Uniform Target Illumination Research Articles

Optimal laser intensity profiles for a uniform target illumination in direct-drive inertial confinement fusion

Abstract A numerical method providing the optimal laser intensity profiles for a direct-drive inertial confinement fusion scheme has been developed. The method provides an alternative approach to phase-space optimization studies, which can prove computationally expensive. The method applies to a generic irradiation configuration characterized by an arbitrary number $N_{B}$ of laser beams provided that they irradiate the whole target surface, and thus goes beyond previous analyses limited to symmetric configurations. The calculated laser intensity profiles optimize the illumination of a spherical target. This paper focuses on description of the method, which uses two steps: first, the target irradiation is calculated for initial trial laser intensities, and then in a second step the optimal laser intensities are obtained by correcting the trial intensities using the calculated illumination. A limited number of example applications to direct drive on the Laser MegaJoule (LMJ) are described.

Proton trajectories and electric fields in a laser-accelerated focused proton beam

The focusing properties of a laser generated proton beam have been investigated using hemispherical targets in both freestanding and enclosed cone-shaped geometries. The proton trajectories and focusing were strongly affected by the electric fields in the beam, bending the trajectories near the axis. In the cone targets, a sheath field effectively channels the proton beam through the open cone tip, substantially improving the beam focusing from ≈90 μm to ≈55 μm diameter for protons with energies >3 MeV. The proton generation and focusing were modeled using 2D hybrid particle-in-cell simulations, which compared well with the experimental results. Simulations predict further improvement in focusing with more uniform target illumination. These results are of significant interest to proton fast ignition and other high energy density physics applications.

Heavy ion hollow beam formation at the energy of 1 AGeV for implosion experiments using an original RF system for fast rotation

For experiments at Facility for Antiprotons and Ion Research (FAIR) on high energy density states in matter produced by heavy ion beams, a hollow cylindrical target combined with cylindrical geometry of the ion beam energy deposition region is required. This combination facilitates extremely high densities and pressures on the axis of the imploding cylinder if a hollow or multi-layered target is imploded using a hollow beam with an annular focal spot. We report investigations on a new method for radio frequency rotation of the ion beam, which is proposed for the Laboratory Planetary Sciences (LAPLAS) experiment in order to produce reliably such a hollow beam of ∼2 mm in diameter. For this purpose the beam delivered from the synchrotron (SIS-100) will be transformed by means of fast rotation around a cylindrical axis. The specified RF system consisting of two deflecting cavities is currently under development. An operating rotation frequency of 325 MHz has been chosen. According to simulations and analytical models, this frequency is sufficient for uniform target illumination at the power deposition level of about 10 TW/g. The RF rotation system will be placed in the beam transport line at a distance from the target, which is equal to one quarter wavelength of the transverse beam oscillations. The design of the rotation system and layout of the target–rotation system and focusing elements are presented.

High resolution of streamer seismic data: obtaining optimal results

Andrew Long, PGS Marine Geophysical, defines the requirements for successful high resolution 3D marine seismic surveys and how these can be met. High resolution of seismic data results when high frequency geological features in close proximity can be precisely focused without any contamination from artefacts or noise (i.e. high signal-to-noise ratio). In general, recoverable frequency bandwidth is an important factor for vertical resolution, and dense 3D spatial sampling is an important factor for spatial resolution. Vertical and horizontal resolution is linked by the success of high frequency focusing and imaging. Without pursuing a theoretical treatise, the following section discusses the geophysical principles that are relevant for high-resolution seismic imaging. High resolution of 3D seismic data is achieved by the application of rigorous geophysical principles. In particular, the systematic pursuit of 1) uniform target illumination, 2) dense spatial sampling of the reflected wavefield, and 3) careful processing and imaging, will collectively deliver an optimum result. Note that the final step can never be successfully completed without the platform of proper target illumination and wavefield sampling. Several 3D data examples are used to demonstrate how flexible survey design and implementation can robustly achieve very high resolution target imaging for all target depths and challenges.

The NIKE KrF laser fusion facility

NIKE is a second generation high power KrF laser now under construction at the Naval Research Laboratory. The project is a collaborative effort between NRL and Los Alamos National Laboratory. NIKE is designed to deliver more than 2 kJ of energy to target in a 600-μm focal spot and a 4-ns pulse duration. Echelon free induced spatial incoherence (ISI) will be used to produce uniform target illumination. Flat targets will be ablatively accelerated to study both Rayleigh-Taylor and parametric instabilities. These results will have direct implications to direct-drive inertial confinement fusion for commercial energy applications. Reliable operation of a high power KrF laser is also an important goal of the NIKE laser, with the objective of 1000 target shots per year. This would be an important step in the development of the KrF laser as an ICF driver. NIKE is cheduled to begin target experiments in early 1994. If successful, these experiments will provide a technical basis to proceed with construction of an ignition facility.

Uniform target illumination by induced spatial incoherence in a multiplexed KrF laser system

A technique for producing uniform target irradiation by applying induced spatial incoherence to a high power multiplexed KrF laser system has been demonstrated. The image of a smooth spatial profile is relayed on four beams through a large aperture, electron beam pumped KrF laser and to the focal plane of a concave mirror. Single pulses of 20 ns duration have been amplified to energies of 30 J while observing the effect of amplification on the output profile uniformity.

Optical beam shaping of a high power laser for uniform target illumination

This paper describes a new approach for transforming a high power laser beam into a beam with a rectangular cross section of uniform intensity. The concept requires both a multimode laser beam and an optical beam shaper to produce an image on target with a trapezoidal intensity profile and better than 10% uniformity.