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In isotope concentration technology, ion extraction current and ion extraction efficiency are the key factors to measure the efficiency of the isotope concentration. In order to increase the ion extraction current, researchers usually hope to produce a plasma source with large initial peak density and width; however, in reality, it is limited by the laser power, and the total number of ions in a plasma produced by laser ionization is almost certain. In this case, how to improve the ion extraction efficiency by choosing the appropriate initial density distribution of plasma has become a difficult problem. In this paper, the effects of the initial density distribution of plasma on the ion extraction characteristics are studied by using the electron equilibrium fluid model. The numerical results suggest that the ion extraction efficiency is independent of the initial density distribution of plasma while the total number of ions in the plasma, the distance between the electrodes, and the electric field intensity are kept constant. When the total number of ions and the electric field intensity are kept constant, the distance between the electrodes is shortened by one time, and the time of ion extraction is also shortened by nearly one time; thus, the plasma source with high initial peak density and small width can be chosen, and the aim of ion extraction can be achieved by shortening the distance between the electrodes. This research results provide an important reference for guiding the experimental parameters such as laser power distribution and the design of ion extraction device. [ABSTRACT FROM AUTHOR]

Plasma-based acceleration (PBA) is being considered for a next generation linear collider (LC). In some PBA-LC designs for the electron arm, the extreme beam parameters are expected to trigger background ion motion within the witness beam, which can lead to longitudinally varying nonlinear focusing forces and result in an unacceptable emittance growth of the beam. To mitigate this, we propose to use quasi-adiabatic plasma density ramps as matching sections at the entrance and exit of each stage. We match the witness electron beam to the low density plasma entrance, where the beam initially has a large matched spot size so the ion motion effects are relatively small. As the beam propagates in the plasma density upramp, it is quasi-adiabatically focused, and its distribution maintains a non-Gaussian equilibrium distribution in each longitudinal slice throughout the process, even when severe ion collapse has occurred. This only causes small amounts of slice emittance growth. The phase mixing between slices with different betatron frequencies leads to additional projected emittance growth within the acceleration stage. A density downramp at the exit of an acceleration section can eliminate much of the slice and projected emittance growth as the beam and ion motion adiabatically defocuses and decreases, respectively. Simulation results from QuickPIC with Azimuthal Decomposition show that within a single acceleration stage with a 25 GeV energy gain, this concept can limit the projected emittance growth to only ∼2% for a 25 GeV, 100 nm emittance witness beam and ∼20% for a 100 GeV, 100 nm normalized emittance witness beam. The trade-off between the adiabaticity of the plasma density ramp and the initial ion motion at the entrance for a given length of the plasma density ramp is also discussed. [ABSTRACT FROM AUTHOR]

The self‐organization of a magnetized multi‐ion plasma, composed of inertialess electrons and inertial H+, He+, and O+ ions, leads to the formation of quadruple Beltrami (QB) field structures. The QB self‐organized state is a linear combination of four single Beltrami fields, and it is a non‐force‐free state that shows strong magnetofluid coupling. Moreover, the QB state is characterized by four relaxed state structures of different length scales. The investigation reveals that the generalized helicities of plasma species and the densities of ion species have a significant impact on the characteristics of the self‐organized vortices in the QB state. The study also highlights the potential consequences of QB field structures on earth's inner magnetosphere, including diamagnetic and paramagnetic trends as well as heating effects resulting from disparate length scales. [ABSTRACT FROM AUTHOR]

A quadruple Beltrami (QB) equilibrium state for a four-component plasma that consists of inertial electrons, positrons, lighter positive (H+) ions and heavier negative ions O 2 − is derived and investigated. The QB relaxed state is a linear superposition of four distinct single Beltrami fields and provides the possibility of the formation of four self-organized vortices of different length scales. In addition, robust magnetofluid coupling characterizes this non-force-free state. The analysis of the QB state also shows that by adjusting the generalized helicities and densities of plasma species, the formation of multiscale structures as well as the paramagnetic and diamagnetic behavior of the relaxed state can be controlled. [ABSTRACT FROM AUTHOR]

