R. Bingham

Electrostatic solitary structures in non‐thermal plasmas

Solitary electrostatic structures involving density depletions have been observed in the upper ionosphere by the Freja satellite [Dovner et al., 1994]. If these are interpreted as ion sound solitons, the difficulty arises that the standard Korteweg-de Vries description predicts structures with enhanced rather than depleted density. Here we show that the presence of non-thermal electrons may change the nature of ion sound solitary structures and allow the existence of structures very like those observed.

Plasma based charged-particle accelerators

Studies of charged-particle acceleration processes remain one of the most important areas of research in laboratory, space and astrophysical plasmas. In this paper, we present the underlying physics and the present status of high gradient and high energy plasma accelerators. We will focus on the acceleration of charged particles to relativistic energies by plasma waves that are created by intense laser and particle beams. The generation of relativistic plasma waves by intense lasers or electron beams in plasmas is important in the quest for producing ultra-high acceleration gradients for accelerators. With the development of compact short pulse high brightness lasers and electron positron beams, new areas of studies for laser/particle beam-matter interactions is opening up. A number of methods are being pursued vigorously to achieve ultra-high acceleration gradients. These include the plasma beat wave accelerator mechanism, which uses conventional long pulse (~100 ps) modest intensity lasers (I ~ 1014–1016 W cm−2), the laser wakefield accelerator (LWFA), which uses the new breed of compact high brightness lasers ( 1018 W cm−2, the self-modulated LWFA concept, which combines elements of stimulated Raman forward scattering, and electron acceleration by nonlinear plasma waves excited by relativistic electron and positron bunches. In the ultra-high intensity regime, laser/particle beam–plasma interactions are highly nonlinear and relativistic, leading to new phenomena such as the plasma wakefield excitation for particle acceleration, relativistic self-focusing and guiding of laser beams, high-harmonic generation, acceleration of electrons, positrons, protons and photons. Fields greater than 1 GV cm−1 have been generated with particles being accelerated to 200 MeV over a distance of millimetre. Plasma wakefields driven by positron beams at the Stanford Linear Accelerator Center facility have accelerated the tail of the positron beam. In the near future, laser plasma accelerators will be producing GeV particles.

On the role of dust in the summer mesopause

Abstract We propose that dust formed at the cool summer mesopause may have optical properties very different from that measured for bulk material of ice. The smallness of the dust and possible surface impurities may lead to high photoelectric yields and low workfunctions. For such reasons the dust in the summer mesopause may, at least occasionally, be charged to substantial positive surface potentials while pure ice, with its high photoelectric workfunction, would be charged to low and negative potentials by collisions with plasma particles. The presence of ‘dressed’ dust particles, with surface potentials of some volts, can lead to enhanced radar backscatter. We also suggest that the apparent reductions in electron density (‘bite-out’), which have been observed in the radar backscatter region, can be caused by the inability of an electrostatic probe to deflect the massive dust particles. The dust density which is required by our model to explain radar backscatter and electron bite-outs is of the order of 10 cm −3 for dust of radius above 5 × 10 −6 cm.

Stimulated Raman forward scattering and the relativistic modulational instability of light waves in rarefied plasma

The modulational instability of copropagating light waves was studied recently by McKinstrie and Bingham [Phys. Fluids B 1, 230 (1989)], who applied their general theory to the study of the relativistic modulational instability (RMI) of light waves in the beat‐wave accelerator. However, in rarefied plasma, the RMI merges with stimulated Raman forward scattering. The longitudinal RMI is suppressed over most of its expected range and the study of secondary instabilities in the beat‐wave accelerator must be amended accordingly. A preliminary analysis indicates that stimulated Raman backward scattering is likely to be important in current beat‐wave experiments, while near‐resonant stimulated Raman forward scattering and near‐forward stimulated Raman scattering could be important in proposed beat‐wave experiments.

Wave collapse at the lower‐hybrid resonance

The modulational instability and collapse of waves in the vicinity of the lower‐hybrid resonance including both magnetosonic and lower‐hybrid waves are investigated by analytical and numerical methods. The mechanism leading to the modulational instability is the nonlinear coupling of lower‐hybrid waves with the much lower‐frequency quasineutral density perturbations via the ponderomotive force. The result is a filamentation of the high‐frequency field producing elongated, cigar‐shaped nonlinear wave packets aligned along the magnetic field with the plasma expelled outside (cavities). The analytical self‐similar solutions describing cavity collapse are obtained and compared with the results of numerical simulation for both two‐ and three‐dimensional cavity geometries. It is shown that in three‐dimensional solutions the transverse, with respect to the magnetic field, contraction remains prevailing. The possibility of ion acceleration as the result of the lower‐hybrid collapse is discussed and detailed comparison is made with the observations of the phenomena in the auroral ionosphere.

