Frederick Osman

Fast ignition by laser driven particle beams of very high intensity

Anomalous observations using the fast ignition for laser driven fusion energy are interpreted and experimental and theoretical results are reported which are in contrast to the very numerous effects usually observed at petawatt-picosecond laser interaction with plasmas. These anomalous mechanisms result in rather thin blocks (pistons) of these nonlinear (ponderomotive) force driven highly directed plasmas of modest temperatures. The blocks consist in space charge neutral plasmas with ion current densities above 1010A∕cm2. For the needs of applications in laser driven fusion energy, much thicker blocks are required. This may be reached by a spherical configuration where a conical propagation may lead to thick blocks for interaction with targets. First results are reported in view of applications for the proton fast igniter and other laser-fusion energy schemes.

Principle of high accuracy for the nonlinear theory of the acceleration of electrons in a vacuum by lasers at relativistic intensities

Acceleration of electrons by lasers in a vacuum was considered impossible based on the fact that plane-wave and phase symmetric wave packets cannot transfer energy to electrons apart from Thomson or Compton scattering or the Kapitza-Dirac effect. The nonlinear nature of the electrodynamic forces of the fields to the electrons, expressed as nonlinear forces including ponderomotion or the Lorentz force, permits an energy transfer if the conditions of plane waves in favor of the beams and/or the phase symmetry are broken. The resulting electron acceleration by lasers in a vacuum is now well understood as free wave acceleration, as ponderomotive scattering, as violent acceleration, or as vacuum beat wave acceleration. The basic understanding of these phenomena relates to an accuracy principle of nonlinearity for explaining numerous discrepancies on the way to the mentioned achievement of vacuum laser acceleration, which goes beyond the well-known experience of necessary accuracy in both modeling and experimental work experiences among theorists and experimentalists in the field of nonlinearity, From mathematically designed beam conditions, an absolute maximum of electron energy per laser interaction has been established. It is shown here how numerical results strongly (both essentially and gradually) depend on the accuracy of the used laser fields for which examples are presented and finally tested by the criterion of the absolute maximum.

Relativistic and ponderomotive self-focusing at laser–plasma interaction

The nonlinear plasma dielectric function due to relativistic electron motion is derived. From this, one can obtain the nonlinear refractive index of the plasma and estimate the importance of relativistic self-focusing in comparison with ponderomotive non-relativistic self-focusing at very high laser intensities. When the laser intensity is very high, ponderomotive self-focusing will be dominant. However, at some point, when the oscillating velocity of the plasma electrons becomes very large, relativistic effects will also play a role in self-focusing.

Fusion energy from plasma block ignition

Generation of high speed dense plasma blocks is well known from hydrodynamic theory and computations (PIC) with experimental confirmation by Badziak et al. (2005) since ps laser pulses with power above TW are available. These blocks may be used for fusion flame generation (thermonuclear propagation) in uncompressed solid state deuterium and tritium for very high gain uncomplicated operation in power stations. Hydrodynamic theory from computations from the end of 1970s to recent, genuine two fluid computations support the skin layer accelerations (SLA), by nonlinear (ponderomotive) forces as measured now in details under the uniquely selected conditions to suppress relativistic self-focusing by high contrast ratio and to keep plane geometry interaction. It is shown how the now available PW-ps laser pulses may provide the very extreme conditions for generating the fusion flames in solid state density DT.

