Plasma Density Gradient Research Articles

Proton acceleration in a laser-induced relativistic electron vortex

We show that when a solid plasma foil with a density gradient on the front surface is irradiated by an intense laser pulse at a grazing angle,${\sim}80^{\circ }$, a relativistic electron vortex is excited in the near-critical-density layer after the laser pulse depletion. The vortex structure and dynamics are studied using particle-in-cell simulations. Due to the asymmetry introduced by non-uniform background density, the vortex drifts at a constant velocity, typically$0.2{-}0.3$times the speed of light. The strong magnetic field inside the vortex leads to significant charge separation; in the corresponding electric field initially stationary protons can be captured and accelerated to twice the velocity of the vortex (100–200 MeV). A representative scenario – with laser intensity of$10^{21}~\text{W}~\text{cm}^{-2}$– is discussed: two-dimensional simulations suggest that a quasi-monoenergetic proton beam can be obtained with a mean energy 140 MeV and an energy spread of${\sim}10\,\%$. We derive an analytical estimate for the vortex velocity in terms of laser and plasma parameters, demonstrating that the maximum proton energy can be controlled by the incidence angle of the laser and the plasma density gradient.

The possibility of using all-sky meteor radar to observe ionospheric E-region field-aligned irregularities

All-sky meteor radars are primarily used for meteor observations. This paper reports the first observations of ionospheric E-region field-aligned irregularities (FAIs) from a conventional all-sky meteor radar located at Wuhan (31°N, 114°E) for the period of March–June 2018. E-region FAI echoes evident in range-time intensity (RTI) maps show quasiperiodic striations with positive and negative slopes, which are consistent with multiple FAI structures moving across the wide beam of the meteor radar. A statistical analysis shows that out of a total of 111 d, there are 73 d with E-region FAI echoes detected by the meteor radar. The FAI events correspond well with the presence of sporadic-E layers which provide the necessary plasma density gradient for the development of gradient drift instability producing FAIs. The results demonstrate the capability of conventional meteor radars to make simultaneous routine observations of meteors and ionospheric E-region FAIs through incorporating RTI and spectral analysis into the online realtime data processing. Meteor radar observations could potentially address the limitations of ionospheric radars, which cannot provide simultaneous measurements of neutral winds and irregularity structures, and thereby contribute to better understanding of the dynamical processes producing E-region irregularities.

Micro instabilities and rotating spokes in the near-anode region of partially magnetized plasmas

Electron and ion transport in the near-anode region of a partially magnetized plasma under conditions typical of Hall thrusters or magnetron discharges is studied with fully kinetic, Particle-In-Cell Monte Carlo Collision (PIC-MCC) simulations assuming a uniform magnetic field and no ionization. We derive a simple relation that defines the magnetic field at the transition point between negative and positive sheaths. For magnetic fields around or above this transition point, PIC-MCC simulations show the development of short wavelength azimuthal instabilities that cascade to longer wavelengths (“rotating spokes”) as the magnetic field is increased. Both short-wavelength and large-wavelength fluctuations can coexist under some conditions. A detailed study of the fluid dispersion relation is used to analyze the PIC-MCC results. Small coherent structures can be associated with the destabilization of ion sound waves by density gradient and collisions. Longer wavelengths or rotating spokes are characteristic of the collisionless Simon-Hoh instability. The small structures are dominant for larger plasma density gradients, while the larger structures correspond to smaller density gradients and larger magnetic fields. Anomalous transport associated with these instabilities can be significant, with effective collision frequencies larger than 2×107 s−1 in xenon for magnetic fields above the transition point.

On the Balance Between Plasma and Magnetic Pressure Across Equatorial Plasma Depletions