Physics of Mass Loaded Plasmas

In space plasmas the phenomenon of mass loading is common. Comets are one of the most evident objects where mass loading controls to a large extent the structure and dynamics of its plasma environment. New charged material is implanted to the fast streaming solar wind by planets, moons, other solar system objects, and even by the interstellar neutral gas flowing through our solar system. In this review we summarize both the current observations and the relevant theoretical approaches. First we survey the MHD methods, starting with a discussion how mass loading affects subsonic and supersonic gasdynamics flows, continuing this with single and multi-fluid MHD approaches to describe the flow when mass, momentum and energy is added, and we finish this section by the description of mass loaded shocks. Next we consider the kinetic approach to the same problem, discussing wave excitations, pitch angle and energy scattering in linear and quasi-linear approximations. The different descriptions differ in assumptions and conclusions; we point out the differences, but it is beyond the scope of the paper to resolve all the conflicts. Applications of these techniques to comets, planets, artificial ion releases, and to the interplanetary neutrals are reviewed in the last section, where observations are also compared with models, including hybrid simulations as well. We conclude the paper with a summary of the most important open, yet unsolved questions.

Vortex streets driven by sheared flow and applications to black aurora

Black aurora often exhibits the signature of vortex structures. In this paper, it is proposed that pseudo-three-dimensional drift-like electrostatic modes can play a very important role in those phenomena. Employing a two-fluid model, we derive a set of nonlinear equations governing the dynamics of long wavelength inertial convective cells and coupled drift-acoustic waves in the presence of geomagnetic field-aligned plasma flows. It is shown that free energy stored in the latter can cause purely growing and oscillatory instabilities in the auroral ionosphere. The threshold conditions as well as expressions for the linear growth rates are presented. Furthermore, it is found that possible stationary solutions of the nonlinear equations can be represented in the form of vortex structures. It is, therefore, likely that the nonlinear low-frequency electrostatic modes, as described here, may possibly account for the coherent vortex structures within black aurora, as they are often observed during the late recovery phase of a substorm.

Generation of scaled protogalactic seed magnetic fields in laser-produced shock waves

The standard model for the origin of galactic magnetic fields is through the amplification of seed fields via dynamo or turbulent processes to the level consistent with present observations. Although other mechanisms may also operate, currents from misaligned pressure and temperature gradients (the Biermann battery process) inevitably accompany the formation of galaxies in the absence of a primordial field. Driven by geometrical asymmetries in shocks associated with the collapse of protogalactic structures, the Biermann battery is believed to generate tiny seed fields to a level of about 10−21 gauss (refs 7, 8). With the advent of high-power laser systems in the past two decades, a new area of research has opened in which, using simple scaling relations, astrophysical environments can effectively be reproduced in the laboratory. Here we report the results of an experiment that produced seed magnetic fields by the Biermann battery effect. We show that these results can be scaled to the intergalactic medium, where turbulence, acting on timescales of around 700 million years, can amplify the seed fields sufficiently to affect galaxy evolution.

Electromagnetic wave scattering in dusty plasmas

The cross section for transition scattering of electromagnetic waves on charged dust particles in a plasma is calculated, extending the results of a previous paper [J. Plasma Phys. 42, 429 (1989)] where the case of longitudinal waves has been considered. For the case of nonlinear screening of the charged dust by the plasma particles (i.e., ‖eφ0/Te‖ ≫ 1, where φ0 is the dust grain surface potential and Te is the electron plasma temperature), numerical and analytical results are presented, showing a significant enhancement, proportional to the square of the grain surface charge, in the cross section with respect to scattering by free electrons. The effect is independent of the sign of the charge for wavelengths larger than the Debye length.

Radiation sources based on laser-plasma interactions

Plasma waves excited by intense laser beams can be harnessed to produce femtosecond duration bunches of electrons with relativistic energies. The very large electrostatic forces of plasma density wakes trailing behind an intense laser pulse provide field potentials capable of accelerating charged particles to high energies over very short distances, as high as 1 GeV in a few millimetres. The short length scale of plasma waves provides a means of developing very compact high-energy accelerators, which could form the basis of compact next-generation light sources with unique properties. Tuneable X-ray radiation and particle pulses with durations of the order of or less than 5 fs should be possible and would be useful for probing matter on unprecedented time and spatial scales. If developed to fruition this revolutionary technology could reduce the size and cost of light sources by three orders of magnitude and, therefore, provide powerful new tools to a large scientific community. We will discuss how a laser-driven plasma wakefield accelerator can be used to produce radiation with unique characteristics over a very large spectral range.