-speed translated θ-pinch plasma was conducted in the Keda Mirror with AXisymmetry device. The translated north FRC was decelerated and compressed by an oncoming θ-pinch plasma stream, and the dynamic process was revealed by a 2D magnetic probe array. The FRC separatrix length is compressed to one-third of the initial value while the radius expands by ∼57%, resulting in the ∼16% increase in the electron temperature and ∼26% in the density, which matches the calculation from an adiabatic compression model. The good agreement is explained by the fast compression and particle supplementation owing to the compression with plasma. The results reported in this work may contribute to the understanding of electron heating in collision-merged FRC and provide a new compression method for the magneto-inertial fusion concept.

Electron-acoustic solitary potential in nonextensive streaming plasma

The linear/nonlinear propagation characteristics of electron-acoustic (EA) solitons are examined in an electron-ion (EI) plasma that contains negative superthermal (dynamical) electrons as well as positively charged ions. By employing the magnetic hydrodynamic (MHD) equations and with the aid of the reductive perturbation technique, a Korteweg-de-Vries (KdV) equation is deduced. The latter admits soliton solution suffering from the superthermal electrons and the streaming flow. The utility of the modified double Laplace decomposition method (MDLDM) leads to approximate wave solutions associated with higher-order perturbation. By imposing finite perturbation on the stationary solution, and with the aid of MDLDM, we have deduced series solution for the electron-acoustic excitations. The latter admits instability and subsequent deformation of the wave profile and can’t be noticed in the KdV theory. Numerical analysis reveals that thermal correction due to superthermal electrons reduces the dimensionless phase speed (bar{U}_{ph}) for EA wave. Moreover, a random motion spread out the dynamical electron fluid and therefore, gives rise to bar{U}_{ph}. A degree enhancement in temperature of superthermal (dynamical) electrons tappers of (increase) the wave steeping and the wave dispersion, enhancing (reducing) the pulse amplitude and the spatial extension of the EA solitons. Interestingly, the approximate wave solution suffers oscillation that grows in time. Our results are important for understanding the coherent EA excitation, associated with the streaming effect of electrons in the EI plasma being relevant to the earth’s magnetosphere, the ionosphere, the laboratory facilities, etc.

Parametric studies of stream instability-induced higher harmonics in plasma ionization breakdown near an emissive dielectric surface

In this work, we comprehensively investigate the generation of higher harmonic (HH) electric fields normal to the applied rf electric field in multipactor-coexisting plasma breakdown by fully kinetic particle-in-cell (PIC) simulations and a theoretical model. Firstly, a base case at driving frequency 1 GHz, transverse rf electric field amplitude 3 MV m−1, and background gas pressure 0.2 Torr, is studied in detail. The enhanced harmonic frequency observed is around ten times the fundamental rf frequency, significantly lower than the Langmuir frequency. A theoretical model reveals that the fundamental mechanism of HHs generation is stream–plasma instability, which originates from stream-like secondary electron emission interacting with plasma. The resulting HH frequency and the growth rate of its oscillating amplitude from the theoretical model, agree well with the PIC simulations. With increasing pressure, the HH oscillations are found to be significantly reduced. This is because at higher pressure the gas ionization rate is higher, which causes a more rapidly increasing plasma density, leaving less time for the growth of instability. Furthermore, the parameter space in terms of background gas pressure and rf field amplitude within which the HHs appear is revealed. Finally, the effect of the driving rf frequency on HHs is also investigated, and it is found that the instability-induced oscillating HHs field remains when the driving frequency is increased to 2 GHz, however, it is highly reduced at higher driving frequency of 5 GHz, as oscillations at the fundamental frequency start playing a more important role.

Magnetised and unmagnetized axisymmetric particle-in-cell simulations of ion energy distributions in cathodic vacuum arcs

Cathodic arcs are electrical discharges consisting of a succession of discrete pulses of energetic plasma travelling from the surface of a cathode toward an anode. Currently, there are no vacuum arc simulations where spots are continuously generated with a set frequency, including the far-field plasma jet, with the inclusion of kinetic behaviour for both ions and electrons. The VSim 11 particle-in-cell software was used to simulate specific vacuum arcs as axisymmetric, electrostatic, and fully kinetic, from the initial generation of each cathode spot to the streaming plasma discharge at a far field, validating the predictions against experimental data. The models were configured to match the experimental arc gun of Zohrer et al and the Mevva V experiment with the cathode materials Al and Nb. The ion and electron velocity data were collected at the far edge of the simulation domain, analogous to a physical energy detector. The simulations successfully predicted the evolution of ion charge state energy distributions, showing peak unmagnetized ion energies that agree with prior experimental data, resulting in a mean error of 3% for Al and Nb. A peak in the electrostatic potential is observed above the cathode surface, supporting the potential hump theory as the cause of the higher-than-expected ion energies observed in cathodic arc discharges. Lower than expected relative energies between ion charge states are observed, matching prior experimental results, with this coupling of ion charge states attributed to non-stationary electrostatic wave–particle interactions, as the use of collisionless simulations rules out Coulombic ion friction. Magnetised simulations incorporated a statically powered short solenoid equivalent to the coil used in the Mevva V experiment to create a diverging magnetic nozzle. The magnetised simulation results demonstrate an annular jet of magnetically confined plasma and indicate an increase in nonstationary electrostatic effects including wave–particle interactions.