Layers from initial Rayleigh density profiles by directed nonlinear force driven plasma blocks for alternative fast ignition

Measurement of extremely new phenomena during the interaction of laser Pulses with terawatt and higher power and picosecond; with plasmas arrived at drastically different anomalies in contrast to the usual observations if the laser pulses were very clean with a contrast ratio higher than 10(8). This was guaranteed by the Suppression of prepulses during less than dozens of ps before the arrival of the main pulse resulting in the suppression of relativistic self-focusing. This anomaly was confirmed in many experimental details, and explained and numerically reproduced as a nonlinear force acceleration of skin layers generating quasi-neutral plasma blocks with ion current densities above M, I A/cm(2). This may support the requirement to produce a fast ignition deuterium tritium fusion at densities not much higher than the solid state by a single shot PW-ps laser Pulse. With the aim to achieve separately studied ignition conditions, we are studying numerically how the necessary nonlinear force accelerated plasma blocks may reach the highest possible thickness by using optimized dielectric properties of the irradiated plasma. The use of double Rayleigh initial density profiles results in many wavelength thick low reflectivity directed plasma blocks of modest temperatures. Results of computations with the genuine two-fluid model are presented.

Analytical description of rippling effect and ion acceleration in plasma produced by a short laser pulse

In this paper we present the analytical description of two processes dealing with the skin-layer ponderomotive acceleration method of fast ion generation by a short laser pulse: ion density rippling in the underdense plasma region and generation of ion beams by trapped electromagnetic field in plasma. Some numerical examples of hydrodynamic simulation illustrating these processes are shown. The effect of using the laser pulse consisting of different frequency components on the ion density rippling and on phenomena connected with trapped electromagnetic field is analyzed.

Computations for nonlinear force driven plasma blocks by picosecond laser pulses for fusion

The concept of the fast ignitor for laser fusion has led to some rnodifications in applying petawatt-picosecond (PW-ps) laser-produced high intensity particle beams to ignite deuterium-tritium (DT) fuel. Some very anomalous measurements of ion emission from targets irradiated by picosecond laser pulses led to the development of a skin depth interaction scheme where a defined control of prepulses is necessary. Based on these experimental facts, we have applied a one-dimensional two-fluid hydrodynamic code to understand how the nonlinear ponderomotive force generates two plasma blocks, one moving against the laser light (ablation) and the other moving into the target. This compressed block produces ion current density of above 10(11) A cm(-2) and an ion energy of about 100 keV. This may be a rather promising option to use PW-ps laser pulses for igniting fusion in solid density DT fuel, realizing very high gain controlled fusion reactions.

Solutions of the nonlinear paraxial equation due to laser plasma–interactions

This article presents a numerical and theoretical study of the generation and propagation of oscillation in the semiclassical limit ħ → 0 of the nonlinear paraxial equation. In a general setting of both dimension and nonlinearity, the essential differences between the “defocusing” and “focusing” cases are observed. Numerical comparisons of the oscillations are made between the linear (“free”) and the cubic (defocusing and focusing) cases in one dimension. The integrability of the one-dimensional cubic nonlinear paraxial equation is exploited to give a complete global characterization of the weak limits of the oscillations in the defocusing case.

Focusing and defocusing of the nonlinear paraxial equation at laser–plasma interaction

This paper presents a numerical and theoretical study of the generation and propagation of oscillation in the semiclassical limit T → 0 of the nonlinear paraxial equation at laser-plasma interaction. In a general setting of both dimension and nonlinearity, the essential differences between the focusing and defocusing cases is identified due to the nonlinearity, and dispersion effects involved in the propagation of solitons at laser plasma interaction. A sequence of codes has been developed in mathematics to explore the focusing and defocusing of the soliton formation and propagation.

Laser-generated pair production and Hawking-Unruh radiation

Laser-produced electron-positron pair production has been under discussion in the literature since 1969. Large numbers of positrons have been generated by lasers for a few years in studies which are also related to the studies of the physics of the fast ignitor laser fusion concept. For electron-positron pair production in vacuum due to vacuum polarization as predicted by Heisenberg (1934) with electrostatic fields, high-frequency laser fields with intensities around 10 28 W/cm 2 are necessary and may be available within a number of years. A similar electron acceleration by gravitation near black holes denoted as Hawking-Unruh radiation was discussed in 1985 by McDonald. The conditions are considered in view of the earlier work on pair production, change of statistics for electrons in relativistic black body radiation, and an Einstein recoil mechanism with a consequence of a physical foundation of the fine structure constant.