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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.

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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.

There is a large region of density-temperature space in which the effects of a magnetic field on heat transport and alpha-particle mobility are significant and the magnetic pressure is small compared with the pressure of a deuterium-tritium plasma. Spherical fusion burn in this regime is examined. It is found that for volume burn, magnetic fields can greatly increase the yield. In regimes where propagating burn does not occur, the burn can be enhanced by a magnetic field. In regimes where propagating deflagration would normally occur in the absence of a magnetic field, magnetic fields actually degrade the cross-field propagation. A detonation wave is harder to ignite in the presence of a magnetic field. Once a detonation wave is ignited, no change in the propagation speed is produced by applying a magnetic field.

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. Simple expressions describe the ablation rate and pressure as a function of laser intensity and wavelength. The analytic results are supported by extensive comparisons with numerical hydrodynamic simulations.PACS numbers: 52.50.Jm Many workers have discussed scaling laws for laser-driven "exploding-pusher" targets, where heat is carried predominantly by "suprathermal" electrons. 1 In the present paper, we derive scaling laws for "ablative" targets, where heat is carried predominately by "thermal"-electron conduction.We calculate analytically for spherical geometry in steady state the ablation rate, ablation pressure, and critical radius as functions of laser intensity, wavelength, and target size. We explicitly include effects of inhibited heat transport in a form suggested by a large body of experimental data. 2 Previous work on spherical laser-driven ablation was largely computational, 3 and assumed that thermal transport inhibition is not important. 4 ' 5 The present model may be roughly applied to planar targets by setting the effective ablation radius equal to the laser-spot diameter. 6 The ablative flow.-Classical expressions 7 for conductivity and heat flux q are valid when the scale length for temperature variations, L T , is much longer than the electron-ion mean free path. When gradients are so steep that L T becomes less than a mean free path, the classical expression for q implies that the characteristic speed for heat flow is much faster than the electron thermal speed. This seems physically unreasonable, at least for electron distribution functions close to Maxwellian. A common remedy has been to postulate an upper limit on the heat flux in this regime. Frequently one expresses this "saturated" magnitude of the heat flux, q^, in terms of the electron thermal speed v te = (kT e /m e ) 1/2 :

Disk targets of Be, CH, Ti, and Au have been irradiated with 0.53-pm laser light in 3-30-J, 600-ps pulses, at intensities from 3&&10 to -4&&10 W/om . The measured absorptions, hard-x-ray fluxes, and sub-keV emission properties are compared with hydrocode simulations. The results show strong collisional absorption, some Brillouin scattering, little suprathermal electron production, and efficient conversion of absorbed energy into sub-keV x rays, in general accord with wavelength-scaling predictions.PACS numbers: 52.50.Jm Many different driver-and target-design options have been proposed for achieving inertial confinement fusion. The use of submicron-wavelength laser drivers has been predicted to offer several advantages over longer-wavelength lasers. ' ' With a submicron laser, energy deposition occurs in higher-density, cooler plasma and this should improve the laser-plasma coupling characteristics. The absorption region is calculated to be more collisional, increasing the inverse bremsstrahlung absorption and reducing many of the deleterious effects of plasma instabilities. " In addition, overdense plasma heating and ablation rates should be increased, " and the sensitivity of these rates to electron transport inhibition decreased. ' However, the subtlety of competing processes makes it very important to study submicron laser-plasma interactions with use of experiments and hydrodynamic-code calculations to test these predictions.Experiments performed at Ecole Polytechnique were the first to indicate increased absorption, enhanced thermal heating, and reduced suprathermal-electron temperatures. More recently, other laboratories have presented further evidence for enhancement of laser-plasma coupling at shorter wavelengths 'The w. ork reported here is the first to study the interaction of submicron laser light with high-Z targets, to measure the efficiency of subsequent conversion into sub-keV x rays, and to make comparisons with detailed computer hydrodynamics calculations.Measurements have been performed to investigate absorption, hot-electron production, and conversion of absorbed energy into sub-keV x rays. We have irradiated 600-p m-diam by 20-p. m-thick disk targets of Be, CH, Ti, and Au with 600-ps near-Gaussian pulses of 0.53-p m light from the frequency-doubled Argus Nd-glass laser.With f/2 focusing optics, intensities were varied from 3 &&10" to -4 &&10" W/cm' by using spot diameters of -50 to 450 pm, and laser energies from 3 to 35 J. 6 Throughout this work, we have used LASN~X ' one-and two-dimensional Lagrangian hydrodynamics simulations to help in understanding the basic interaction processes. The modeling used here, including non-local-thermodynamic-equilibrium atomic physics, single-group thermal electron transport, and multigroup flux-limited diffusion for the suprathermal (and all laser-heated) electrons, is largely consistent with that used previously. ' Inverse bremsstrahlung absorption is calculated by using the classical absorption opacity, though some nonlinear effects remain to be incorpora...

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