Higher Harmonics in Multipactor Induced Plasma Ionization Breakdown near a Dielectric Surface.

In this Letter, the novel physics of higher harmonic (HH) generation of the normal electric field near a dielectric surface is reported in multipactor induced plasma ionization breakdown, as determined by kinetic particle-in-cell simulations. The observed HH frequency is around ten times the fundamental rf driving frequency, but lower than the electron plasma frequency. A theory is constructed which indicates that stream plasma interaction-induced instability is the mechanism of HH generation in the collisional regime. The HH frequency and its corresponding growth rate of the HH oscillation amplitude from the theory are in good agreement with kinetic particle-in-cell simulations.

A “Dam” in the Corona May Make the Solar Wind Gain Its Unusual Speeds

A new study supports the idea of a “helicity barrier” influencing the fluctuating stream of interplanetary plasma.

Measurement of Plasma Stream Parameters by Triple Probe in a Pulsed Plasma Accelerator

Measurement of Plasma Stream Parameters by Triple Probe in a Pulsed Plasma Accelerator

Frequency Transition From Weak to Strong Turbulence in the Solar Wind

During a specific time window while approaching the Sun, the longitudinal speed of Parker Solar Probe matches that of the Sun’s rotation. The spacecraft therefore co-rotates with the Sun, and as long as it does so, it is immersed in the solar-wind plasma of the same flow tube. This unique feature of the Parker Solar Probe’s orbital configuration is exploited in this work for the first time, to investigate the spectral properties of the turbulence of the same plasma stream, from large to small scales, very close to the Sun. Standard diagnostics for spectral power, compressibility, and intermittency are applied to the magnetic field data acquired by Parker Solar Probe during its seventh encounter with the Sun. The results clearly show the presence of a frequency transition (at about 5 × 10–3 Hz in the spacecraft frame) within the inertial range, where the spectrum steepens from an Iroshnikov-Kraichnan-like 3/2 to a Kolmogorov-like 5/3 scaling, the Alfvénic content decreases, whereas intermittency grows. This observational evidence is interpreted as the transition from scales dominated by Alfvénic fluctuations (and thus poorly intermittent and turbulent) to scales dominated by nonlinear interactions (and thus more intermittent and turbulent). To the author’s knowledge, this is the first time that such a transition from weak to strong turbulence in the inertial range has been observed, and it certainly deserves further investigation, both from an observational and theoretical perspective.

Determination of Electrons Temperature and Density For Ag, Zn, and Cu metals using Plasma jet System at atmospheric pressure

Under atmospheric pressure, an argon plasma stream was sustained and its plasma characteristics were examined. The emission spectra of plasma created in a plasma jet system using argon gas were observed for three metals (Ag, Zn, and Cu) for the anode and varied flow rates ranging from 1–4 L/min. at constant voltage, and normal atmospheric pressure. The spectral lines of excited Ar, Ag, Zn, and Cu species were identified at a wavelength of (650–900) nm .The Debye length, sphere, and temperature of an electron are all measured. Optical emission spectrometer (OES) equipment was used to capture the spectrum produced by the plasma at various argon gas flow rates.The temperature and density of the electron (Te) and (ne) ranges for Ar-gas, Ag, Zn, and Cu-anode increased as the stream pace of argon gas to the plasma made by the release current (D.C.) expanded.(1.241- 1.473)eV and (1.93 x1018–6.38 x1018 ) cm-3 , (1.187– 1.245) eV and (4.32 x1016–6.23 x1016 ) cm-3 , (1.374 -1.631)eV and (4.01 x1018 – 12.1 x1018 ) cm-3 respectively . The intensity of spectral lines, on the other hand, increased.

