M. J. Herbst

Evidence in the second‐harmonic emission for self‐focusing of a laser pulse in a plasma

Short‐pulse (300 psec), high‐intensity (1014−1015 W/cm2) Nd‐laser light was propagated into variable scale length plasmas (Ln≡n/∇n=200–400 μm at 0.1 critical density) preformed by long‐pulse (4 nsec), low‐intensity (≂6×1012 W/cm2) irradiation of planar targets. For high short‐pulse intensities (≥5×1014 W/cm2), time‐integrated images show filament‐shaped regions of second‐harmonic (2ω0) emission from the low density (0.01≤ne/nc≤0.2) region of the ablation plasma. Two‐dimensional computer calculations of the hyrodynamics and laser beam propagation indicate that these filaments are consistent with ponderomotive self‐focusing of the short pulse. A theoretical model that explains the 2ω0 generation mechanism within low‐density filaments is also presented.

Ablative acceleration of planar targets to high velocities

Laser irradiated targets are ablatively accelerated to velocities near those required for fusion pellet implosions while remaining relatively cool and uniform. The target velocities and velocity profiles are measured using a double-foil method, which is described in detail. Also, the ablation plasma flow from the target surface is spatially resolved, and the scalings with absorbed irradiance of the ablation pressure, ablation velocity, and mass ablation rate are determined. Results are compared with hydrodynamic code calculations.

Laser interaction in long-scale-length plasmas

Absorption of a short‐pulse, high‐intensity Nd‐laser beam (vacuum irradiance of 1014 to 1015 W/cm2) by preformed plasmas of different density scale lengths is investigated. Increased effects of plasma instabilities are found at longer scale lengths. The amount of backscattered light increases with plasma scale length and limits the absorption fraction at the longest scale length. The onset of suprathermal electron production, deduced from observations of energetic (20 to 50 keV) x rays, occurs at lower laser irradiance for longer‐scale‐length plasmas. A correlation between energetic x rays and 3ω0/2 emission suggests that the suprathermal electrons are produced by a plasma instability at quarter‐critical density. At higher intensities there is evidence for severe perturbations of the preformed plasma and for self‐focusing of the incident beam.

New Measurement Techniques Using Tracers Within Laser-Produced Plasmas

The use of locally embedded tracers within laser-irradiated solid targets has led to a new class of diagnostic methods for laser-produced plasmas. Demonstrated uses of tracers include the first visualizations of hydrodynamic flow of laser-ablated materials and improved spectroscopic measurements of plasma density and temperature profiles; comparisons with a two-dimensional hydrodynamics computer code are shown. Proposed future uses of tracers include the first measurements of fluid velocity profiles and improved determinations of mass ablation rates.

Analysis of Stability and Symmetry Implications for ICF

Pellet gains in excess of 100 will probably be necessary for most applications of inertial fusion.1 In order to achieve these high gains a number of critical physics elements must be controlled. These include (1) high coupling efficiency, (2) low fuel preheat, (3) implosion symmetry, (4) implosion stability (ablation pressure) and (5) an ignition scheme. These factors interact with each other providing conflicting requirements. In particular the first two items are directly in conflict with the second two. For example, high coupling efficiency and low fuel preheat requires control of deleterious plasma instabilities. These instabilities generally scale as Iλ2. Therefore they are usually controlled by the use of lower intensities or shorter laser wavelength. But lower laser intensities are generally associated with thinner shell or double shell target designs,2 and these higher aspect ratio designs place severe requirements on laser symmetry and target stability. Smoothing out laser nonuniformities by lateral thermal conduction3 requires that the absorption-to-ablation distance be on the order of the target radius, leading to longer, not shorter wavelengths. This separation distance also produces a lower hydro-dynamic efficiency.

Laser-plasma interaction experiments and diagnostics at NRL

Laser plasma interaction experiments have now advanced to the point where very quantitative measurements are required to elucidate the physic issues important for laser fusion and other applications. Detailed time-resolved knowledge of the plasma density, temperature, velocity gradients, spatial structure, heat flow characteristics, radiation emission, etc, are needed over tremendou ranges of plasma density and temperature. Moreover, the time scales are very short, aggrevating the difficulty of the measurements further. Nonetheless, such substantial progress has been made in diagnostic development during the past few years that we are now able to do well diagnosed experiments. In this paper the authors review recent diagnostic developments for laser-plasma interactions, outline their regimes of applicability, and show examples of their utility. In addition to diagnostics for the high densities and temperature characteristic of laser fusion physics studies, diagnostics designed to study the two-stream interactions of laser created plasma flowing through an ambient low density plasma will be described.

Diagnostics Of Laser Fusion Physics Experiments

Laser fusion involves the compression using very high power laser beams of a pellet containing fusionable fuel, such as a deuterium-tritium mixture, to such high densities and temperatures that it ignites and yields a net energy gain. The deposited energy causes a plasma to ablate from the target surface which drives the implosion. The physics issues to achieve success are numerous; they include: the laser absorption and pellet surface acceleration processes must be benign and efficient; uniform megabar pressures must be gen-erated by the ablating plasma to accelerate the target shell inward with a velocity over 150 km/sec and with about 1% accuracy; throughout this implosion the fuel must remain cold. To study these physics issues a number of novel diagnostics gre required. They involve measurements of photon and particle energies from 1 eV to 10 5 eV with subnanosecond time-resolution and micron spatial resolution. Many of these diagnostic techniques and their applications in the NRL laser fusion experiment are described.