Target designs for energetics experiments on the National Ignition Facility

The goal of the first hohlraum energetics experiments on the National Ignition Facility (NIF) [G. H. Miller et al , Optical Eng. 43, 2841 (2004)] is to select the hohlraum design for the first ignition experiments. Sub-scale hohlraums heated by 96 of the 192 laser beams on the NIF are used to emulate the laser-plasma interaction behavior of ignition hohlraums. These ''plasma emulator'' targets are 70% scale versions of the 1.05 MJ, 300 eV ignition hohlraum and have the same energy-density as the full-scale ignition designs. Radiation-hydrodynamics simulations show that the sub-scale target is a good emulator of plasma conditions inside the ignition hohlraum, reproducing density ne within 10% and temperature Te within 15% along a laser beam path. Linear backscatter gain analysis shows the backscatter risk to be comparable to that of the ignition target. A successful energetics campaign will allow the National Ignition Campaign to focus its efforts on optimizing ignition hohlraums with efficient laser coupling

Laser Plasma instability reduction by coherence disruption

The saturation levels of stimulated scattering of intense laser light in plasmas and techniques to reduce these levels are of great interest. A simple model is used to highlight the dependence of the reflectivity on the coherence length for the density fluctuations producing the scattering. Sometimes the coherence lengths can be determined nonlinearly. For NIF hohlraum plasmas, a reduction in the coherence lengths might be engineered in several ways. Finally, electron trapping in ion sound waves is briefly examined as a potentially important effect for the saturation of stimulated Brillouin scattering

How much laser power can propagate through fusion plasma?

Propagation of intense laser beams is crucial for inertial confinement fusion, which requires precise beam control to achieve the compression and heating necessary to ignite the fusion reaction. The National Ignition Facility (NIF), where fusion will be attempted, is now under construction. Control of intense beam propagation may be ruined by laser beam self-focusing. We have identified the maximum laser beam power that can propagate through fusion plasma without significant self-focusing and have found excellent agreement with recent experimental data, and suggest a way to increase that maximum by appropriate choice of plasma composition with implication for NIF designs. Our theory also leads to the prediction of anti-correlation between beam spray and backscatter and suggests the indirect control of backscatter through manipulation of plasma ionization state or acoustic damping.Comment: 15 pages, 4 figures, submitted to Plasma Physics and Controlled Fusio

Laser-beam propagation in high temperature hohlraum plasmas

The authors have developed a new target platform to study propagation and backscatter of a frequency-doubled (2{omega}) laser beam through large-scale length plasmas at ignition-design densities, intensities and temperatures above 3 keV. The plasma is created by heating a gas filled hohlraum target with 37 heater beams that deliver a total energy of up to 15 kJ in a 1 ns square pulse. They measure a factor of two higher temperatures than in open geometry gasbag targets investigated earlier. This new temperature regime with a measured beam transmission of up to 80% suggests we can expect good laser coupling into ignition hohlraums at the National Ignition Facility (NIF) using 2{omega} light

Predictive three dimensional modeling of Stimulated Brillouin Scattering in ignition-scale experiments

The first three-dimensional (3D) simulations of a high power 0.351 {micro}m laser beam propagating through a high temperature hohlraum plasma are reported. We show that 3D linear kinetic modeling of Stimulated Brillouin scattering reproduces quantitatively the experimental measurements, provided it is coupled to detailed hydrodynamics simulation and a realistic description of the laser beam from its millimeter-size envelop down to the micron scale speckles. These simulations accurately predict the strong reduction of SBS measured when polarization smoothing is used

Laser-plasma interaction in ignition relevant plasmas: benchmarking our 3D modelling capabilities versus recent experiments