Magnetosonic (MS) waves are electromagnetic emissions from a few to 100 Hz primarily confined near the magnetic equator both inside and outside the plasmasphere. Previous studies proved that MS waves can transport equatorially mirroring electrons from an equatorial pitch angle of 90° down to lower values by bounce resonance. However, the dependence of the bounce resonance effect on wave or background plasma parameters is still unclear. Here, we applied a test particle simulation to investigate electron transport coefficients, including diffusion and advection coefficients in energy and pitch angle, due to bounce resonance with MS waves. We investigate five wave field parameters, including wave frequency width, wave center frequency, latitudinal distribution width, wave normal angle root-mean-square of wave magnetic amplitude, and two background parameters, L-shell value and plasma density. We find different transport coefficient peaks resulting from different bounce resonance harmonics. Higher-order harmonic resonances exist, but the effect of fundamental resonance is much stronger. As the wave center frequency increases, higher-order harmonics start to dominate. With wave frequency width increasing, the energy range of effective bounce resonance broadens, but the effect itself weakens. The bounce resonance effect will increase when we decrease the wave normal angle, or increase the wave amplitude, latitudinal distribution width, L-shell value, and plasma density. The parametric study will advance our understanding of the favorable conditions of bounce resonance. [ABSTRACT FROM AUTHOR]

The resonant field penetration to magnetic islands is the central MHD mechanism of non-axisymmetric plasma responses in a tokamak such as disruptive locking or favorable ELM stabilization. The resonant field penetration can be induced by any non-ideal processes as manifested in the delicate balance under the generalized Ohm's law. Here, we show that the viscous effects by electrons are not ignorable in the field penetration unlike previous presumption, even if the electron viscosity is as small as the square root of its mass compared to the ions. It is clear that its effects become only bigger if the electron viscosity becomes anomalously large. The work strictly follows the three-field model in the linear regime targeting the prediction of the onset of the field penetration and successfully extending it with electron viscosity and identifying new regimes. The results also indicate that the error field thresholds become more strongly dependent on plasma density than ones predicted in the linear regimes without the electron viscosity, which is consistent with experimental observations and thus a significant implication. [ABSTRACT FROM AUTHOR]

Through resonant wave‐particle interactions with electrostatic electron cyclotron harmonic (ECH) waves, low‐energy plasma sheet electrons can be scattered into the atmospheric loss cone and precipitate. This process can be investigated by calculating bounce‐averaged quasi‐linear diffusion coefficients. However, the numerical calculation of ECH wave‐induced scattering rates requires the specification of several parameters, including the properties of the hot plasma sheet electrons responsible for the wave excitation. The dependence of the diffusion coefficients on these parameters and the exact contribution of scattering by ECH waves to diffuse auroral precipitation is still poorly understood and has not been carefully quantified yet. In this study, we calculate bounce‐averaged quasi‐linear scattering rates and compare the resulting lifetimes to the strong diffusion limit. We vary the temperature, plasma density, and loss cone parameters of the hot component in the electron loss cone distribution over a large range based on previous studies and observations. By adopting a new model that derives the wave normal angle from the midpoint and the angular width of the growth rate profile, our findings suggest that the electron lifetime near the loss cone is not significantly affected by the hot electron temperature and loss cone parameters. However, there is an increase in electron lifetimes with increasing plasma density. For an electric field amplitude of 1 mV/m, the electron lifetime is below or equal to the lifetime calculated using the strong diffusion limit up to E = 0.25 keV when n = 1.5 cm−3, and up to E = 0.22 keV when n = 9 cm−3. Key Points: Plasma density has the greatest impact on electron loss among the tested parametersIncreasing trend in electron lifetimes with plasma density which becomes more pronounced at higher electron energiesResults depend crucially on adapted wave normal angle model which represents growth rate profile more consistently than in previous works [ABSTRACT FROM AUTHOR]