Surface waves in a streaming plasma propagating in a duct: kinetic theory

The dispersion relation of a surface wave generated by a drifting plasma in an infinite duct surrounded by vacuum is derived non-relativistically by means of the Vlasov equation. The kinematic boundary condition imposed on the distribution function, the specular reflection conditions on the four sides of duct, can be satisfied by placing an infinite number of fictitious surface charge sheets spaced by the duct widths. The surface wave mode is specifically the transverse magnetic mode, often called the surface polariton, which propagates with phasor $\exp ({{\rm i}k_zz-{\rm i}\omega t})$ . The method of placing appropriate fictitious surface charge sheets enables one to treat the surface waves in semi-infinite, slab and duct plasmas simultaneously on an equal footing, kinetically. The streaming effect manifests itself through the Doppler-shifted frequency and a correction-like term ${u^2}/{c^2}$ , where u is the streaming velocity and c is the speed of light.

The impact of various instances of solar wind speed on the fluctuations of cosmic radiation in the solar minima (23, 24, and 25)

Its key role in space weather determines the importance of solar wind study. The solar wind is made up of various plasma streams with varying hydrodynamic and magnetic properties, which has been known for a long time. Because the distinction between the sorts of flows is conditional rather than absolute, there is still no widely agreed terminology, which varies according to the author. The number of these kinds as well as how they are classified is varied. We investigated data on solar wind speed and cosmic ray intensity for four stations, Apatity, Fort Smith, Hermanus, and Moscow, collected from the National Oceanic and Atmospheric Administration and neutron monitors, from 1998 to 2020 for three solar cycles (23, 24, and 25). Slow and rapid wind, as well as disturped streams, are the three basic forms of solar wind. The sluggish solar wind has a typical speed of 300–500 km per second. At speeds of 700–800 km/s, rapid solar wind is emitted. In all cases of solar wind speed, the amplitude and phase of daily variability on selected sites were investigated. The presence of the solar wind was recorded every day during the study time, with low solar wind events occurring more frequently than the rapid and disturped streams.

Externally driven plasma models as candidates for pulsar radio emission

ABSTRACT Coherent radio emission from pulsars originates from excited plasma waves in an ultra-relativistic and strongly magnetized electron–positron pair plasma streaming along the open magnetic field lines of the pulsar. Traditional coherent radio emission models have relied on instabilities in this pair plasma. Recently, alternative models have been suggested. These models appeal to direct coupling of the external electromagnetic field to the superluminal O-mode (lt2 mode) during the time-dependent pair cascade process at the polar gap. The objective of this work is to provide generic constraints on plasma models based on lt2 mode using realistic pulsar parameters. We find that the very short time-scale associated with pair cascades does not allow lt2 mode to be excited at radio frequencies and the impulsive energy transfer can only increase the kinetic spread (‘temperature’) of the pair plasma particles. Moreover, under homogeneous plasma conditions, plasma waves on both branches of O mode (i.e. superluminal lt2 and subluminal lt1) cannot escape the plasma. In the strongly magnetized pair plasma, only the extraordinary mode (t mode) can escape freely. We show that any generic fictitious mechanisms do not result in the wave electric field of t mode to have predominant orientation either parallel or perpendicular to the magnetic field plane as observed. Such fictitious mechanisms will inevitably lead to depolarization of signals and cannot account for the highly polarized single pulses observed in pulsars. We suggest coherent curvature radiation as a promising candidate for pulsar radio emission mechanism.

Experimental Evaluation of the Effect of Argon Cold Plasma on Oxidative Metabolism of the Blood.

We studied the effect of course exposure to argon cold plasma (ten 1- and 2-min procedures) on some parameters of the oxidative metabolism of rat blood plasma. The intensity of free radical processes, the total antioxidant activity, and malondialdehyde concentration in rat plasma were evaluated. It was found that 2-min exposure to argon cold plasma and nonionized argon stream produce a prooxidant effect, while 1-min exposure argon plasma led to stimulation of the antioxidant reserves of the blood.

Waiting times between gamma-ray flares of flat spectrum radio quasars, and constraints on emission processes

Context.The physical scenario responsible for gamma-ray flaring activity and its location for flat spectrum radio quasars is still debated.Aims.The study of the statistical distribution of waiting times between flares, defined as the time intervals between consecutive activity peaks, can give information on the distribution of flaring times and constrain the physical mechanism responsible for gamma-ray emission.Methods.We adopt here a scan statistic-driven clustering method (iSRS) to recognize flaring states within theFermi-LAT archival data, and identify the time of activity peaks.Results.We obtained that waiting times between flares can be described with a Poissonian process, consisting of a set of overlapping bursts of flares, with an average burst duration of ∼0.6 year and average rate of ∼1.3 y−1. For short waiting times (below 1 d host-frame) we found a statistically relevant second population, the fast component, consisting of a few tens of cases, most of them revealed for CTA 102. Interestingly, the period of conspicuous detection of the fast component of waiting times for CTA 102 coincides with the reported crossing time of the superluminal K1 feature with the C1 stationary feature in radio.Conclusions.To reconcile the recollimation shock scenario with the bursting activity, we have to assume that plasma streams with a typical length of ∼2 pc (in the stream reference frame) reach the recollimation shock. Otherwise, the distribution of waiting times can be interpreted as originating from relativistic plasma moving along the jet for a deprojected length of ∼30−50 pc (assuming a bulk Γ = 10) that sporadically produces gamma-ray flares. In the magnetic reconnection scenario, reconnection events or plasma injection to the reconnection sites should be intermittent. Individual plasmoids can be resolved in a few favourable cases only, and could be responsible for the fast component.