We have developed a new target platform to study Laser Plasma Interaction in ignition-relevant condition at the Omega laser facility (LLE/Rochester)[1]. By shooting an interaction beam along the axis of a gas-filled hohlraum heated by up to 17 kJ of heater beam energy, we were able to create a millimeter-scale underdense uniform plasma at electron temperatures above 3 keV. Extensive Thomson scattering measurements allowed us to benchmark our hydrodynamic simulations performed with HYDRA [1]. As a result of this effort, we can use with much confidence these simulations as input parameters for our LPI simulation code pF3d [2]. In this paper, we show that by using accurate hydrodynamic profiles and full three-dimensional simulations including a realistic modeling of the laser intensity pattern generated by various smoothing options, fluid LPI theory reproduces the SBS thresholds and absolute reflectivity values and the absence of measurable SRS. This good agreement was made possible by the recent increase in computing power routinely available for such simulations

Three dimensional modeling of Laser-Plasma interaction: benchmarking our predictive modeling tools vs. experiments

We have developed a new target platform to study Laser Plasma Interaction in ignition-relevant condition at the Omega laser facility (LLE/Rochester)[1]. By shooting an interaction beam along the axis of a gas-filled hohlraum heated by up to 17 kJ of heater beam energy, we were able to create a millimeter-scale underdense uniform plasma at electron temperatures above 3 keV. Extensive Thomson scattering measurements allowed us to benchmark our hydrodynamic simulations performed with HYDRA[2]. As a result of this effort, we can use with much confidence these simulations as input parameters for our LPI simulation code pF3d[3]. In this paper, we show that by using accurate hydrodynamic profiles and full three-dimensional simulations including a realistic modeling of the laser intensity pattern generated by various smoothing options, whole beam three-dimensional linear kinetic modeling of stimulated Brillouin scattering reproduces quantitatively the experimental measurements(SBS thresholds, reflectivity values and the absence of measurable SRS). This good agreement was made possible by the recent increase in computing power routinely available for such simulations. These simulations accurately predicted the strong reduction of SBS measured when polarization smoothing is used

Neutron time-of-flight measurements of charged-particle energy loss in inertial confinement fusion plasmas

Neutron spectra from secondary ^{3}H(d,n)α reactions produced by an implosion of a deuterium-gas capsule at the National Ignition Facility have been measured with order-of-magnitude improvements in statistics and resolution over past experiments. These new data and their sensitivity to the energy loss of fast tritons emitted from thermal ^{2}H(d,p)^{3}H reactions enable the first statistically significant investigation of charged-particle stopping via the emitted neutron spectrum. Radiation-hydrodynamic simulations, constrained to match a number of observables from the implosion, were used to predict the neutron spectra while employing two different energy loss models. This analysis represents the first test of stopping models under inertial confinement fusion conditions, covering plasma temperatures of k_{B}T≈1-4  keV and particle densities of n≈(12-2)×10^{24}  cm^{-3}. Under these conditions, we find significant deviations of our data from a theory employing classical collisions whereas the theory including quantum diffraction agrees with our data

Measurement of the dispersion of thermal ion-acoustic fluctuations in high-temperature laser plasmas using multiple wavelength Thomson-scattering

The dispersion of ion-acoustic fluctuations has been measured using a novel technique that employed multiple color Thomson scattering to measure the frequency spectrum for two separate thermal ion-acoustic fluctuations with significantly different wave vectors. The plasma fluctuations are shown to become dispersive with increasing electron temperature. They demonstrate that this technique allows a time resolved local measurement of electron density and temperature in inertial confinement fusion plasmas

Local Measurement of Electron Density and Temperature in High Temperature Laser Plasma Using the Ion-Acoustic Dispersion

The dispersion of ion-acoustic fluctuations has been measured using a novel technique that employs multiple color Thomson-scattering diagnostics to measure the frequency spectrum for two separate thermal ion-acoustic fluctuations with significantly different wave vectors. The plasma fluctuations are shown to become dispersive with increasing electron temperature. We demonstrate that this technique allows a time resolved local measurement of electron density and temperature in inertial confinement fusion plasmas