Energetic electron precipitation from the equatorial magnetosphere into the atmosphere plays an important role in magnetosphere‐ionosphere coupling: precipitating electrons alter ionospheric properties, whereas ionospheric outflows modify equatorial plasma conditions affecting electromagnetic wave generation and energetic electron scattering. However, ionospheric measurements cannot be directly related to wave and energetic electron properties measured by high‐altitude, near‐equatorial spacecraft, due to large mapping uncertainties. We aim to resolve this by projecting low‐altitude measurements of energetic electron precipitation by ELFIN CubeSats onto total electron content (TEC) maps serving as a proxy for ionospheric density structures. We examine three types of precipitation on the nightside: precipitation of <200 keV electrons in the plasma sheet, bursty precipitation of <500 keV electrons by whistler‐mode waves, and relativistic (>500 keV) electron precipitation by EMIC waves. All three types of precipitation show distinct features in TEC horizontal gradients, and we discuss possible implications of these features. Plain Language Summary: Bursty precipitation of energetic electrons, via pitch‐angle scattering by whistler‐mode waves from the magnetosphere to the ionosphere, is an important factor in the global magnetosphere‐ionosphere coupling. It induces local modifications of ionospheric density and chemical composition. A recurrent problem in the investigation of this process is the presence of large uncertainties in the field‐line mapping between ionospheric density structures and high altitude satellites measuring electron fluxes in the magnetosphere. In the present study, such uncertainties are significantly reduced by making use of precipitating electron fluxes recorded by ELFIN CubeSats at low altitudes (450 km) just above the ionosphere and comparing them with maps of the corresponding ionospheric density structures. We identify three different types of electron precipitation on the nightside, corresponding to low, moderate, and high energy precipitating electrons. We show that each type of the precipitation is characterized by particular plasma density gradients in the ionosphere, suggesting a key role of wave ducting by plasma density gradients in fostering the precipitation of 300–500 keV electrons by whistler‐mode waves, and the potential importance of midnight plasma injections in generating EMIC waves that can further precipitate 0.5–2 MeV electrons far away from the plasmasphere. Key Points: Total electron content (TEC) maps are examined during energetic electron precipitation at the conjugate low altitudeNight‐side whistler‐mode wave driven precipitation is often associated with local horizontal TEC gradientsNight‐side EMIC wave driven precipitation is often poleward of the TEC minima associated with the plasmapause projection [ABSTRACT FROM AUTHOR]

We have surveyed 21 reconnection exhaust events observed by Magnetospheric MultiScale in the low-plasma- β and high-Alfvén-speed regime of the Earth's magnetotail to investigate the scaling of electron bulk heating produced by reconnection. The ranges of inflow Alfvén speed and inflow electron β e covered by this study are 800–4000 km s−1 and 0.001–0.1, respectively, and the observed heating ranges from a few hundred electronvolts to several kiloelectronvolts. We find that the temperature change in the reconnection exhaust relative to the inflow, Δ T e, is correlated with the inflow Alfvén speed, V Ax,in, based on the reconnecting magnetic field and the inflow plasma density. Furthermore, Δ T e is linearly proportional to the inflowing magnetic energy per particle, m i V Ax , in 2 , and the best fit to the data produces the empirical relation Δ T e = 0.020 m i V Ax , in 2 , i.e., the electron temperature increase is on average ∼2% of the inflowing magnetic energy per particle. This magnetotail study extends a previous magnetopause reconnection study by two orders of magnitude in both magnetic energy and electron β, to a regime that is comparable to the solar corona. The validity of the empirical relation over such a large combined magnetopause–magnetotail plasma parameter range of V A ∼ 10–4000 km s−1 and β e ∼ 0.001–10 suggests that one can predict the magnitude of the bulk electron heating by reconnection in a variety of contexts from the simple knowledge of a single parameter: the Alfvén speed of the ambient plasma. [ABSTRACT FROM AUTHOR]