AbstractIn magnetized plasmas such as the ionosphere, electric currents develop in regions of strong density gradients to balance the resulting plasma pressure gradients. These currents, usually known as diamagnetic currents decrease the magnetic pressure where the plasma pressure increases, and vice versa. In the low‐latitude ionosphere, equatorial plasma depletions (EPDs) are well known for their steep plasma density gradients and adverse effect on radio wave propagation. In this paper, we use continuous measurements of the magnetic field and electron density from the European Space Agency's Swarm constellation mission to assess the balance between plasma and magnetic pressure across large‐scale EPDs. The analysis is based on the magnetic fluctuations related to diamagnetic currents flowing at the edges of EPDs. This study shows that most of the EPDs detected by Swarm present a decrease of the plasma pressure relative to the ambient plasma. However, EPDs with high plasma pressure are also identified mainly in the vicinity of the South Atlantic magnetic anomaly. From the electron density measurements, we deduce that such an increase in plasma pressure within EPDs might be possible by temperatures inside the EPD as high as twice the temperature of the ambient plasma. Due to the distinct location of the high‐pressure EPDs, we suggest that a possible heating mechanism might be due to precipitation of particle from the radiation belts. This finding corresponds to the first observational evidence of plasma pressure enhancements in regions of depleted plasma density in the ionosphere.

Generation of a strong reverse shock wave in the interaction of a high-contrast high-intensity femtosecond laser pulse with a silicon target

We present ultrafast pump-probe reflectivity and Doppler spectrometry of a silicon target at relativistic laser intensity. We observe an unexpected rise in reflectivity to a peak approximately ∼9 ps after the main pulse interaction with the target. This occurs after the reflectivity has fallen off from the initially high “plasma-mirror” phase. Simultaneously measured time-dependent Doppler shift data show an increase in the blue shift at the same time. Numerical simulations show that the aforementioned trends in the experimental measurements correspond to a strong shock wave propagating back toward the laser. The relativistic laser-plasma interaction indirectly heats the cool-dense (ne≥1023 cm−3 and Te∼10 eV) target material adjacent to the corona, by hot electron induced return current heating, raising its temperature to around 150 eV and causing it to explode violently. The increase in reflectivity is caused by the transient steepening of the plasma density gradient at the probe critical surface due to this explosive behavior.

Sub-femtosecond electron bunches in laser wakefield acceleration via injection suppression with a magnetic field

It is shown that electron injection into a laser-driven plasma bubble can be manipulated by applying an external magnetic field in the presence of a plasma density gradient. The down-ramp of the density-tailored plasma locally reduces the plasma wave phase velocity, which triggers injection. The longitudinal magnetic field dynamically induces an expanding hole in the electron density distribution at the rear of the wake bubble, which reduces the peak electron velocity in its vicinity. Electron injection is suppressed when the electron velocity drops below the phase velocity, which depends on the size of the density hole. This enables the start and end of electron injection to be independently controlled, which allows generation of sub-femtosecond electron bunches with peak currents of a few kilo-Ampere, for an applied magnetic field of ∼10 Tesla.

Generation of second harmonics by a self-focused Hermite-Gaussian laser beam in collisionless plasma

In this work, the generation of second harmonics of a Hermite–Gaussian laser beam in collisionless plasma has been presented. On incidence of the Hermite–Gaussian laser beam in plasma, the charge carriers shift from the high field region to the low field region on account of the ponderomotive force which results in the generation of a transverse density gradient in the background plasma which in turn generates plasma waves at incident beam frequency ω0. Interaction of this plasma wave with the pump beam generates the second harmonics of the incident laser beam with frequency 2ω0. The moment theory approach has been used to derive the coupled differential equations for the beam widths of the laser beam in the transverse x- and y-directions which are further solved numerically. The effect of different modes and initial beam widths of the Hermite–Gaussian beam in the x- and y-directions has been investigated for the self-focusing and second harmonic yield (SHY) of the laser beam in plasma. Also, the effect of increasing plasma density is visualized on the self-focusing and SHY of the beam. It has been observed that the SHY significantly depends on different modes and initial widths of the Hermite–Gaussian laser beam as well as on plasma density.

Generation of a controllable electron density gradient using a single plasma source

In this work, an experimental design for generation of the controllable electron density gradient is achieved through a large area oxide coated cathode and two stainless meshes. The large area oxide coated cathode is adopted to produce a large area, uniform and high density plasma, and these two metal meshes are specially designed to control the density and potential of the plasma. A boundary layer with independent electron density gradient and uniform plasma potential is subsequently generated. The controllable electron density gradient can be achieved, and the accompanied electric field can be compensated simultaneously using this design, which could be potentially extended to study the excitation of the plasma instabilities when the strong plasma density gradient exists.

