Beam Spray Thresholds in ICF-Relevant Plasmas.

Beam spray measurements suggest thresholds that are a factor of ≈2 to 15× less than expected based on the filamentation figure of merit often quoted in the literature. In this moderate-intensity regime, the relevant mechanism is forward stimulated Brillouin scattering. Both weak ion acoustic wave damping and thermal enhancement of ion acoustic waves contribute to the low thresholds. Forward stimulated Brillouin scattering imparts a redshift to the transmitted beam. Regarding the specific possibility of beam spray occurring outside the laser entrance holes of an indirectly driven hohlraum, this shift may be the most concerning feature owing to the high sensitivity of crossed-beam energy transfer to the interacting beam wavelengths in the subsequent overlap region.

Bootstrap estimation of the effect of instrument response function uncertainty on the reconstruction of fusion neutron sources.

Neutron imagers are important diagnostics for the inertial confinement fusion implosions at the National Ignition Facility. They provide two- and three-dimensional reconstructions of the neutron source shape that are key indicators of the overall performance. To interpret the shape results properly, it is critical to estimate the uncertainty in those reconstructions. There are two main sources of uncertainties: limited neutron statistics, leading to random errors in the reconstructed images, and incomplete knowledge of the instrument response function (the pinhole-dependent point spread function). While the statistical errors dominate the uncertainty for lower yield deuterium-tritium (DT) shots, errors due to the instrument response function dominate the uncertainty for DT yields on the order of 1016 neutrons or higher. In this work, a bootstrapping method estimates the uncertainty in a reconstructed image due to the incomplete knowledge of the instrument response function. The main reconstruction is created from the fixed collection of pinhole images that are best aligned with the neutron source. Additional reconstructions are then built using subsets of that collection of images. Variations in the shapes of these additional reconstructions originate solely from uncertainties in the instrument response function, allowing us to use them to provide an additional systematic uncertainty estimate.

A pulsed-laser calibration system for the laser backscatter diagnostics at the Omega laser.

A calibration system has been developed that allows a direct determination of the sensitivity of the laser backscatter diagnostics at the Omega laser. A motorized mirror at the target location redirects individual pulses of a millijoule-class laser onto the diagnostic to allow the in situ measurement of the local point response of the backscatter diagnostics. Featuring dual wavelength capability at the second and third harmonics of the Nd:YAG laser, both spectral channels of the backscatter diagnostics can be directly calibrated. In addition, channel cross-talk and polarization sensitivity can be determined. The calibration system has been employed repeatedly over the last two years and has enabled precise backscatter measurements of both stimulated Brillouin scattering and stimulated Raman scattering in gas-filled Hohlraum targets that emulate conditions relevant to those in inertial confinement fusion targets.

Controlling stimulated brillouin backscatter with beam smoothing in weakly damped systems.

We derive an analytical estimate of the effect of temporal smoothing of laser beams on stimulated Brillouin scattering (SBS) in a regime relevant to indirect drive ignition. We predict a strong reduction of SBS in the gold plasma expanding from the Hohlraum wall with temporal smoothing. This is a new regime far above threshold where the time to reach convective saturation allows for an effective contrast reduction of the beam intensity driving the instability. This result agrees with three dimensional simulations. Polarization smoothing is shown to double the effective bandwidth.

Breakdown of hot-spot model in determining convective amplification in large homogeneous systems.

Convective amplification in large homogeneous systems is studied, both analytically and numerically, in the case of a linear diffraction-free stochastic amplifier. Overall amplification does not result from successive amplifications in small scale high intensity hot spots, but from a single amplification in a delocalized mode of the driver field spreading over the whole interaction length. For this model, the hot-spot approach is found to systematically underestimate the gain factor by more than 50%.

Near-threshold reflectivity fluctuations in the independent-convective-hot-spot-model limit of a spatially smoothed laser beam.

In the framework of the independent-hot-spot model, it is shown that the reflectivity resulting from scattering instabilities when a spatially smoothed laser beam interacts with a plasma exhibits large statistical fluctuations near threshold. The importance of the fluctuations is discussed in terms of a confidence interval for the reflectivity, which is more relevant to experimental measurements than the average reflectivity. An analytical model for the fluctuating reflectivity is developed and shown to be in good agreement with numerical simulations. The influence of the transverse size of the interaction region is studied.