Recent MMS observations confirmed the existence of the quasi‐stationary electron‐scale super‐thin current sheets (STCSs) with a half‐thickness of few electron gyroradii. STCSs contain the electron population consisting of magnetized and demagnetized particles. To study the role of the demagnetized electrons, we developed a self‐consistent combined model of a STCS in which the dynamics of a part of the electron population is described by the quasi‐adiabatic approximation, while the motion of the other part is described by the guiding center approximation. Demagnetized ion population can be described in a frame of quasi‐adiabatic approach. In the presented model we introduced the weight coefficient of the demagnetized electron plasma density χ depending on the magnetic field normal component Bn/B0 and compared the characteristic profiles of two STCSs differing by the presence (χ ≠ 0) and absence (χ = 0) of demagnetized electrons. It is shown that the presence of demagnetized electrons makes the current density profile of STCS significantly more intense and narrower. The thickness and intensity of STCS depend on the anisotropy coefficient of magnetized electrons and the incoming drift velocity of demagnetized electrons. The maximum current density continuously increases with the growth of the demagnetized electron population and is controlled by the ratio of the thermal to the incoming drift electron velocities. Thus intense current sheets can be formed even in the absence of strong electron anisotropy, which is confirmed by MMS observations. Key Points: Demagnetized electrons makes the current sheets more intense and narrower with sharper gradients of magnetic field in the centerAn increase in the anisotropy coefficient of magnetized electrons and the field‐aligned flow of demagnetized electrons can lead to the increase in the intensity and thin‐ning of the electron current sheetIntense super thin current sheets can be formed even in the presence of a moderate electron anisotropy (pǁe ≈ p⊥e), which is confirmed by MMS observations [ABSTRACT FROM AUTHOR]

The formation of high-energy electrons and ion fluxes induced by an abnormal electron heating mode in asynchronous pulse-modulated plasma was investigated using particle-in-cell simulation. We demonstrate that the abnormally high electron heating mode was induced only for a short time in the asynchronous pulsed plasmas. Furthermore, enhanced production of energetic electrons accompanies this electron heating. In particular, the higher energy electrons (ε > 20 eV) are mainly produced by the abnormal electron heating during the first period of the abrupt sheath expansion phase in the asynchronous pulsed plasma with α1 = α3 = 0.05. These high-energy electrons are crucial for tailoring the expansion of plasma density and neutralizing the surface charging for the HARC etching process. A synergy of higher energy electrons and higher density ion fluxes in asynchronous pulsed plasma can be a promising solution to reduce statistical variation and charging-induced profile deterioration without the etch rate reduction in 3D NAND fabrication. [ABSTRACT FROM AUTHOR]

Time-domain structures (TDS), manifested as ≥ 1 ms pulses with significant parallel electric fields, play an important role in accelerating electrons in the field-aligned direction. These precipitated electrons contribute to the formation of aurora. In this study, we present observations of time-domain structures that occurred in the plasmaspheric plumes at the post-midnight to dawn sector. The close correlation between TDS and plasmaspheric plumes implies that the generation of TDS might be modulated by plasma density. During the wave occurrence, protons with an energy level below 1 keV show the enhanced field-aligned pitch-angle distributions, and the electron fluxes with the energies ranging from tens to hundreds of eV are also significantly enhanced. The correlation between TDS and scattered particles indicates the importance of including time-domain structures in future studies of radiation belt dynamics. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the production of high energy gamma photons at the interaction between an ultra-high intensity laser pulse with an energetic electron beam and with a near-critical density plasma for the laser intensity varying between 10 19 – 10 23 W/cm 2 . In the case of the interaction with an electron beam, and for the highest laser intensities considered, the electrons lose almost all their energy to emit gamma photons. In the interaction with a near-critical density plasma, the electrons are first accelerated by the laser pulse up to GeV energies and further emit high energy radiation. A maximum laser-to-photons conversion coefficient of 30 % is obtained. These results can be used for the preparation of experiments at the Apollon and ELI laser facilities for the investigation of the emission of high energy γ -photons and to study the electron-positron pair creation in the laboratory. [ABSTRACT FROM AUTHOR]

In this paper, we present a new method to compute rapidly, wave particle interaction induced pitch angle and momentum diffusion coefficients. Those terms are normally obtained by the numerical solving of equations based on the quasi-linear theory. However, this bulk resolution leads to a high computational cost, preventing the integration of plasma density and VLF waves nowcasts and forecasts. Therefore, and in the context of the SAFESPACE project (H2020), we implemented a new wave particle interaction code called FARWEST (FAst Radiation diffusion with Waves ESTimator), with an efficient interpolation based method. The proposed implementation was validated against reference results obtained by WAPI, ONERA's wave particle interaction legacy code, with a substantial reduction of the computing time while conserving the physical accuracy of the generated coefficients. The FARWEST code is later used to measure the impact of a time-dependent plasma density distribution (numerically computed by BIRA's plasmaspheric model SPM) on the coefficients and on the representation of the electron flux map by Salammbô, ONERA's radiation belt code. [ABSTRACT FROM AUTHOR]