ДРЕЙФОВАЯ АЛЬФВЕНОВСКАЯ НЕУСТОЙЧИВОСТЬ В ДВУМЕРНОЙ КОНФИГУРАЦИИ МАГНИТНОГО ХВОСТА - КОЛЕБЛЮЩИЕСЯ ЭЛЕКТРОНЫ В ДОПОЛНЕНИИ

To explain the possible destabilization of a 2D magnetic equilibrium such as the Near-Earth magnetotail, we developed a kinetic model describing the resonant interaction of electromagnetic fluctuations and bouncing electrons trapped in the magnetosphere, characterized by a high plasma density gradient. A small-β approximation is used in agreement with a small field line curvature. It has been found that for a quasi-dipole configuration, unstable electromagnetic modes may develop in the current sheet in westward direction with a growth rate of the order of a few tenth of seconds provided that the typical scale of density gradient slope responsible for the diamagnetic drift effects is over one Earth radius or less. This instability growth rate is large enough to destabilise the current sheet on time scales often observed during substorm onset.

Hepatitis E Virus (HEV) Open Reading Frame 2 Antigen Kinetics in Human-Liver Chimeric Mice and Its Impact on HEV Diagnosis.

Hepatitis E virus infection (HEV) is an emerging problem in developed countries. Diagnosis of HEV infection is based on the detection of HEV-specific antibodies, viral RNA, and/or antigen (Ag). Humanized mice were previously reported as a model for the study of HEV infection, but published data were focused on the quantification of viral RNA. However, the kinetics of HEV Ag expression during infection remains poorly understood. Plasma specimens and suspensions of fecal specimens from HEV-infected and ribavirin-treated humanized mice were analyzed using HEV antigen-specific enzyme-linked immunosorbent assay, reverse transcription-quantitative polymerase chain reaction analysis, density gradient analysis, and Western blotting. Open reading frame 2 (ORF2) Ag was detected in both plasma and stool from HEV-infected mice, and levels increased over time. Contrary to HEV RNA, ORF2 Ag levels were higher in mouse plasma than in stool. Interestingly, ORF2 was detected in plasma from mice that tested negative for HEV RNA in plasma but positive for HEV RNA in stool and was detected after viral clearance in mice that were treated with ribavirin. Plasma density gradient analysis revealed the presence of the noninfectious glycosylated form of ORF2. ORF2 Ag can be used as a marker of active HEV infection and for assessment of the effect of antiviral therapy, especially when fecal samples are not available or molecular diagnostic tests are not accessible.

Few-cycle laser wakefield acceleration on solid targets with controlled plasma scale length

We measure the emission of energetic electrons from the interaction between relativistic-intensity ultrashort laser pulses and a solid density plasma with a tunable density gradient scale length. We detect an electron beam that only appears with few-cycle pulses (<10 fs) and large plasma scale lengths (L > λ0). Numerical simulations, in agreement with the experiments, reveal that these electrons are accelerated by a laser wakefield. Plasma waves are indeed resonantly excited by the few-cycle laser pulses in the near-critical density region of the plasma. Electrons are then injected by ionization into the plasma waves and accelerated to relativistic energies. In this laser wakefield acceleration regime, the plasma waves are rotated by the plasma density gradient, which results in the electrons not being emitted in the same direction as the driving laser pulse.

Generation of second harmonics of intense Hermite–Gaussian laser beam in relativistic plasma

AbstractIn this paper, the scheme of generation of second harmonics of incident electromagnetic wave having a Hermite–Gaussian intensity profile in an under dense relativistic plasma has been presented. The relativistic mass variation of electrons by the intense electric field of incident beam generates the density gradients in background plasma which further excites the electron plasma wave (EPW) at resonant frequency and coupling of the EPW with the incident beam results in the generation of second harmonics of incident beam. Propagation dynamics of the Hermite–Gaussian laser beam in plasma has been studied by the formulation of differential equation for the spot size of the laser beam with the help of method of moments. Numerical simulations have been carried out to solve the differential equation for the dimensionless beam width parameters. Solution of the nonlinear wave equation for the electric field vector of second harmonics of incident beam gives the expression for second-harmonic yield. It has been observed that second-harmonic yield is affected by the different modes of Hermite–Gaussian laser beam in relativistic plasma.

