Heinrich Hora

Self-focusing of laser beams in a plasma by ponderomotive forces

Self-focusing of a laser beam in a plasma is treated in terms of the ponderomotive acceleration due to the gradient of the light intensity. The focusing of radiation within the first minima of diffraction sets a lower limit to the laser power which is of the order of 1 MW for the usual lasers if cut-off density and a plasma temperature of about 10 eV are assumed.

NONLINEAR CONFINING AND DECONFINING FORCES ASSOCIATED WITH THE INTERACTION OF LASER RADIATION WITH PLASMA.

The nonlinear interaction of an intense light wave with an inhomogeneous plasma produces a macroscopic motion. A rigorous treatment of this interaction based on the ponderomotive force description leads to a general equation of motion. In a plasma with collisions and high electron density, the resulting force density can be interpreted as an expression of the radiation pressure. Below special densities there results a nonlinear collisionless force whose direction is toward decreasing density, and which produces a deconfinement. The magnitude of this force has a polarization dependence only in the third‐order terms of the spatial dependence of the density. The total deconfining recoil momentum transferred to the inhomogeneous transition layer is evaluated. The theory of the nonlinear collisionless acceleration is used to explain the experimentally observed properties of the fast part of plasmas produced by lasers from isolated single small aluminum balls and thick solid targets.

MODULATION OF AN ELECTRON WAVE BY A LIGHT WAVE

Electron transmission diffraction patterns of thin monocrystal films are made visible on a nonluminescent target, if the films are illuminated by a 107 Wcm−2 argon laser irradiation. It appears that the 50‐keV electrons pick up an oscillation of the light frequency within the crystal film and preserve this oscillation state until they reach the nonluminescent target.

Striated Jets Due to Nonlinear Ponderomotive Forces in Laser Produced Plasmas at Obliquely Incident Light

An analysis is made of one of the expected nonlinear and anomalous mechanisms and instabilities in the corona of a laser irradiated plasma. As is well known, nonlinear ponderomotive forces generate a net acceleration of the plasma toward lower densities. It is demonstrated that the same forces at oblique incidence of the radiation, can cause a net motion perpendicular to the well‐known forces along the density gradient: The standing wave generated that is split into zones (slabs) of the thickness of a quarter of a wave length, where the plasma is moving in the plane of incidence parallel and antiparallel to the slabs. This striated motion can also occur in an underdense plasma, while for the net acceleration toward lower density the strong change of the refractive index near the cutoff density is necessary. The striated parallel and antiparallel motion of the slabs is limited by the mean free path, by the velocity toward lower density and is determined by the condition of laminar and turbulent motion. The...

Momentum of Photons and the Nonlinear Force of Laser-Plasma Interaction

The momentum of photons in a collisionless plasma reproduces the Fresnel formulas only if the energy exchange with electrons is included. The transferred momenta correspond exactly to those derived from the theory of the nonlinear force of a laser‐plasma interaction.

Momentum transfer to laser‐irradiated targets, indicating the nonlinear interaction force

Measurements by Metz of the momentum ρ transferred to laser‐irradiated targets are evaluated and compared with the results of the nonlinear force of laser‐plasma interaction. The measured thresholds for the intensity I*, estimated to be 2 × 1014 W/cm2, agree with the threshold of the nonlinear force. The measured highly superlinear relation P ≈ I4 agrees very well with the theory for not too large intensities I≳ I*. A consequence would be an electron energy of coherent oscillation near 104 eV, which may cause a nonthermal origin of fusion neutrons in other experiments.

Influence of Fast Ion Losses in Inertially Confined Nuclear Fusion Plasma

The gain G of the thermonuclear fusion energy of very dense spherical plasmas heated by lasers in times of about 1 nsec is treated theoretically. Assuming as a pessimistic estimate that fast ions leave the plasma without contributing to fusion neutrons, we find that for G = 1 laser energies of 3 × 107 joules are needed. A kinetic theory for a one species gas starting with a Gaussian density and velocity profile yields almost the same result as found by a hydrodynamic model with adiabatic expansion and without fast ion losses taken into account. In the latter case the even-point of G = 1 is reached at the well-known value for the laser energy of 4.6 × 106 joules for a D-T reaction.

Volume Ignition in Pellet Fusion to Overcome the Difficulties of Central Ignition

Since C. Yamanaka et al. demonstrated that the best fusion gains from laser irradiated pellets result only when central shocks are avoided and an ideal volume compression is achieved, the problems o f the central (spark) ignition with necessary densities of 1000 times the solid state may be overcome. Based on an analytical formula of volume ignition, the new conditions should provide reactor adequate laser fusion with compression to 50 to 100 times solid state.

Calculations of Laser Exitation in a GaAs Anode by Slow Electrons

The laser mechanism in a p-type GaAs slab with no junction and a hole concentration higher than 7 x 1018 cm-3 is investigated theoretically for the case that the excited electrons are injected from a gas discharge outside the slab. With such an excitation by slow electrons a lower bound of the current density is calculated from the Schawlow-Townes condition. The losses of diffraction and absorption mechanisms are taken into account. Since the slab can be very thin, minimum current densities with laser action of 10 amp./cm2 are possible. Several absorption and excitation mechanisms are discussed. Their relative importance may be found from the spectrum of the laser light and the depth of the activation zone of the slab.

Turbulence Limits in Rotating Plasmas for Isotope Separation

Abstract With the aim of using rotating plasmas (driven by j x B or E x B forces) for isotope separation, several experimental configurations have been studied and theory has been developed with respect to numerical models, the Alfvén ionization limit, DAngelos shear instability, while turbulence was suggested qualitatively only. A more detailed study of the limit of laminar flow is presented taking into account the magnetic anisotropy of viscosity and interaction with neutrals. In all cases of experiments, the Reynolds numbers are below critical values of 10 4 by at least a factor 100. The pessimistic conclusions of Boeschoten for his experiment can there-fore not be shared with respect to the turbulence problems. The published cases of isotope separation by rotating plasmas are then discussed extensively.