Sharp Plasma Boundary Research Articles

Electron Surfing Acceleration at Rippled Reconnection Fronts

The reconnection front (RF), one of the most efficient accelerators of particles in the terrestrial magnetosphere, is a sharp plasma boundary resulting from transient magnetic reconnection. It has been both theoretically predicted and observationally confirmed that electron-scale substructures can develop at the RFs. How such electron-scale structures modulate the electron energization and transport has not been fully explored. Based on high-resolution data from MMS spacecraft and particle tracing simulations, we investigate and compare the electron acceleration across two typical RFs with or without rippled electron-scale structures. Both observations and simulations reveal that high-energy electron flux behind the RF increases more dramatically if the electrons encounter a rippled RF surface, as compared to a smooth RF surface. The main acceleration mechanism is electron surfing acceleration, in which electrons are trapped by the ripples, due to the large local magnetic field gradient, and therefore undergo surfing motion along the motional electric field.

Trapping and Acceleration of Electron Bunches Generated by a Laser Pulse Moving through the Sharp Plasma Boundary

The formation and acceleration of electron bunches resulting from the self-injection of electrons into the wake wave from the laser pulse moving through a sharp plasma boundary are investigated in one-dimensional geometry. It is shown that electron trapping in the accelerating wakefield is governed by the electron energy and has a threshold character. The acceleration of the trapped bunch is numerically simulated.

Physical mechanism of electron bunch generation by an ultrarelativistic-intensity laser pulse passing through a sharp plasma boundary

The physical mechanism of the interaction process of a laser pulse of ultrarelativistic intensity with semi-bounded plasma having a sharp boundary is studied in one-dimensional geometry. It is shown that electron bunches are generated by the laser pulse due to the multiflow motion of plasma electrons with crossing trajectories. An analytical relation that allows an electron bunch charge to be estimated as a function of plasma characteristics and electron trajectory parameters is derived and confirmed by the results of numerical simulations.

Laser second-harmonic generation from an overdense plasma slab

AbstractA s-polarized short-pulse laser impinged obliquely on an overdense plasma slab is shown to produce very significant second harmonic in the direction of specular reflection and transmission. The laser induces a non-linear current on electrons, which is curl free. However, with sharp plasma boundary, it gives rise to electromagnetic radiation at the second harmonic. Our formalism includes multiple reflections of the incident and second-harmonic waves from both the front and rear surfaces. The present work includes finiteness of the slab. The normalized second-harmonic amplitude acquires a sharp peak at some specific angle of incidence for a particular set of parameters dependent on thickness of the slab and plasma density.

Generation of attosecond electron bunches upon laser pulse propagation through a sharp plasma boundary

The formation of an electron bunch produced due to electron self-injection into a wake wave that is generated by a relativistic-intensity laser pulse propagating through a sharp boundary of semi-bounded plasma is studied in one-dimensional geometry. Analytical expressions are obtained for estimating the bunch length and the energy spread of electrons in the bunch. Numerical simulation is performed which confirms the results of analytical consideration.

Preliminary evidences of AC-run-away in femtosecond laser-matter interaction

The role of collisional absorption is shown to be important at the beginning of the interaction between a laser pulse and matter, i.e., a plasma, while later on collective absorption phenomena take place. The scope of this paper is to present first results about the long predicted AC-run-away effect, defined as a sudden transition from a collisional absorption regime to a collective one, in ideal circumstances: for an ultrashort laser pulse, i.e., subpicosecond pulses, and with a sharp plasma boundary, i.e., without a laser prepulse. For this purpose, we use a kinetic model based on the ballistic theory. The model is of relevance to laser-matter interaction including laser nuclear fusion or laser particle acceleration.

A simple model of an asymmetric capacitive plasma with dual frequency

Recently, simulations have been carried out to investigate a dual frequency capacitive plasma in a system consisting of a pair of coaxial cylindrical electrodes. Considering the same asymmetric configuration, it is shown here that a sharp plasma–boundary model may be used to capture the essential dynamical features of the simulations.