Temporal Spectrum of a Scattered Electromagnetic Waves in the Conductive Collision Turbulent Magnetized Plasma

Using WKB method the peculiarities of the temporal spectrum of an ordinary and extraordinary electromagnetic wave scattered in a weakly randomly inhomogeneous three-dimensional nonstationary and conductive magnetized plasma are investigated. On the basis of the stochastic differential transport equation for the frequency fluctuation the broadening and the displacement of the temporal spectrum for both waves are obtained for the polar terrestrial ionosphere. These statistical characteristics contains anisotropic parameters: velocity of a plasma stream, conductivities of the ionosphere, elongated electron density irregularities are characterized by the anisotropy factor and inclination angle with respect to the geomagnetic lines of forces. The analysis of the power spectrum of the waves as a function of the propagation distance and the nondimensional frequency parameter containing the carrier frequency and characteristic temporal scale of electron density fluctuations are carried out. Analytical and numerical calculations have shown that the terrestrial conductivity and anisotropy factors exert a substantial influence on evaluation of the temporal spectrum than the inclination angle. It was found that the wave spectrum increases initially as the square root of the propagation distance, but at large distances it approaches a limiting value. Statistical moments of a scattered ordinary and extraordinary waves do not depend on an absorption sign and valid for both absorptive and active media.

Laboratory modelling of equatorial ‘tongue’ accretion channels in young stellar objects caused by the Rayleigh-Taylor instability

Context. The equatorial accretion scenario, caused by the development of the Rayleigh-Taylor (RT) instability at the disk edge, was suggested by accurate three-dimensional magnetohydrodynamic (MHD) modelling, but no observational or experimental confirmation of such phenomena has been evidenced yet. Aims. We studied the propagation of a laterally extended laser-generated plasma stream across a magnetic field and investigated if this kind of structure can be scaled to the case of equatorial ‘tongue’ accretion channels in young stellar objects (YSOs); if so, this would support the possibility of equatorial accretion in young accreting stars. Methods. We conducted a scaled laboratory experiment at the PEARL laser facility. The experiment consists in an optical laser pulse that is focused onto the surface of a Teflon target. The irradiation of the target leads to the expansion of a hot plasma stream into the vacuum, perpendicularly to an externally applied magnetic field. We used a Mach-Zehnder interferometer to diagnose the plasma stream propagation along two axes, to obtain the three-dimensional distribution of the plasma stream. Results. The laboratory experiment shows the propagation of a laterally extended laser-generated plasma stream across a magnetic field. We demonstrate that: (i) such a stream is subject to the development of the RT instability, and (ii) the stream, decomposed into tongues, is able to efficiently propagate perpendicular to the magnetic field. Based on numerical simulations, we show that the origin of the development of the instability in the laboratory is similar to that observed in MHD models of equatorial tongue accretion in YSOs. Conclusions. As we verify that the laboratory plasma scales favourably to accretion inflows of YSOs, our laboratory results support the argument in favour of the possibility of the RT-instability-caused equatorial tongue accretion scenario in the astrophysical case.

Occurrence of microstructures on steel surface under the influence of plasma focus discharge

The surface of metals after irradiation by intense streams of argon plasma arising in the plasma focus was studied. Metal samples of steel, copper, tungsten and molybdenum were investigated. Different types of microstructures were recorded. The surfaces of steel samples contain micropores with sizes of 0.1-3 μm with a shape close to circular. On copper and tungsten plates the appearance of fine-dispersed structures with sizes of 0.5-20 μm is observed. The possible mechanisms of the appearance of these microstructures are discussed. Keywords: plasma focus, intense plasma flows, micropores, filamentous discharge structure.

Возникновение микроструктур на поверхности стали под воздействием разряда плазменного фокуса

The surface of metals after irradiation by intense streams of argon plasma arising in the plasma focus was studied. Metal samples of steel, copper, tungsten and molybdenum were investigated. Different types of microstructures were recorded. The surfaces of steel samples contain micropores with sizes of 0.1-3 μm with a shape close to circular. On copper and tungsten plates the appearance of fine-dispersed structures with sizes of 0.5-20 μm is observed. The possible mechanisms of the appearance of these microstructures are discussed.