Terahertz radiation generated by shell electrons in the bubble regime via the interaction between an intense laser and underdense plasma

Backward terahertz radiation can be produced by a high-intensity laser normally incident upon an underdense plasma. It is found that terahertz radiation is generated by electrons refluxing along the bubble shell. These shell electrons have similar dynamic trajectories and emit similar backward radiations to vacuum. This scheme has been proved through electron dynamic calculations and by using an ionic sphere model. In addition, the bubble shape is found to influence the radiation frequency, and this scheme can be implemented in both uniform and up-ramp density gradient plasma targets. The terahertz radiation may be used for diagnosing the electron bubble shape in the interaction between an intense laser and plasma. All the results are presented via 2.5 dimensional particle-in-cell simulations.

Gradient-drift instability applied to Hall thrusters

The stability of gradient-drift waves in a Hall-type plasma thruster is investigated within the framework of two-fluid ideal magnetohydrodynamics. The analysis is based on the dispersion relation, which includes the effects of equilibrium electron current, finite ion flow velocity, electron inertia, electron temperature, magnetic field and plasma density gradients, and also the Debye length effects. The features of unstable modes are calculated along the thruster channel. Three spatially separated areas of instability are revealed: (i) the near-anode region with long-wavelength azimuthal oscillations, (ii) the main part of the acceleration channel with short-wavelength axial modes destabilized by macroscopic ion flow, and (iii) the plume region characterized by short-wavelength oblique waves.

Ionospheric scintillation suppression based on chemical release

There occur frequently the ionospheric scintillation events at low and middle latitudes, which seriously affect the radio transmission process of satellite link, resulting in the decline of satellite communication and navigation signal quality and even interrupt. During the gestation period before the ionospheric scintillation, the growth rate of plasma instability can be reduced and thus suppress the scintillation events by releasing the electron density-enhancing chemicals in the ionosphere plasma bubble, filling with plasma bubble, changing the plasma environmental characteristics, and regulating the ionospheric dynamics process. The theory and method of suppressing the ionospheric scintillation based on chemical release are tnvestigated. According to the change of the plasma environment caused by the chemical release, and the quantitatively calculating of the contribution of control factors to the growth rate of instability, an ionospheric scintillation suppression model is built, which is based on chemical release into ionosphere. The process of plasma bubble filling out is simulated and the results of the simulation show that the plasma cloud is completely filled with plasma bubbles after 1200 seconds, which reduces the plasma density gradient and suppresses the growth of plasma instability. The growth of plasma instability decreases from 0.2 before releasing to about 0.0004 after releasing, and no new instability is excited within 20 minutes after the plasma bubble has been filled up. Guangdong, South China Sea and other regions in China are at the peak of equatorial anomalies, and the occurrence rate and severity of scintillation are more significant than those in the equatorial and Polar Regions, thus these regions become the regions where there occur most frequently the scintillation and the most serious influence globally. The research work of this paper will lay a solid theoretical foundation for the technology of suppressing the satellite signal ionospheric scintillation in middle and low latitude area of China.

Large‐Scale Ducting of Pc1 Pulsations Observed by Swarm Satellites and Multiple Ground Networks

AbstractLow Earth orbit satellites frequently encounter Pc1 pulsations, but most have been observed with limited latitudinal extent or short lifetime. In this study we analyze two large‐scale Pc1 pulsations (both latitudinally wide and long‐lasting) generated by ionospheric ducting effect using Swarm and ground magnetometers on 25 June and 3 September 2015. Swarm observed the 25 June pulsations on both dayside and nightside during the storm time substorm (a strong geomagnetic storm on 23 June with Dst = − 204 nT). We found the Pc1 pulsations were pervasive in both magnetic local time sectors of dayside and nightside for at least 2 hr. Another large Pc1 pulsation on 3 September was observed during a nonstorm substorm period. We conclude that (1) ionospheric ducting can transmit Pc1 waves to a wide range of L shells, (2) geomagnetic storm is not a prerequisite for such large‐scale ducting, and (3) wave intensity can abruptly decrease across sharp gradients in the ionospheric plasma density.