Comparison between the Maxwell demon and a planar electrode has been revisited with an in-depth analysis of whether the angular momentum trap of the Maxwell demon indeed provides better energy selectivity than a small planar electrode that absorbs electrons indiscriminately. The evolutions of the EEDF under the influence of these heating techniques is directly analyzed, as well as the resultant plasma parameters. Experimental results show that the Maxwell demon indeed provides better energy selectivity as shown by its better retention of hot electrons than an indiscriminative absorption surface, which in turn results in smaller disturbance to the plasma potential a smaller reduction of the plasma density in the heating process. Experimental result also shows no electron heating when the demon is replaced by an ion-sheath forming large electrode, this is consistent with Mackenzie’s original results (MacKenzie et al 1971 App. Phys. Lett. 18 529). While it is possible to obtain the exact same plasma parameters replacing the Maxwell demon with a suitably sized planar plate and additional plasma parameters control, for experiments sensitive to the exact processes from which plasma parameters are formed, one should not overlook the physical differences of these heating methods. [ABSTRACT FROM AUTHOR]

Recent studies have shown that the stabilization of neoclassical tearing modes (NTMs) by electron cyclotron current drive (ECCD) in ITER may be impeded when the injected electron cyclotron wave beam is broadened by plasma density fluctuations. This paper starts with the analysis of NTM stabilization by ECCD using the modified Rutherford equation (MRE). It is shown that with a wide wave deposition, the most effective approach for mode stabilization is to apply modulated ECCD early during the mode growth when the magnetic island is small, as expected. Numerical simulations based on reduced magnetohydrodynamic equations have been further carried out, and the results show the same trend as those obtained from the MRE. However, the perpendicular transport of fast electrons, taken into account in numerical simulations, is found to significantly degrade the mode stabilization by modulated ECCD. The modulated driven current required for mode stabilization is proportional to the square root of the perpendicular diffusivity of fast electrons and is increased by several times when the diffusivity reaches the anomalous transport level due to plasma turbulence. If the radial misalignment of the driven current from the resonant surface reaches ∼4% of the plasma minor radius, the mode stabilization can be degraded to a great extent, depending on the wave deposition width and the perpendicular diffusivity of fast electrons. [ABSTRACT FROM AUTHOR]

The concept of electron acceleration by a laser beam in vacuum is attractive due to its seeming simplicity, but its implementation has been elusive, as it requires efficient electron injection into the beam and a mechanism for counteracting transverse expulsion. Electron injection during laser reflection off a plasma mirror is a promising mechanism, but it is sensitive to the plasma density gradient that is hard to control. We get around this sensitivity by utilizing volumetric injection that takes place when a helical laser beam traverses a low-density target. The electron retention is achieved by choosing the helicity, such that the transverse field profiles are hollow while the longitudinal fields are peaked on central axis. We demonstrate using three-dimensional simulations that a 3 PW helical laser can generate a 50 pC low-divergence electron beam with a maximum energy of 1.5 GeV. The unique features of the beam are short acceleration distance (∼100 μm), compact transverse size, high areal density, and electron bunching (∼100 as bunch duration). Plasma mirrors have become the preferred method for electron injection to laser-based accelerators, but the optimal configuration is difficult to achieve. Here, an alternative injection method employing a low-density foam target and a helical laser pulse is investigated numerically. [ABSTRACT FROM AUTHOR]

