F. C. Young

Laser‐plasma interaction and ablative acceleration of thin foils at 1012–1015 W/cm2

The interaction physics and hydrodynamic motion of thin‐foil targets irradiated by long, low‐flux Nd‐laser pulses (3 nsec, 1012–1015 W/cm2) are studied experimentally and compared with theoretical models. Laser light absorption is high (80%–90%) and thin‐foil targets are accelerated up to 107 cm/sec with good (20%) hydrodynamic efficiency in the 1012–1013 W/cm2 range. These results agree with a simple rocket ablation model. Details of thermal heat flow, both axially (related to ablation depth) and laterally (related to beam uniformity requirements), are also presented.

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.

Laser‐produced‐plasma energy transport through plastic films

The transport of energy from a 1.06‐μm, 95‐psec laser pulse at an irradiance of 1015 W/cm2 through a thin layer of polystyrene into an Al substrate was studied by x‐ray, ion, and scattered‐light measurements. The intensities of the following quantities were measured as a function of polystyrene thickness: (1) x‐ray line radiation from the Al backing, (2) bremsstrahlung continuum from 3 to 88 keV, (3) ions of several keV energy, and (4) scattered laser light. The results indicate that a polystyrene thickness of no more than 0.5 μm is sufficient to inhibit substantial heating of the Al substrate.

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.

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.