K. G. Estabrook

Hot electron production and heating by hot electrons in fast ignitor research

In an experimental study of the physics of fast ignition the characteristics of the hot electron source at laser intensities up to 10(to the 20th power) Wcm{sup -2} and the heating produced at depth by hot electrons have been measured. Efficient generation of hot electrons but less than the anticipated heating have been observed.

J×B heating by very intense laser light

Plasma heating by the oscillating component of the pondermotive force of a very intense light wave is discussed.

Two‐dimensional relativistic simulations of resonance absorption

Resonant absorption has been simulated for radiation of energy density E20/4πnT ranging from much less than to somewhat greater than unity. Characteristic features of the absorption process are an absorption efficiency of approximately 50%, generation of suprathermal particles, and strong modification of the density profile in the vicinity of the critical density. The latter effect, the appearance of a finite density variation over the distance of a few Debye lengths, is forced by strong gradients in the plasma wave intensity and electron temperature. Such a density discontinuity greatly enhances the range of incidence angles for which resonance absorption is effective and decreases the effects of the oscillating two‐stream and ion‐acoustic decay instabilities.

Theory and simulation of one‐dimensional Raman backward and forward scattering

Computer simulations and theory are used to illustrate the linear and nonlinear behavior or Raman backward and forward scattering in laser‐irradiated plasmas. The heated‐electron distributions, nonlinear Landau damping, bandwidth of the scattered light, and other results of nonlinear simulations in homogeneous and nonhomogeneous plasmas are described. The characteristics of Raman scattering in a plasma with a quite high background temperature (64 keV) are examined, which are relevant to CO2‐heated plasmas. Finally the effects of self‐generated magnetic fields are pointed out, which can reduce Raman scattering by inhibiting the transport of hot electrons, and the reduction of Raman scattering by use of laser light with a large bandwidth is described.

Supernova hydrodynamics experiments on the Nova laser

In studying complex astrophysical phenomena such as supernovae, one does not have the luxury of setting up clean, well-controlled experiments in the universe to test the physics of current models and theories. Consequently, creating a surrogate environment to serve as an experimental astrophysics testbed would be highly beneficial. The existence of highly sophisticated, modern research lasers, developed largely as a result of the world-wide effort in inertial confinement fusion, opens a new potential for creating just such an experimental testbed utilizing well-controlled, well-diagnosed laser-produced plasmas. Two areas of physics critical to an understanding of supernovae are discussed that are amenable to supporting research on large lasers: (1) compressible nonlinear hydrodynamic mixing and (2) radiative shock hydrodynamics.

Laser–plasma interactions in ignition‐scale hohlraum plasmas

Scattering of laser light by stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS) is a concern for indirect drive inertial confinement fusion (ICF). The hohlraum designs for the National Ignition Facility (NIF) raise particular concerns due to the large scale and homogeneity of the plasmas within them. Experiments at Nova have studied laser–plasma interactions within large scale length plasmas that mimic many of the characteristics of the NIF hohlraum plasmas. Filamentation and scattering of laser light by SBS and SRS have been investigated as a function of beam smoothing and plasma conditions. Narrowly collimated SRS backscatter has been observed from low density, low‐Z, plasmas, which are representative of the plasma filling most of the NIF hohlraum. SBS backscatter is found to occur in the high‐Z plasma of gold ablated from the wall. Both SBS and SRS are observed to be at acceptable levels in experiments using smoothing by spectral dispersion (SSD).

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...

Stimulated Raman scattering, two‐plasmon decay, and hot electron generation from underdense plasmas at 0.35 μm

Experimental studies of stimulated Raman back and side scattering, two‐plasmon decay, and the generation of high‐energy electrons in 0.35 μm laser plasma interaction are presented. To isolate the various phenomena occurring at different densities, we have attempted to control the maximum plasma density by varying the thickness of the foil targets. The scattered light frequency is used as a diagnostic to measure the peak plasma density. Time integrated and time resolved scattered spectra for variable plasma densities are discussed. Effects of self‐generated magnetic fields and plasma temperature on SRS and 2ωp decay, respectively, are examined as possible mechanisms responsible for splitting of the backscattered spectrum at ω0/2. A discussion of the effects of a parabolic density profile on the SRS threshold is also included. Finally, the measured energy and angular distribution of the high‐energy electrons are discussed. Two‐plasmon decay is suggested as the probable mechanism generating the hot electrons.

Thomson scattering from laser plasmas

Thomson scattering has recently been introduced as a fundamental diagnostic of plasma conditions and basic physical processes in dense, inertial confinement fusion plasmas. Experiments at the Nova laser facility [E. M. Campbell et al., Laser Part. Beams 9, 209 (1991)] have demonstrated accurate temporally and spatially resolved characterization of densities, electron temperatures, and average ionization levels by simultaneously observing Thomson scattered light from ion acoustic and electron plasma (Langmuir) fluctuations. In addition, observations of fast and slow ion acous- tic waves in two-ion species plasmas have also allowed an independent measurement of the ion temperature. These results have motivated the application of Thomson scattering in closed-geometry inertial confinement fusion hohlraums to benchmark integrated radiation-hydrodynamic modeling of fusion plasmas. For this purpose a high energy 4{omega} probe laser was implemented recently allowing ultraviolet Thomson scattering at various locations in high-density gas-filled hohlraum plasmas. In partic- ular, the observation of steep electron temperature gradients indicates that electron thermal transport is inhibited in these gas-filled hohlraums. Hydrodynamic calcula- tions which include an exact treatment of large-scale magnetic fields are in agreement with these findings. Moreover, the Thomson scattering data clearly indicate axial stagnation in these hohlraums by showing a fast rise of the ion temperature. Its timing is in good agreement with calculations indicating that the stagnating plasma will not deteriorate the implosion of the fusion capsules in ignition experiments.

Stimulated Raman scattering in large plasmas

In long pulse, high‐energy experiments (4000 J, 2 nsec, 5×1014 W/cm2, 1.064 μm) at the Shiva laser facility, several percent of the laser light has been observed to be Raman scattered. The spectrum of the Raman‐scattered light was measured from 1.48 to 2.17 μm. The Raman scattering occurred principally at electron densities much lower than the quarter‐critical electron density. The high‐energy electrons expected in Raman scattering were observed indirectly in these experiments via their bremsstrahlung radiation. Additional experiments show that the Raman instability has a much lower intensity threshold for longer laser pusle length and larger laser spot size. Raman light measurements for 5320 A irradiated disk target experiments are also reported. The light near 2λ0 or 1.064 μm had both a red‐ and blue‐shifted component. At high intensities, Raman scattering also occurred in the very underdense plasma.