Based on 10 years' of vertical total electron content (VTEC) data in solar cycle 24 from Ahmedabad (23.0°N, 72.6°E, dip angle 35.2°), a station under the crest region of equatorial ionization anomaly (EIA), it is shown that both the integrated residual and total post‐sunset enhancements in VTEC are conspicuous during Equinox and December solstice when solar flux level exceeds 110 solar flux unit (sfu) with the exception of the year 2012–2013. The post‐sunset enhancements are absent in June solstice at this local time even if the solar flux level exceeds 110 sfu. The integrated residual and total VTEC enhancements during post‐sunset hours are found to be linearly correlated with the solar flux level in Equinox and December solstice. It is noted that a parabolic fit works better for the integrated total VTEC enhancement during December solstice suggesting a possible saturation of VTEC enhancements at high solar flux levels. Based on these observations and Thermosphere Ionosphere Electrodynamics‐General circulation model (TIE‐GCM) outputs, it is argued that the pre‐reversal enhancement (PRE) in the equatorial F region zonal electric field works in tandem with latitudinal gradient in the F region plasma density to determine the degree of VTEC enhancement over the EIA crest region during post‐sunset hours. These results highlight the solar flux dependence of the post‐sunset enhancement of VTEC over the crest region and show that sudden stratospheric warming events in 2012–2013 suppressed these enhancements in December solstice even if solar flux levels exceeded 110 sfu. Plain Language Summary: The low latitude region is one of the most L‐band scintillation affected region in the whole globe. In addition, owing to the equatorial plasma fountain process, the low latitude F region ionosphere is also marked by steep latitudinal plasma density gradient during post‐sunset hours. It is shown that the evening plasma fountain driven by pre‐reversal enhancement of the zonal electric field controls the post‐sunset enhancement of the F region plasma density over the crest region of equatorial ionization anomaly (EIA) region with the assistance from the latitudinal plasma density gradient. This result is important to forecast the ionospheric condition over the EIA crest region during post‐sunset hours. Key Points: Post‐sunset enhancements in vertical total electron content (VTEC) over the equatorial ionization anomaly crest region are significant only above 110 solar flux unit level during December solstice and EquinoxPre‐reversal enhancement (PRE) of equatorial zonal electric field is shown to primarily drive the post‐sunset enhancements in VTECThermosphere Ionosphere Electrodynamics‐General circulation model simulations reveal the additional importance of latitudinal plasma density gradients over low latitudes [ABSTRACT FROM AUTHOR]

The X-ray spike structure for solar flares recorded by the CGRO/BATSE spectrometer is discussed, and the kinetics of accelerated electrons propagating in the plasma of a magnetic loop with a spike temporal structure is simulated. The time resolution of the BATSE full-Sun spectrometer varied from 0.016 to 2.048 s during the flare. The total energy range 20 keV–1 MeV was divided into 16 channels. The spike structure of hard X-ray radiation is considered for three powerful flares: M1.4 SOL1991-12-25, M7.8 SOL1991-12-28, and Х12 SOL1991-06-06. The X-ray time profiles recorded with a time resolution of 16 ms represent a sequence of numerous pulses (spikes) with a duration of about 0.1 s. The growth and decay times of the spike are approximately the same, and the profiles of many spikes are nearly triangular. No strict temporal correspondence of spikes of different energies was revealed with such a resolution. Modeling of the propagation of a sequence of accelerated electron beams in the plasma of flare loops confirms the preservation of a similar spike structure of the hard X-ray radiation generated in the footpoints of the loops. Moreover, the pulse structure is preserved independently of the pitch-angular distribution of electrons and the loop plasma parameters. In the coronal part of the loop (looptop), hard X-ray spikes are generated only in the case of increase in plasma density. The spike structure of accelerated electrons is discussed within the so-called bursty reconnection model. [ABSTRACT FROM AUTHOR]

The secondary electron asymmetry effect (SEAE) provides the opportunity to generate the dc self-bias voltage and asymmetric plasma response, where secondary electron emission coefficients at both electrodes are unequal. In this work, we use one-dimensional implicit particle-in-cell/Monte Carlo collision simulation to investigate the effects of the homogeneous and inhomogeneous magnetic field on the SEAE. The magnetic field is applied parallel to electrodes, and the discharge is operated in a geometrically and electrically symmetric capacitively coupled argon plasma. By comparing the simulation results of the effects of the homogeneous and inhomogeneous magnetic field on the SEAE, the homogeneous magnetic field can increase the dc self-bias voltage to a certain extent and has little effect on the plasma density distribution. The inhomogeneous magnetic field is more advantageous in generating the dc self-bias but at the cost of uneven plasma density distribution. In addition, by comparing the results of inhomogeneous magnetic fields with opposite gradients, we found that the value of the self-bias voltage can be changed by adjusting the magnetic field gradient. Aside from that the roles of two electrodes can be easily reversed by changing the slopes of the magnetic field gradient. The results suggest that such a configuration could be desired in the microelectronics industry, since the controllable self-bias voltage is needed for deposition and etching processes. [ABSTRACT FROM AUTHOR]