Generation of short pulse radiation from magnetized wake in gas-jet plasma and laser interaction

New features of a high power tunable radiation from the magnetized plasma wakes are studied. This Letter covers some aspects of the problem, which was previously discussed in details in [Phys. Rev. E 68 (2003) 026409]. A gas-jet flow is used to generate the sharp boundary plasma. Wakefield is excited by a mode locked Ti:sapphire laser beam operating at 800 nm wavelength with the pulse width of 100 fs (FWHM) and maximum energy of 100 mJ per pulse with 10 Hz repetition rate. The neutral density of gas-jet flow is measured with a Mach–Zehnder interferometer. Strength of the applied external dc magnetic field normal to the direction of laser pulse propagation varies from 0 to 8 kG in the interaction region. Radiation is observed, in the forward direction due to the axial component of the magnetized wakefield and in the normal direction due to the radial component of the magnetized wakefield, both perpendicular to the direction of the applied magnetic field. The frequency of the emitted radiation with the pulse width of 200 ps (detection limit), measured by the method of time-of-flight, is in the millimeter wave range. Radiations are polarized perpendicularly to the laser pulse propagation direction and dc magnetic field lines as is expected from the theory. Electrodynamic properties of the radiation have been studied at different plasma densities and magnetic field strengths.

A one-dimensional model of plasma expansion

We consider a system consisting of a vacuum gap separated by two electrodes. A plasma is injected at the cathode and expands. From the sharp plasma boundary, electrons are accelerated towards the anode, forming an electron beam. In this paper, from a two-fluid isentropic Euler system for each species of particles coupled with the Poisson equation, we perform a formal asymptotic analysis, yielding a quasi-neutral model for the plasma region and a Child-Langmuir law for the beam region. These two models are connected through a transmission layer problem. Finally, a numerical validation of this asymptotic model against the original two-fluid model is given.

Transition Radiation of Photons and Neutrinos at a Plasma Boundary

It is shown that photons and neutrinos with nearly constant velocity can radiate a large spectrum of electromagnetic waves when they cross the boundary between a plasma and vacuum. This is a direct consequence of the existence of an equivalent electric charge of photons or neutrinos in a plasma. The general expression for the radiation field in the vacuum region far away from the boundary is established, and the particular case of a sharp plasma boundary is discussed.

Damping of Surface Alfvén Wave in a Slab Plasma

The MHD wave is studied when two steep density gradient regions exist at surfaces of slab plasma. In such a case, it is shown that the surface Alfven wave has two branches with nearly the same damping rates, since the steep density gradients are located closely each other. However, for the sharp boundary plasma, the surface Alfven wave does not damp. As the density profile is relaxed, the damping rates become larger, pass via extremum, and again they become small when the scale length of the density gradients extremely increases. These damping rates seem consistent with behavior of magnetic fluctuations observed in the Heliotron-E pellet injection experiment.

Interaction of an ultrashort, relativistically strong laser pulse with an overdense plasma

The results of an analytical description and of a particle-in-cell simulation of the interaction of an ultrashort, relativistically intense laser pulse, obliquely incident on a nonuniform overdense plasma, are presented and several novel features are identified. The absorption and reflection of the ultraintense electromagnetic laser radiation from a sharp-boundary plasma, high harmonic generation, and the transformation into low-frequency radiation are discussed. In the case of weak plasma nonuniformity the excitation of nonlinear Langmuir oscillations in the plasma resonance region and the resulting electron acceleration are investigated. The vacuum heating of the electrons and the self-intersection of the electron trajectories are also studied. In the case of a sharp-boundary plasma, part of the energy of the laser pulse is found to be converted into a localized, relativistically strong, nonlinear electromagnetic pulse propagating into the plasma. The expansion of the hot electron cloud into the vacuum region and the action of the ponderomotive force of the laser pulse in the localized longitudinal electric field of the Langmuir oscillations lead to ion acceleration. The energy increase of a minority population of multicharged ions is found to be much greater than that of the ambient ions.

There is no “cometopause” at comet Halley

Immediately after the flybys at comet Halley by a fleet of spacecraft in 1986, Gringauz et al. (1986a) reported the detection by the Vega‐ 2 spacecraft of a chemical and sharp plasma boundary, which they named the “cometopause,” at a distance of ∼1.6 × 105 km from the nucleus. Gringauz and Verigin (1991) presented the “cometopause” as a permanent feature of the solar wind ‐ Halley type comet interaction at ∼1 UA from the Sun. This permanent boundary presumably separates an upstream region dominated by the solar wind from the downstream region where heavy cometary ions dominate. We present here the analysis of the results of the Giotto positive ion cluster composition analyzer ‐ Rème plasma analyzer (PICCA‐RPA2) ion mass spectrometer and electron electrostatic analyzer ‐ Rème plasma analyzer (EESA‐RPA1) electron spectrometer data, which clearly show that there is no such boundary at comet Halley.

