P. K. Kaw

RELATIVISTIC NONLINEAR PROPAGATION OF LASER BEAMS IN COLD OVERDENSE PLASMAS.

The nonlinear propagation of a very intense laser beam in a cold overdense plasma is analytically investigated; the laser beam is assumed to be intense enough to cause the directed component of electron velocity to be comparable to the velocity of light. Special attention has been given to the case when coupled longitudinal‐cum‐transverse modes propagate in the plasma. An interesting result is that the beam can readily propagate through the overdense plasma, in contrast to what would be expected on the basis of a linear theory of laser propagation.

Coalescence instability of magnetic islands

The stability of a periodic magnetic island structure using the ideal magnetohydrodynamic equations has been investigated. An instability is found which describes the tendency toward coalescence of parallel currents in the neighboring islands. It is expected that this instability will proceed at a fast hydromagnetic rate as long as the forces driving the instability can overcome the stabilizing forces due to the compression of the magnetic field between the islands. Beyond that phase, resistivity is expected to dominate the tendency toward island coalescence. Island coalescence of this kind can explain why in the observation of tearing mode instabilities in tokamaks, only the modes with minimum values of m and n are seen.

Laser‐Induced Anomalous Heating of a Plasma

It is shown that a sufficiently intense laser beam can drive low‐frequency instabilities in a fully ionized plasma; these instabilities may cause considerable enhancement of the high‐frequency resistivity of the plasma around certain frequencies and thus lead to an anomalous heating effect.

Optical Absorption and Expansion of Laser‐Produced Plasmas

Calculations of the absorption of laser radiation by an inhomogeneous overdense plasma have been carried out. Such a plasma is produced when a high‐powered laser irradiates a solid particle. Nearly complete absorption is found as long as 2R is greater than c⊥ here, R is the scale length for density falloff, c is the velocity of light, and ⊥ is the electron‐ion collision time for a plasma frequency equal to the incident laser frequency. For hydrogen this gives complete absorption up to temperatures of 5 keV if R is greater than 5u2009×u200910−2u2009cm; higher Z materials absorb better. The expansion of a laser‐produced plasma was also investigated. In the case of a plasma produced from a thin metallic film, it is found that the expansion preferentially takes place normal to the film which is in agreement with observation.

Surface Waves on a Plasma Half‐Space

The effect of density gradients and a finite temperature on the dispersion relation for surface waves on a plasma half‐space has been investigated analytically. The full set of Maxwells equations is used to obtain the dispersion of surface waves on a warm homogeneous plasma, thus complementing earlier work on the electrostatic mode. The full surface‐wave dispersion relation is then derived for a cold plasma with arbitrary but weak density profile in the WKB limit. Finally, the dispersion of electrostatic surface modes on a cold plasma with a linear density profile of arbitrary strength is obtained. It is shown that when the density variation over a wavelength is very large, a new type of damped surface wave with a frequency higher than the surface plasma frequency is possible.

Quasiresonant mode coupling of electron plasma waves

A finite amplitude ion fluctuations (wave number ki) couples a primary electron plasma wave (wave number k0) to a whole spectrum of plasma oscillations (wave numbers k0u2009±u2009nu2009ku2009i). Simple fluid equations are used to analyze the effect and computer simulation is used to verify the theory, and show the limits of the analysis. If the variation in plasma frequency Δωp associated with the ion fluctuation exceeds the frequency mismatch between the primary wave and the nth mode, then a large fraction of the energy can be transferred to that mode, while if the mismatch in frequency is greater than Δωp only a relatively small energy transfer occurs. Because the frequency of plasma oscillations varies only weakly with wavelength, very small ion fluctuations are sufficient to couple long‐wavelength waves to short‐wavelength waves which are Landau damped. This phenomenon may also account for some experimental observations of fluctuations in which the frequency and wave number do not appear to satisfy the normal plasma ...

Parametric Instabilities of Ion Cyclotron Waves in a Plasma

An externally applied large amplitude circularly polarized ion cyclotron wave of long wavelength propagating along the static magnetic field can drive instabilities of ion cyclotron waves and ion acoustic waves. The dispersion relation is first derived using a kinetic equation approach. Later, it is shown that the same relation can be derived on the basis of a “KΔX” analysis. The instabilities can be classified into three classes: |u2009ku2009|⋙k0 corresponds to the usual purely growing instability; |u2009ku2009|u2009∼k0 corresponds to the decay instability, and |u2009ku2009|u2009≫k0 corresponds to slightly oscillatory instability which is actually the finite k0 modification of the purely growing instability. The dependence of both, the threshold and maximum growth rates, on various parameters is discussed. The relevance of the results to the anomalous ion heating for fusion research is also pointed out.

Ponderomotive Force on Laser‐Produced Plasmas

The deconfining ponderomotive force exerted by an intense electromagnetic wave on a linearly inhomogeneous plasma layer has been analytically investigated. Since the ponderomotive force maximizes near the region where the dielectric constant e′u2009→u20090, an exact solution of the wave equation is required for a correct estimate of the maximum force. For the case of oblique incidence with the electric vector in the plane of incidence, the solution of the exact wave equation leads to an interesting resonance effect which predicts a force higher than that obtained by earlier workers using a WKB procedure. Physically, this enhanced ponderomotive force arises because of the large nonuniform longitudinal fields that are generated in the region e′u2009≈u20090 for this geometry.

Lower hybrid parametric instabilities: Nonuniform pump waves and tokamak applications

Electrostatic lower hybrid ’’pump’’ waves are often launched into tokamak plasmas by structures (e.g., waveguides) whose dimensions are considerably smaller than characteristic plasma sizes. Such waves propagate in well‐defined resonance cones and give rise to parametric instabilities driven by electron E×B velocities. The finite size of the resonance cone region determines the threshold for both convective quasi‐mode decay instabilities and absolute instabilities. The excitation of absolute instabilities depends on whether a traveling or standing wave pump model is used; traveling wave pumps require the daughter waves to have a definite frequency shift. Altogether, parametric instabilities driven by E×B velocities occur for threshold fields significantly below the threshold for filamentation instabilities driven by pondermotive forces. Applications to tokamak heating show that nonlinear effects set in when a certain power‐per‐wave‐launching port is exceeded. For sufficiently high powers, these instabilit...

Relativistic acceleration of charged particles by superintense laser beams

The relativistic motion of charged particles in very intense electromagnetic wave pulses with slowly space‐varying amplitude has been analytically investigated. The results of the analysis have been used to discuss the conditions under which currently available lasers can be used for producing MeV electrons in the laboratory.