Flapping of the current sheet (CS) associated with the propagation of high‐velocity bulk flows allowed to observe the super thin current sheet (STCS) in the magnetotail plasma sheet (PS). During the interval of interest, 111 crossings of the CS neutral plane were detected. In 95 crossings, the STCSs with a current density J ≥ 20 nA/m2 were observed. A half‐thickness (LSTCS) of the STCSs was about a few electron gyroradii or less. In a number of the STCSs the parameter of adiabaticity (κe) was <1.0 for suprathermal electron population (>1 keV). Our analysis has shown that the electric current in such STCSs is carried by unmagnetized electrons (κe < 1), and the stress balance is supported by the off‐diagonal terms of their pressure tensor. In this sense, the underlying physics of the formation of STCSs at electron scales by unmagnetized electrons is similar to the mechanism of ion‐scale thin CS formation by the quasi‐adiabatic ions. The low‐energy population of magnetized electrons is also crucially important since it keeps the STCS stable and allows their observation as a quasi‐stationary structure. We compare the observed half‐thickness of the STCS with that predicted by a new kinetic theory (λ) considering the coupling between ion‐scale TCS and electron‐scale STCS. We found a reasonable agreement between both values: LSTCS ∼ (0.3–1)λ. Further improvement in the theory taking into account the dynamics of unmagnetized electrons may provide better agreement with observations. Key Points: Super thin current sheets (STCSs) with half‐thickness about few electron gyroradii or less are observed during high‐velocity bulk flowsIn the STCSs, the current is carried by unmagnetized electrons and stress balance is supported by off‐diagonal terms of their pressure tensorIn the STCSs, the higher energy electrons carry the current, while the low‐energy electrons support the stability of the STCS [ABSTRACT FROM AUTHOR]

Two-dimensional particle-in-cell simulations are used to explore laser-driven collisionless shock acceleration of ions in a multi-species plasma. Simple plasma slab simulations consisting of electrons, protons, and fully ionized carbon are used, varying the carbon ionization state, the relative fraction of ions, and the ratio of downstream to upstream plasma density. We find that two shocks can simultaneously propagate with different velocities defined by the dominant ion species reflected by each shock. The appearance of two shocks allows for ions to be accelerated twice, but can also cause trapping and heating of ions. We modify the current collisionless electrostatic shock theory where ions are treated as a single fluid to include a second ion fluid. This fluid model is unable to calculate the Mach number at which both ions will reflect, therefore we propose a kinetic model that may better model multi-species shocks. Scans are also performed in simulations with a laser pulse and realistic density profile that show reduced proton peak energies with the inclusion of carbon ions. Double shocks are only seen in simulations with steep density profiles, demonstrating the experimental importance of tailored density profiles. [ABSTRACT FROM AUTHOR]

Structure and transportation of electron vortices in near-critical density plasmas driven by ultrashort intense laser pulses have been studied by multi-dimensional particle-in-cell simulations. Dimensional features of electron vortices are revealed. In two-dimensional geometry, two electron vortices and a quasi-static magnetic dipole are closely coupled. In three-dimensional geometry, a moving electron vortex ring associated with a closed magnetic ring moves in near-critical density plasmas. Such structure can transport some energy to the region where the laser pulse cannot reach. It is found that the motion of plasma ions makes the vortex magnetic energy dissipate quickly. These studies provide possible connection of electron vortices in nature with laser plasma experiments. [ABSTRACT FROM AUTHOR]

The ion (Ti) and electron (Te) temperatures at the last closed flux surface (LCFS) during density scanning discharges and in the scrape-off layer (SOL) with a constant plasma density have been measured in the J-TEXT tokamak using a retarding field analyzer and four-tip Langmuir probe, respectively. Both the LCFS Ti and Te are found to reduce with increasing central line-averaged density (). The temperature ratio (τ = Ti/Te) is about 2 for a low (1.5 → 2.5 × 1019 m−3), decays in a moderate (2.5 → 3.5 × 1019 m−3) and tends to unity at a high (3.5 → 4.2 × 1019 m−3). This suggests that the ions and electrons at the LCFS (even in the SOL) are thermally decoupled at low or moderate and tend to full thermal coupling in the high-density plasmas. In addition, both Ti and Te decrease with the radius and Te falls more steeply than Ti due to faster parallel energy loss of electrons in the SOL. In contrast, τ rises with the radius, which indicates that the ions and electrons are much more thermally decoupled in the deeper SOL. [ABSTRACT FROM AUTHOR]

