Raman backscatter as a remote laser power sensor in high-energy-density plasmas.

Stimulated Raman backscatter is used as a remote sensor to quantify the instantaneous laser power after transfer from outer to inner cones that cross in a National Ignition Facility (NIF) gas-filled hohlraum plasma. By matching stimulated Raman backscatter between a shot reducing outer versus a shot reducing inner power we infer that about half of the incident outer-cone power is transferred to inner cones, for the specific time and wavelength configuration studied. This is the first instantaneous nondisruptive measure of power transfer in an indirect drive NIF experiment using optical measurements.

Self-organized bursts of coherent stimulated Raman scattering and hot electron transport in speckled laser plasma media.

Nonlinear electron trapping physics governs the onset and saturation of stimulated Raman scattering (SRS) in laser beams with many speckles. Hot electrons from intense speckles, produced during SRS daughter electron-plasma-wave bowing and filamentation, seed and enhance the growth of SRS in neighboring speckles by reducing Landau damping. Trapping-induced nonlinearity and speckle interaction through transport of hot electrons and back- and sidescattered SRS waves enable the system of speckles to self-organize and exhibit coherent, sub-ps SRS bursts with more than 100% instantaneous reflectivity, consistent with a SRS transverse coherence width much larger than a speckle width.

A dual high-energy radiography platform with 15 µm resolution at the National Ignition Facility.

To study matter at extreme densities and pressures, we need mega laser facilities such as the National Ignition Facility as well as creative methods to make observations during timescales of a billionth of a second. To facilitate this, we developed a platform and diagnostic to characterize a new point-projection radiography configuration using two micro-wires irradiated by a short pulse laser system that provides a large field of view with up to 3.6 ns separation between images. We used tungsten-carbide solid spheres as reference objects and inferred characteristics of the back-lighter source using a forward-fitting algorithm. The resolution of the system is inferred to be 15 µm (using 12.5 µm diameter wires). The bremsstrahlung temperature of the source is 70-300 keV, depending on laser energy and coupling efficiency. By adding the images recorded on multiple stacked image plates, the signal-to-noise of the system is nearly doubled. The imaging characterization technique described here can be adapted to most point-projection platforms where the resolution, spectral contrast, and signal-to-noise are important.

Amplification of light in a plasma by stimulated ion acoustic waves driven by multiple crossing pump beams.

Experiments demonstrate the amplification of 351 nm laser light in a hot dense plasma similar to those in inertial confinement fusion ignition experiments. A seed beam interacts with one or two counter-propagating pump beams, each with an intensity of 1.2×10(15) W/cm2 at 351 nm, crossing the seed at 24.8° at the position where the flow is Mach 1, allowing resonant stimulation of ion acoustic waves. Results show that the energy and power transferred to the seed are increased with two pumps beyond the level that occurs with a single pump, demonstrating that, under conditions similar to ignition experiments where each beam has a low gain exponent, the total scatter produced by the multiple beams can be significantly larger than that of the individual beams. It is further demonstrated that the amplification is greatly reduced when the pump polarization is orthogonal to the seed, as expected from models of stimulated scatter.

Green frequency-doubled laser-beam propagation in high-temperature hohlraum plasmas.

We demonstrate propagation and small backscatter losses of a frequency-doubled (2omega) laser beam interacting with inertial confinement fusion hohlraum plasmas. The electron temperature of 3.3 keV, approximately a factor of 2 higher than achieved in previous experiments with open geometry targets, approaches plasma conditions of high-fusion yield hohlraums. In this new temperature regime, we measure 2omega laser-beam transmission approaching 80% with simultaneous backscattering losses of less than 10%. These findings suggest that good laser coupling into fusion hohlraums using 2omega light is possible.

Intensity limits for propagation of 0.527 microm laser beams through large-scale-length plasmas for inertial confinement fusion.

We have established the intensity limits for propagation of a frequency-doubled (2omega, 527 nm) high intensity interaction beam through an underdense large-scale-length plasma. We observe good beam transmission at laser intensities at or below 2x10(14) W/cm(2) and a strong reduction at intensities up to 10(15) W/cm(2) due to the onset of parametric scattering instabilities. We show that temporal beam smoothing by spectral dispersion allows a factor of 2 higher intensities while keeping the beam spray constant, which establishes frequency-doubled light as an option for ignition and burn in inertial confinement fusion experiments.

Thomson scattering measurements of saturated ion waves in laser fusion plasmas.

We have measured the characteristics of saturated ion-acoustic waves in inertial confinement fusion plasmas. A 263-nm probe laser has been applied to simultaneous Thomson scatter on both ion-acoustic waves excited by thermal electrostatic fluctuations and by stimulated Brillouin scattering of a kilojoule laser beam of varying intensity. The Thomson scattering spectra show saturated ion-wave amplitudes for intensities above 5x10(14) W cm(-2) consistent with three dimensional nonlinear wave modeling.

Anomalous absorption of high-energy green laser light in high- z plasmas.

We observe strong anomalous absorption of green laser light in mm-scale high-temperature gold plasmas. Both the laser light absorption and the resulting increase of the electron temperature, which was measured independently with Thomson scattering, have been successfully modeled by including enhanced collisions due to heat-flux driven ion acoustic fluctuations. Calculations that include only inverse bremsstrahlung significantly underestimate the experimental laser absorption and the electron temperature.

Backscatter reduction using combined spatial, temporal, and polarization beam smoothing in a long-scale-length laser plasma.

Spatial, temporal, and polarization smoothing schemes are combined for the first time to reduce to a few percent the total stimulated backscatter of a NIF-like probe laser beam (2x10(15) W/cm (2), 351 nm, f/8) in a long-scale-length laser plasma. Combining temporal and polarization smoothing reduces simulated Brillouin scattering and simulated Raman scattering (SRS) up to an order of magnitude although neither smoothing scheme by itself is uniformly effective. The results agree with trends observed in simulations performed with the laser-plasma interaction code F3D simulations [R. L. Berger et al., Phys. Plasma 6, 1043 (1999)].

Observation of saturation of energy transfer between copropagating beams in a flowing plasma.

Experiments demonstrate energy and power transfer between copropagating, same frequency, beams crossing at a small angle in a plasma with a Mach 1 flow. The process is interpreted as amplification of the low intensity probe beam by the stimulated scatter of the high intensity pump beam. The observed probe amplification increases slowly with pump intensity and decreases with probe intensity, indicative of saturation limiting the energy and power transfer due to ion-wave nonlinearities and localized pump depletion. The results are consistent with numerical modeling including ion-wave nonlinearities.