K. Mizuno

Distributed absorption model for moderate to high laser powers

An analytic model is presented of a planar plasma heated by moderate to high laser powers such as are used to drive fusion pellets. Unlike previous models, this model explicitly includes the temporal evolution of the heat conduction region. It is shown that previous steady‐state models apply only to a narrow range of the parameters (laser energy flux and pulse width) used in laser pellet fusion. The new model is shown to agree with flux‐limited hydrodynamics simulations. The model and hydrodynamic simulations for classical heat transport show that the mass ablation rate and ablation pressure are essentially independent of laser wavelength for parameters relevant to laser fusion.

Density and temperature profiles in strongly absorbing plasma with distributed absorption

A model is presented of the spatial structure of a laser‐heated, strongly absorbing, planar plasma. Inverse bremsstrahlung is very strong so that laser light absorption is not localized. Absorption is distributed over densities from well below critical to the critical density. It is shown that the spatial structure of the plasma is self‐consistent with laser energy deposition and heat transport so measurement of the plasma structure can be used as diagnostic of absorption and transport. Nonphysical discontinuities in density and temperature at the critical surface that are predicted by previous local absorption models for strongly flux‐limited heat transport are reduced. These jumps persist in the present model. It is shown that an improved flux‐limited heat transport model, strongly limited in the underdense plasma and weakly limited in the overdense plasma, results in continuous density and temperature profiles.

High-power and superpower Cerenkov masers

The linear and nonlinear theory of the efficient operation of high-power (gigawatt) and superpower (50 GW) Cerenkov masers is presented. Issues such as breakdown, plasma production, and coupling to the output device are discussed. The relative merits of dielectric Cerenkov masers and plasma Cerenkov masers are considered. The principal design tool is a new particle simulation model that was developed to investigate Cerenkov masers. The novel aspects of this model are briefly described along with a comparison of calculated and experimental results. The agreement between calculations and measurements is generally good. Designs for a high-power and superpower plasma Cerenkov masers are also described. >

Anomalous thermal electron heating and heat transport inhibition due to parametric instabilities

Measurements are presented of strong thermal electron heating and the heat transport inhibition. An electron plasma wave heats hot electrons near the critical density. A return current is induced to keep charge neutrality. Thermal electrons are heated strongly by the resistivity of parametrically excited isotropic ion wave turbulence (anomalous joule heating). The heat transport of thermal electrons is also inhibited by the resistivity. The experimental results agree reasonably well with theory.

Final report of investigation of the Acoustic Decay Instability in laser plasma interaction

we have made extensive studies of the Ion Acoustic Decay Instability (IADI) in laser-produced plasmas using the Janus (Phoenix) laser at LLNL. We found that the threshold is quite low and that, in planar plasmas, it can be reduced to homogeneous-plasma, collisional values. These observations are consistent with the plasma-density profiles calculated by hydrodynamic simulations using the LASNEX computer code run with a flux limiter of f = 0.1. We have designed experiments to study the IADI in larger plasmas using the Nova laser. 2 refs., 1 fig.

Spectroscopic measurements of ion acoustic decay instabilities in laser produced plasma

Summary form only given. Extensive studies of spectroscopic measurements of IADI (ion-acoustic decay instability) in laser-produced plasma have been made. The experiments were performed using the GDL (gas dynamic laser) laser facility at LLE and the Janus (Phoenix) laser and NOVA laser facilities at LLNL. The laser is incident normally onto a planar target (CH, Al, Cu, Mo and Au of 50-μm thickness) with a 1-ns FWHM Gaussian pulse and a maximum energy of 200 J. The IADI was studied by monitoring the Stokes sideband of the backscattered (0 and 45°) spectrum near the second harmonic of the laser light. Two spectrometers were used to measure the angular dependence of the time integrated 2ω signal. Extensive studies of IADI have been made: (1) measurements of the threshold vs. laser spot size, (2) laser wavelength scaling (1-μm and 0.5-μm laser irradiations), (3) plasma flow effect, measurements of the angular dependence of 2ω signal, (4) measurements of IADI in high-Z target, ionic charge state Z, and IADI vs. target material, and (5) the measurements of IADI in the high-laser-intensity regime. The thresholds of the IADI are quite low, even with the high-Z target, so that IADI is potentially important in laser-produced plasma

Finite bandwidth drive effect on the parametric decay instability near the lower hybrid frequency

The effect of a finite bandwidth driver on hot electron production near the lower hybrid frequency is studied experimentally. The heating rate slowly decreases and the threshold for hot electron production slowly increases with the increasing driver bandwidth.