The optical thickness of the plasma is often insufficient to fully absorb the microwaves during heating at the second harmonic of the electron cyclotron frequency. An analysis of the experiments to the T-10 tokamak allows us to find the criteria for the full absorption, and to construct a canonical profile transport model for the full and partial absorptions of microwaves. The conditions to the equivalence of discharges in different tokamaks, and in a pair of tokamaks with the optimized W7-X stellarator are formulated. For equivalent discharges, calculations to the T-15MD tokamak under construction with the obtained model coincide with measurements of electron and ion temperatures in the W7-X over a wide range of plasma densities. The validated model is used to analyze future shots of T-15MD. [ABSTRACT FROM AUTHOR]

The dynamics of an electron avalanche starting from the flat cathode surface and from the tops of microtips of different heights on the cathode surface is compared. The dynamics of avalanches was simulated in a changing electric field of the forming cathode sheath of a self-sustained high-pressure volume discharge with preliminary ionization of the gaseous medium. The aim of this work was to study the possibility of the development of instability during the formation of the cathode sheath. It is shown that the start from the top of the microtip gives the avalanche an advantage both in the increase in the number of electrons and in the distance traveled in comparison with the avalanche that started from a flat surface. This advantage is due to the region of enhancement of the electric field produced by the microtip and increases with the height of the tip. Areas of increased plasma density formed by avalanches that start from microtips can create an initial inhomogeneity necessary for the development of instability during the formation of the cathode sheath. The possible mechanisms of the development of instability in the cathode sheath are analyzed. [ABSTRACT FROM AUTHOR]

The large differential drift motion between electrons and ions that is created by the E × B current can produce different instabilities, such as the electron cyclotron drift instability, perpendicular to the magnetic field, and the Modified Two-Stream Instability (MTSI), with a component along the magnetic field. In this paper, we derive and validate a stability condition for the apparition of the MTSI modes in 2D particle-in-cell simulations of E × B discharges in the radial-azimuthal plane of a Hall thruster. We verify that, by choosing properly the domain dimensions, it is possible to capture correctly the MTSI growth and its corresponding number of azimuthal periods. In particular, we show that an azimuthal length that is smaller than a certain threshold prevents the MTSI from growing. Moreover, we show that the MTSI growth does not depend on the plasma density, but is affected by the axial electric field (perpendicular to the simulation domain). Additionally, we show that during its linear growth in the early times of the simulations, the MTSI produces an enhanced heating of the electrons in the magnetic field direction as well as an increased cross field mobility. For longer times, in the nonlinear regime, the system evolves toward a more chaotic state with the presence of structures that mostly exhibit large azimuthal wavelengths. [ABSTRACT FROM AUTHOR]

In this paper, stopping power as a result of free and bound electrons in a fully or partially ionized plasma will be studied. The free electron stopping power will be analyzed using the dielectric formalism, whereas bound electron stopping power will be calculated through atomic approximations. The data used for the calculations came from experiments in which hydrogen and argon hot plasmas are shot with up to 10 MeV protons. Plasma ion densities are ranged between 1 0 19 and 1 0 22 cm−3 and electron temperatures from 10 to 300 eV. In this experimental setup, the data show a regime of reduced stopping power that will be reproduced early exactly by our theoretical model. [ABSTRACT FROM AUTHOR]

Computer simulations based on the particle in cell/Monte Carlo collision method are performed to study the plasma characteristics and especially the transition in electron heating mechanisms in a hybrid direct current (dc)/dual-frequency (DF) capacitively coupled CF4 discharge. When applying a superposed dc voltage, the plasma density first increases, then decreases, and finally increases again, which is in good agreement with experiments. This trend can be explained by the transition between the four main heating modes, i.e., DF coupling, dc and DF coupling, dc source dominant heating, and secondary electron dominant heating. [ABSTRACT FROM AUTHOR]