A preformed plasma channel of light ion species, with a small percentage of high-Z atoms, can be produced by a laser prepulse. When an intense short laser pulse propagates through the channel, high-Z atoms are stripped of most of their electrons. After the passage of the pulse, recombination of electrons with high-Z ions produces population inversion for the generation of coherent X rays. However, the gain is strongly dependent on the radial coordinate r. A paraxial-ray theory of X-ray amplification in such a situation reveals that gain focusing and refraction guiding play an important role in determining the gain and the spot size of the X-ray laser.

A high-power millimetre wave (ω1, k1) propagating through a magnetized plasma at an angle to a density ripple (0, k0) produces density perturbations (ω1, k1, k0). The density perturbations couple with the oscillatory velocity at (ω1, k1) to produce a nonlinear current at (2ω1, 2k1+k0) driving second-harmonic electromagnetic radiation. The efficiency of the process is sensitive to the angle between the density ripple and the millimetre wave.

We demonstrate fast mixing of vortex–current filaments by means of numerical simulations of collision (strong interaction) between two straight filaments. The two filaments mutually approach, collide, and are rapidly tangled with each other. In fact, the instantaneous Lyapunov exponent shows that the dynamics becomes chaotic. Then there appear many small regions where the two filaments overlap. We consider each overlapping region to be equivalent to the traditional resistive diffusion region. We assume that the overall ‘reconnection rate’ of the two filaments is proportional to the product of the traditional (non-chaotic) resistive reconnection rate and the normalized overlapping volume. The overlapping volume rapidly increases on the time scale of ideal MHD. When many overlapping regions are produced, the overall reconnection probability, i.e. the sum of the probabilities of reconnection in every overlapping region, should be increased compared with that of the single overlapping region. Thus the overall reconnection rate becomes sufficiently large, although the basic reconnection process in each overlapping region is resistive and slow. We conclude that the fast mixing due to chaos may enhance the conventional resistive reconnection. We call this process ‘chaotic reconnection’.

In this work, we analyse the sequence of bifurcated equilibria in two-dimensional reduced magnetohydrodynamics. Diamagnetic effects are studied under the assumption of a constant equilibrium pressure gradient, not altered by the formation of a magnetic island. The formation of an island when the symmetric equilibrium becomes unstable is studied as a function of the tearing-mode stability parameter Δ′ and of the diamagnetic frequency, by employing fixed-point numerical technique and an initial-value code. At larger values of Δ′, a tangent bifurcation takes place, above which no small-island solutions exist. This bifurcation persists up to fairly large values of the diamagnetic frequency (of the order of one-tenth of the Alfvén frequency). The implications of this phenomenology for the intermittent MHD dynamics observed in tokamaks is discussed.

We derive a system of nonlinear equations that govern the dynamics of low-frequency short-wavelength electromagnetic waves in the presence of equilibrium density, temperature, magnetic field and velocity gradients. In the linear limit, a local dispersion relation is obtained and analyzed. New ηe-driven electromagnetic drift modes and instabilities are shown to exist. In the nonlinear case, the temporal behaviour of a nonlinear dissipative system can be written in the form of Lorenz- and Stenflo-type equations that admit chaotic trajectories. On the other hand, the stationary solutions of the nonlinear system can be represented in the form of dipolar and vortex-chain solutions.

The method of matched asymptotic expansions is used to examine the structure of the plasma sheath of the positive column at low pressure in electronegative gases using the fluid model to describe the positive-ion motion. It is shown that at low negative-ion concentrations, and at high concentrations, the structure is that of a plasma joined to a thin sheath, but that for the electron/negative-ion temperature ratio Te/Tn ≡ ε > 5 + √24, and for a well-defined range of A ≡ nn0/ne0 (the central negative ion to electron density ratio) and for small Debye length, there is a more complex structure with a central negative-ion-dominated plasma surrounded by a quasiplasma in which density oscillations may occur before joining to a sheath. This is in agreement with recent computations using the same model.