W. C. Mead

A model for laser driven ablative implosions

A theoretical model is presented describing the spatial structure and scaling laws of laser driven ablative implosions. The effect of inhibited electron thermal transport is explicitly included. The theory is in excellent agreement with results from a computer hydrodynamics code, under conditions when heat flow is flux‐limited at the critical surface and suprathermal electrons do not form a dominant energy transport mechanism.

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.

Laser irradiation of disk targets at 0.53 μm wavelength

Results and analyses are presented for laser irradiation of Be‐, CH‐, Ti‐, and Au‐disk targets with 0.53 μm light in 3–200 J, 600–700 psec pulses, at nominal incident intensities from 3×1013 to 5×1015 W/cm2. The measured absorptions are higher than observed in similar 1.06 μm irradiations, and are largely consistent with modeling which shows the importance of inverse‐bremsstrahlung and Brillouin scattering. Observed red‐shifted back‐reflected light shows that Brillouin scattering occurs at low to moderate levels. Backscattering fractions up to 30% were observed in the f/2 focusing lens. The measured fluxes of multi‐keV x rays indicate hot‐electron fractions of 1% or less, with temperatures of 6 to 20 keV which are consistent with resonance absorption or perhaps 2ωpe. Measurements show 30%–50% efficient conversion of absorbed light into sub‐keV x rays, with time‐, angular‐, and spatial‐emission distributions which are generally consistent with non‐local‐thermodynamic‐equilibrium modeling using inhibited th...

Irradiation of parylene disks with a 1.06 μm laser

Parylene (C8H8) disks have been irradiated with Nd:YAG‐glass laser pulses focused to flux levels in the 1015 to 1017 W/cm2 range. The flux level was varied by changing the pulse length (50–150 psec), the laser energy (5–15 J), and the axial position of the target with respect to the f/1.1 focusing lens. An extensive array of diagnostics was used to measure the temporal and energy distribution of the focused laser light at the target, the temporal and angular distribution of the scattered laser light, the x‐ray spatial and spectral emission characteristics, and the emitted ion and electron energy distributions. The experimental results, together with two‐dimensional numerical simulations imply absorption via collective processes, laser generation of suprathermal electrons, and transport inhibition consistent with the presence of mega‐Gauss level thermoelectric magnetic fields.

Characteristics of lateral and axial transport in laser irradiations of layered‐disk targets at 1.06 and 0.35 μm wavelengths

Results and analysis are presented for Be‐on‐Al disk target irradiations at 1.06 and 0.35 μm laser wavelengths with 600–700 psec pulses, 240 μm spot diameter, and 1×1014 W/cm2 absorbed intensity. Absorptions of 32%–39% (1.06 μm) and 90% (0.35 μm) are largely due to inverse bremsstrahlung. The hard x‐ray spectra indicate low hot‐electron fractions of 10−2 (1.06 μm) and 10−4 (0.35 μm). Backreflected light shows strong hot spots for 0.35 μm irradiations. Multiple absolute and relative x‐ray measurements are compared with one‐ and two‐dimensional computer hydrodynamics calculations. Only weak indications of lateral transport are found and limits are set from x‐ray imaging and spectral data from targets with and without a surrounding Ti shield. Axial transport appears strongly inhibited at 1.06 μm and mildly inhibited at 0.35 μm wavelength. Measured shock‐wave transit times and velocities imply ablation pressures of 7 Mbar (1.06 μm) and 11 Mbar (0.35 μm).

Theoretical interpretation of angle- and polarization-dependent laser light absorption measurements

It is shown that recently published observations of angle‐ and polarization‐dependent absorption of intense laser light are consistent with computer simulations of resonance absorption in a steepened plasma profile, with the additional assumption of a modestly rippled critical surface.

Effect of symmetry requirements on the wavelength scaling of directly driven laser fusion implosions

The creation of a large, hot atmosphere surrounding a laser fusion target can produce symmetrization of incident beam illumination non-uniformities. This paper discusses the effect of laser wavelength on atmosphere formation and it is shown that for short-wavelength lasers there can be a significant penalty in implosion efficiency when the atmosphere is kept large enough to produce adequate symmetrization. Several ways to improve this situation are considered.

Streaked Spectrometry Using Multilayer X‐Ray Interference mirrors to investigate Energy Transport in Laser Plasma Applications

Transport of energy in laser-produced plasmas is scrutinized by devising spectrally and temporally identifiable characteristics in the x-ray emission history which identify the heat-front position at various times in the heating process. Measurements of the relative turn-on times of these characteristics show the rate of energy transport between various points. These measurements can in turn constrain models of energy transport phenomena. We are time-resolving spectrally distinguishable subkilovolt x-ray emissions from different layers of a disk target to examine the transport rate of energy into the target. A similar technique is used to measure the lateral expansion rate of the plasma spot. A soft x-ray streak camera with 15-psec temporal resolution is used to make the temporal measurements. Spectral discrimination of the incident signal is provided by multilayer x-ray interference mirrors.

Scaling of laser-plasma interactions with laser wavelength and plasma size

Laboratory experiments on the wavelength scaling of laser-plasma interactions have thus far been restricted to relatively modest laser energies (EL ≤ 200 J), short pulse lengths (τL ≤ 2 nsec), and small focal spot diameters (d ≲, 200 µm). Thus the plasma sizes have been quite modest. Under these conditions, wavelength scaling data using Livermore’s Argus laser show classical absorption physics and little evidence of instabilities, in good agreement with simple models and with computer hydrodynamics calculations. Absorption and preheat behave much more favorably at short laser wavelengths.

Observations of Parametric Instabilities in Long Scalelength Plasmas

The role of laser wavelength in laser-plasma coupling physics has been extensively studied at many laboratories for the past several years.(1) These studies have been motivated by theoretical considerations, which indicate improved absorption, reduced suprathermal electron production, and increased hydrodynamic efficiencies for laser wavelengths ≲ 1 µm.(2) Indeed, while such improvements have been experimentally verified, the limited energy and power presently available (EL ≲ 150 Joules, PL ≲ 1011W) have restricted the studies to plasma coronas of dimensions ≲ 100 λo (λo is the vacuum laser wavelength). Such small coronas are not characteristic of near-term and future ICF targets designed for ignition and gain.(3) These targets will have diameters greater than 1 mm and they will be irradiated with laser pulses of several nanosecond duration (3 ≲ τL ≲ 10 ns).(3) Thus the targets will be enveloped by underdense plasma coronas having dimensions of 103–104 vacuum laser wavelengths. In such plasmas, coupling processes, such as inverse bremsstrahlung, Stimulated Raman and Brillouin Scattering, two-plasmon decay, and filamentation (ponder-motive and thermal), should become important.(4)