Intensified proton and carbon ion flux from femtosecond laser driven plasma source

Ion acceleration from aluminium foils irradiated with a 30 fs laser pulse of ∼1020 W/cm2 intensity at an incidence angle of 45° was investigated. Laser intensity contrast enhancement by a factor of 100 resulted in a nearly 7 and 30 times increase in proton and carbon ion flux, respectively, while their maximum energy remains almost unchanged. More than 1013 protons and 1014 carbon C4+ ions per MeV bandwidth per steradian solid angle were measured. Simulations, being in a good agreement with the experimental findings, have revealed that the difference in proton emission between the low and high contrast cases is a narrower angular distribution of protons at high laser pulse contrast. In the low contrast scenario, the plasma density gradient increases the hot electron divergence, leading to the reduction of particle flux in a fixed solid angle. The analytical model verifies the concept of the theoretical limit of particle flux. These results open up the possibility for further optimization of the laser driven bright source of energetic particles.

Experimental observation of drift wave turbulence in an inhomogeneous six-pole cusp magnetic field of MPD

This paper presents a detailed study on the controlled experimental observation of drift wave instabilities in an inhomogeneous six pole cusp magnetic field generated by an in-house developed multi-pole line cusp magnetic field device [Patel et al., Rev. Sci. Instrum. 89, 043510 (2018)]. The device is composed of six axially symmetric cusps and non-cusp (in between two consecutive magnets) regions. The observed instability has been investigated in one of these non-cusp regions by controlling the radial plasma density gradient with changing pole magnetic field which is a unique feature of this device. It has been observed that the frequency of the instability changes explicitly with the density gradient. Moreover, the scale lengths of plasma parameters, frequency spectrum, cross-correlation function, and fluctuation level of plasma densities have been measured in order to identify the instability. The cross field drift velocity due to fluctuation in plasma parameters has been measured from the wave number-frequency S (kz, ω) spectrum and verified with the theoretical values obtained from density scale length formula. Further, from the S (kz, ω) spectrum, it has been found that the drift velocity alternates the sign in the consecutive non-cusp regions.

Enhancement of betatron x rays through asymmetric laser wakefield generated in transverse density gradients

Laser wakefield acceleration of electrons usually offers an axisymmetry around the laser propagation axis. Thus, the accelerating electrons that are focused on axis often execute small transverse oscillations. In this Article, we propose a simple scheme to break this symmetry, which enhances the transverse wiggling of electrons and boosts the betatron radiation emission. Through 3D particle-in-cell simulations, we show that sending the laser with a small angle of incidence on a transverse plasma density gradient generates an asymmetric wakefield. It first provokes injection and then increases the wiggling of the electrons through the transverse shifting of the wakefield axis which occurs when the laser pulse leaves the gradient. Consequently, we show that the radiated energy per unit of charge can increase by a factor $>20$ when using this scheme, and that the critical energy of the radiation quintuples compared with a reference case without the transverse density gradient.

Analysis of the high-energy electron population in surface-wave plasma columns in presence of collisionless resonant absorption

In surface-wave plasmas, the energy can be transferred to the plasma electrons through both ohmic (collisional) and collisionless heating mechanisms. At very low pressure, when the electron–neutral collision frequency is much lower than the wave frequency (collisionless regime), a resonance is excited close to the tube walls where the electron plasma frequency in the radially-inhomogeneous plasma column reaches the wave frequency. In such conditions, the sharp rise of the component of the surface-wave electric field perpendicular to the tube axis can induce transit-time heating. At the resonant point, the long-wavelength electromagnetic surface wave can also be converted into short-wavelength electrostatic Langmuir waves that propagate down the density gradient. In this work, spatially-resolved trace-rare-gases optical emission spectroscopy combined with collisional-radiative modeling is used to analyze the electron energy distribution function (EEDF) and wave–particle interactions in low-pressure argon plasma columns sustained by an electromagnetic surface wave at 600 MHz (over-dense plasma). The EEDF is found to depart from a Maxwellian with the presence of a high-energy tail. The relative population of high-energy electrons increases with the axial distance towards the end of the plasma column where the electron density decreases and the resonance point becomes closer to the discharge axis. Over the range of experimental conditions examined, the high-energy tail increases with the characteristic length of the plasma density gradient at the resonance point; a feature that can be linked to collisionless electron heating by Landau damping of Langmuir waves.