M. J. Boyle

The interaction of 1.06 μm laser radiation with high Z disk targets

Gold disks have been irradiated with 1.06 μm laser light at intensities between 7 × 1013 and 3 × 1015 W/cm2, and pulse lengths between 200 and 1000 psec. Due to the high Z and long pulse, inverse bremsstrahlung becomes an important absorption mechanism and competes strongly with resonance absorption and stimulated scattering. In addition to measured absorptions, data on the temporal, spatial, angular, and spectral characteristics of the x‐ray emission are presented. Temporally and spectrally resolved back‐reflected light, and polarization‐dependent sidescattered light are detected, providing estimates for the amount of stimulated scattering and of the coronal electron temperature. Inhibited electron thermal conduction and nonlocal thermodynamic equilibrium ionization physics play key roles in bringing numerical simulations of these experiments into agreement with all of the above‐mentioned data.

Calibrated ’’four‐color’’ x‐ray microscope for laser plasma diagnostics

Four quartz, orthogonal, cylindrical mirror pairs, two of which are coated with nickel, image the x‐ray emission from laser fusion targets on hard film with a magnification of 3. K‐edge filters used in conjunction with the mirror pairs permit us to take simultaneous pictures in four energy bands between 0.7 and 3.5 keV. We have measured microscope resolution, mirror reflection efficiency, and film sensitivity and used them to deduce the absolute emissivity and spectral characteristics of various laser fusion targets. This instrument is now used routinely for studying laser‐generated plasmas at Lawrence Livermore Laboratory.

Evidence of localized heating in CO2‐laser‐produced plasmas

Polyethylene foils and parylene disks have been irradiated by CO2 (λ∼10.6 μm) and Nd : YAG‐glass (λ∼1.06 μm) laser pulses focused to flux levels in the 1013‐ and 1014‐W/cm2 range. X‐ray pinhole photographs of the CO2‐laser‐produced plasmas exhibit intense localized emission regions whose characteristic dimensions are smaller than the nearly diffraction‐limited focal spot of the laser. Corresponding photographs of the glass‐laser‐produced plasmas show no evidence of localized emission. These experimental results are consistent with theoretical predictions for laser‐beam trapping and/or filamentation in laser‐produced plasmas.

Interaction of 1.06 μm laser radiation with variable Z̃ targets

Parylene (C8H8) and tungsten‐glass (W2O/P2O5) disks have been irradiated with 150–400 psec Nd:YAG‐glass laser pulses focused to diameters of 250–300 μm with flux levels in the 1013–1015 W/cm2 range. An extensive array of diagnostics was used to measure the temporal and energy distributions of the focused laser light at the target, the angular distribution of the scattered laser light, the x‐ray spatial and spectral emission characteristics, and the emitted ion and electron distributions. Analysis of the experimental results indicates that the laser‐plasma interaction was characterized by a variety of collective phenomena which appeared stronger in the tungsten‐glass experiments.

Evidence for profile steepening in laser‐irradiated plasmas

Experimental evidence of a steepened electron density profile near critical density has been obtained by studying the light scattered by targets (10‐μm thick disks and 100‐μm diam glass spherical microshells filled with deuterium and tritium gas) illuminated by linearly polarized, 1.06‐μm light. Scale lengths on the order of 1 μm have been inferred both from the polarization of the reflected light and from the azimuthal asymmetry (asymmetry about the beam axis) of the time‐integrated scattered light with respect to the laser electric field. Azimuthally asymmetric heating of the microshells targets is indicated both by x‐ray micrographs and by the spatial distribution of the plasma blowoff.

Imaging characteristics of an axisymmetric, grazing incidence x‐ray microscope designed for laser fusion research

We report herein the first demonstration of the x-ray imaging characteristics of an axisymmetric x-ray microscope designed for laser fusion diagnostics. The design, fabrication, and evaluation of a grazing incidence hyperboloid/ellipsoid x-ray mirror pair are discussed. Initial data relating the measured mirror surface profiles, the estimated surface roughness, and mechanical alignment of the x-ray mirror pair to the observed spatial resolution are presented. A revised fabrication approach, utilizing a single point diamond turning feedback technique and resulting in improved resolution, is also discussed.

Use of a radioactive tracer to determine the fraction of fusion target debris collected

In order to obtain the pusher attenuation length (ρΔR) of inertial confinement fusion targets by neutron activation, it is necessary to know what fraction of the target debris is counted. One method of measuring this quantity experimentally is by means of a radioactive tracer. We demonstrate the usefulness of this technique by experiments utilizing a 24Na tracer.

Laser‐fusion experiments utilizing a 4π illumination system

A focusing system which utilizes two f/0.47 doublets in conjunction with ellipsoidal mirrors produces two focusing cones with half‐angles of 81.5°. This system has been used with the Lawrence Livermore Laboratory Janus 1.06‐μm laser to irradiate and implode DT‐filled glass microshells of ∼70 μm diameter. The purpose of the system was to provide more uniform heating of the pusher and more uniform compression of the DT fuel than had been obtained when irradiating targets using f/1 lenses. Neutron yields of ∼107 per event have been obtained at power levels of ∼0.4 TW. X‐ray micrographs show that improved uniformity of heating and implosion sphericity are achieved. Data are also presented which add further confirmation of the importance of absorption by plasma wave resonance for non‐normal incidence of the laser light on the target surface.

What is required for collimated point-source x-ray lithography to achieve an economically viable throughput?

The wafer throughput performance of collimated point-source proximity x-ray lithography was evaluated for features sizes that range from 0.1 to 0.25 micrometers . We analyzed the performance of parabolic collimating optics that rely on multilayer coatings for their x-ray reflection properties. The resulting x-ray transport properties of the collimator were then used to simulate the performance of a lithography system that incorporated a laser produced plasma x-ray source, a lithographic mask, photoresist, and staging subsystems. These system studies make it clear that modest advances in source intensity, mask technology, and photoresist sensitivity are required for point source proximity x-ray lithography to achieve economically viable wafer throughputs of 50 six-inch wafer levels per hour.