Alpha loss-cone Alfvén-wave instabilities in a sharp-boundary model of a tandem-mirror central cell

The Alfvén waves of a cylindrical, sharp-boundary plasma, which models the plasma in a tandem-mirror central cell, have been analyzed for instabilities driven by the loss cone of the alpha-particle distribution function. Improved methods for treating the waves, the distribution function, and the instability have been developed and used. The many normal modes of the cylindrical plasma can be found reliably. The diffusion-front method for calculation of the distribution function allows accurate and rapid computation of instability growth rates and marginal-stability boundaries. Present tandem-mirror reactor designs have densities and temperatures that lead to instability. Consequences of the instability should be assessed by studying the quasilinear evolution of the alpha particles.

KINK INSTABILITIES OF A SHARP BOUNDARY PLASMA WITH SMALL ASPECT RATIO

Kink instabilities of a sharp boundary plasma with small aspect ratio are studied. The results of numerical calculations show that a conductive shell has a stabilizing effect to some extent and critical beta of toroidal mode number n=1 is the minimum whether there is a conductive shell or not, i.e. the perturbation with n=1 is the most dangerous one. With decrease of aspect ratio, the critical beta increases initially, when a/R = 0.7, it appears as a maximum, when a/R≥0.6, the critical beta values of high beta limit and low beta limit are nearly equal.

Equilibrium and Adiabatic Compression of a Free Boundary Doublet

The equilibrium shape and location of a plasma doublet is studied by neglecting the toroidal curvature effects. Inflection points are included on the sharp plasma boundary supported in equilibrium by a vacuum magnetic field. Using conformal mapping, a one-parameter family of explicit solutions are obtained. The plasma shape is uniquely determined by the dimensions of the surrounding conducting shell and outer wall current. During adiabatic compression the method of conformal mapping becomes singular at some stage. Effects of various parameters are discussed. Related free boundary problems with infinite domain are included.

Confinement of a high-beta plasma column

Hydromagnetic aspects of confinement of a high-beta linear plasma column, including the reverse field configurations, are investigated. The previous theoretically predicted diverging confinement time of a sharp-boundary plasma column as β→1 is removed by properly taking into account the magnetic tension effect near the column throats. The obtained end loss rate is tested through simulation: the present simulation results in slab geometry are in good agreement with theory and are also close to other simulation and experimental values. The confinement time τ of a plasma with thickness a and length L at β=1 is found to be τ= 3/4 (3/2π)1/2(L/2cs)(L/a)1/2. When the magnetic field inside the column is reversed, the plasma end loss much improves: the end loss induces the reconnection of the field lines near the throats, which produces closed field lines (islands). Those islands are found to be unstable against the tilting instability. The tilting-induced reconnection of the island field line and the mirror field line is a very fast process even for negligibly resistive plasmas and this helps to rapidly spill out the plasma confined in the islands. The significance of these processes for the spheromaks and reversed field (theta) pinches is discussed.

Non-linear analysis of the stability of the Screw Pinch

Non-linear effects on the stability of the Screw Pinch in cylindrical system surrounded by a uniform pitch magnetic field in an incompressible pressureless plasma have been studied. A linear dispersion relation governing the stability of interface has been obtained with the help of normal mode analysis. Perturbation technique has been used to obtain the non-linear Schrodinger equation for the time variation of amplitudeA(t) which evolves according to\(\partial ^2 A/\partial t^2 = \lambda ^2 A + \alpha A^3 \). This can lead to non-linear stabilization, explosive instability or eventual decay, depending on the sign of α. The whole calculations have been made form=1, kink modes in a sharp boundary plasma pinch. Non-linear analysis for θ-pinch (μ→0) andz-pinch (μ→∞) have also been discussed in detail. It has been found that non-linear theory have a destabilizing influence on both θ- andz-pinches.

Magnetohydrodynamic Stability of an Axisymmetric Toroidal Sharp-Boundary Plasma with Flattened Cross Section

A stability of an axisymmetric toroidal sharp-boundary plasma whose cross section is horizontally elongated is presented in the flat-ring cyclide coordinate system. The solution of Laplace's equation referred to as the Wangerin function is used and the energy principle is reduced to the quadratic form without using the tokamak ordering. The marginal stability of the plasma is numerically studied. Numerical results of the kink mode show that an increase of the elongation ratio for a fixed aspect ratio leads to decreases of both the maximum plasma beta and poloidal beta, and an increase of the minimum safety factor. It is also shown that the stable region for a plasma with a circular cross section is wider than with a flattened cross section.

Numerical studies of new stellarator concepts

A three-dimensional computer code has been developed to study the magnetohydrodynamic equilibrium and stability of a diffuse or sharp boundary plasma in toroidal geometry. It is shown how equilibria with net toroidal current identically zero can be determined and how growth rates of instabilities can be calculated. Applications are made to an l = 2, 3 stellarator configuration that offers the possibility of achieving a critical value as high as 10% for the plasma parameter β.