J. M. Dawson

Geodesic Acoustic Waves in Hydromagnetic Systems

In toroidal systems with geodesic curvature an electrostatic acoustic mode occurs with plasma motion in the magnetic surfaces, perpendicular to the field. In typical stellarators this mode should dominate ordinary sound waves associated with motion along the field.

High-Frequency Conductivity and the Emission and Absorption Coefficients of a Fully Ionized Plasma

The problem of the ac conductivity of a fully ionized plasma is investigated for frequencies embracing the plasma frequency. The finite duration of encounters is taken into account in a self‐consistent fashion which includes collective effects. The concomitant processes of absorption and emission of electromagnetic radiation are investigated and in particular the bremsstrahlung emission and absorption coefficients near the plasma frequency are given. The conversion of longitudinal to transverse waves by scattering from ions is discussed.

Nuclear reaction rates and energy in stellar plasmas : The effect of highly damped modes

The effects of the highly damped modes in the energy and reaction rates in a plasma are discussed. These modes, with wave numbers k≫kD, even being only weakly excited, with less than kBT per mode, make a significant contribution to the energy and screening in a plasma. When the de Broglie wavelength is much less than the distance of closest approach of thermal electrons, a classical analysis of the plasma can be made. It is assumed, in the classical analysis, with ℏ→0, that the energy of the fluctuations ℏω≪kBT. Using the fluctuation-dissipation theorem, the spectra of fluctuations with ℏ≠0 is appreciably decreased. The decrease is mainly for the highly damped modes at high frequencies (∼0.5–3kBT). Reaction rates are enhanced in a plasma due to the screening of the reacting ions. This is taken into account by the Salpeter factor, which assumes slow motion for the ions. The implication of including the highly damped modes (with ℏ≠0) in the nuclear reaction rates in a plasma is discussed. Finally, the inves...

Free electron laser

The possibility of a new type of laser has been investigated by computer simulation using a fully relativistic electromagnetic particle code which has one spatial and three velocity dimensions. By passing a relativistic electron beam over a rippled static magnetic field, high frequency electromagnetic radiation is generated. If the ripple wavelength is λ0, the lasing wavelength is roughly λ0/2γ2. Thus, such a laser is continuously tunable by varying γ. It has been observed in simulation that as much as 35% of the beam energy can be converted into radiation, of which as much as nearly 90% can be in the most rapidly growing mode. A theory of the coupling between the negative energy plasma wave and the electromagnetic radiation by means of the rippled magnetic field is presented. Good agreements have been obtained between the simulation and the theory. The saturation mechanism is found to be the trapping of the beam by the unstable plasma wave. A theoretical estimate of the amount of energy that can be converted into radiation from the electron beam is also given.

ON THE PRODUCTION OF PLASMA BY GIANT PULSE LASERS

Calculations are presented which show that a laser pulse delivering powers of the order of 1010 W to a liquid or solid particle with dimensions of the order of 10−2 cm will produce a hot plasma with temperatures in the range of several hundred eV. To a large extent the plasma temperature is held down by its rapid expansion and cooling. This converts much of the energy supplied into ordered energy of expansion. This ordered expansion energy can amount to several keV per ion. If the expanding plasma can be caught in a magnetic field and its ordered motion converted to random motion this might be utilized as a means for filling controlled thermonuclear fusion devices with hot plasma. Further, it should also be possible to do many interesting plasma experiments on such plasmas.

Interpenetrating Plasma Shells: Near-Equipartition Magnetic Field Generation and Nonthermal Particle Acceleration

We present the first three-dimensional fully kinetic electromagnetic relativistic particle-in-cell simulations of the collision of two interpenetrating plasma shells. The highly accurate plasma-kinetic particle-in-cell (with the total of 108 particles) parallel code OSIRIS has been used. Our simulations show (1) the generation of long-lived near-equipartition (electro)magnetic fields, (2) nonthermal particle acceleration, and (3) short-scale to long-scale magnetic field evolution, in the collision region. Our results provide new insights into the magnetic field generation and particle acceleration in relativistic and subrelativistic colliding streams of particles, which are present in gamma-ray bursters, supernova remnants, relativistic jets, pulsar winds, etc.

Correct values for high‐frequency power absorption by inverse bremsstrahlung in plasmas

Numerical corrections are made to previous values for the power absorption coefficient and some common errors noted.

Theory and numerical simulation on plasma diffusion across a magnetic field

The diffusion of two and two and a half‐dimensional plasmas across magnetic fields have been studied theoretically and by numerical simulation. Only diffusion at thermal equilibrium is studied. It is found that there are three regions: for sufficiently weak magnetic fields the diffusion coefficient is the classical one with D⊥ going like B−2; for moderate magnetic fields (ωce≈ωpe) the diffusion rate is enhanced and B−1 is almost independent of B; finally, for very large fields (ωci>ωpi) the diffusion coefficient goes like B−1. The enhanced diffusion at moderate and high magnetic fields is dominated by collective modes; i.e., by thermally excited convective modes. Theory and simulation are in good agreement. It is also shown that the diffusion coefficient behaves essentially the same way for a three‐dimensional plasma when the magnetic field lines are closed.

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.

One‐Dimensional Plasma Model

A one‐dimensional plasma model consisting of a large number of identical charge sheets embedded in a uniform fixed neutralizing background is investigated by following the sheet motions on a high‐speed computer. The thermalizing properties and ergodic behavior of the system are examined and found to be in agreement with the assumption that one is equally likely to find the system in equal volumes of the available phase space. The velocity distribution, Debye shielding, drag on fast and slow sheets, diffusion in velocity space, the Landau damping of the Fourier modes, the amplitude distribution function for the Fourier modes, and the distribution of electric fields felt by the sheets were obtained for the plasma in thermal equilibrium and compared with theoretically predicted values. In every case, except one, the drag on a slow sheet, the numerical results agreed with theory to within the statistical accuracy of the results. The numerical results for the drag on a slow sheet were about a factor of 2 lower...