Session GP18: Poster Session: ICF: Laser-Plasma Interactions (9:30am - 12:30pm)

Modeling many-beam laser plasma interactions using an adaptive paraxial scheme

Modeling the interaction of many laser beams in a hohlraum geometry via crossed-beam energy transfer (CBET) necessitates propagating beams with crossing angles surpassing 100 degrees. We demonstrate a modified paraxial scheme where many beams with large crossing angles may be propagated on a shared numerical grid, permitting efficient and accurate computation of energy exchange and polarization rotation due to CBET, diffraction, and refraction. Far-field boundary conditions are generated using numerical reproductions of experimentally characterized phase plates installed at the National Ignition Facility, allowing a faithful representation of the speckled laser spots. The numerical scheme adapts to the evolving direction of propagation of each beam, permitting beams to refract through large angles. We demonstrate laser beams undergoing ``glint’’ from the hohlraum wall, featuring a 100 degree change in beam propagation direction. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.   

Higher Dimensional Effects in Laser Plasma Interactions Relevant to Inertial Fusion

In inertial confinement fusion, laser plasma interactions, where the incident laser decays into a backward going light wave and a collective mode of the plasma can reduce laser coupling by reflecting the incident laser and also cause pre-heat which can can degrade compression. In SRS and SBS, the instability itself is primarily one dimensional, meaning that the scattered light and the plasma waves both travel in the same direction as the laser. However, higher dimensional effects, which can be caused by laser speckles used by laser smoothing schemes, or higher dimensional effects in laser plasma interactions near the quarter critical surface such as side-scatter or the two plasmon decay, requires two- or even three-dimensional simulations. In this work, we will present two dimensional and three dimensional multi-speckle simulations of laser plasma interactions relevant to current and future ICF experiments and demonstrate the kinetic nature of these instabilities.   

Suppression of power losses during EM pulse propagation in underdense plasma slab

Maximizing the power propagation of intense electromagnetic waves through an underdense uniform plasma slab underlies many important applications, including laser-based particle acceleration, radiation sources, and plasma-based amplifiers. For current state-of-the-art terawatt lasers, significant loss mechanisms in plasma include Forward Raman Scattering and the self-modulational instability. Using 2D PIC simulations with the code EPOCH, we characterize these loss mechanisms and investigate various means to suppress undesirable laser scattering.   

Simulations of Laser Plasma Interactions for Moderate to High Laser Intensities

Stimulated Raman scattering (SRS) and Brillouin scattering (SBS) have been important topics throughout ICF history. For NIF-relevant regimes, there has been a pressing need to accurately model and avoid these instabilities, and there has grown a rich collection of simulation studies focusing on the details of kinetic physics and nonlinear interactions that influence SRS and SBS. Even though the driving laser frequency for ICF was decreased from 1$\omega$ to 3$\omega$ in an attempt to decrease the growth rate of these detrimental instabilities, there are many fundamentally interesting topics in the LPI dynamics of lasers with longer wavelengths and/or higher intensities ($I\lambda_0^2 \approx 10^{16}-10^{17}$ W $\mu$m$^2$/cm$^2$) driving high-energy density plasmas similar to NIF plasmas ($n_e \approx$ 5-20$\%$ $n_{cr}$, and $T_e$ of order keV). In these regimes, LPI can be strongly driven but nonlinearly saturate, multiple scattering instabilities can co-exist, SRS and SBS may be in the strongly driven regime, and other instabilities like Compton scattering might arise. We present theory and particle-in-cell simulations from this regime.   

Limiting cross-beam energy transfer with laser beam spatial structure

Schemes to implement laser bandwidth wide enough to mitigate laser-plasma instabilities will be both intrusive and expensive. As an alternate approach, work is presented that investigates the mitigating effects of spatial, rather than temporal, laser beam conditioning on cross-beam energy transfer (CBET) [I.V. Igumenshchev {\it et al.,} Phys. Plasmas {\bf 19}, 056314 (2012)]. Such conditioning might be generated by phase plates alone and could therefore be implemented more easily. We have quantified the energy exchange occurring between crossing laser beams that possess orbital angular momentum (OAM) as the amount of OAM exchange between the beams is varied [{\it cf. e.g.,} M. Padgett {\it et al.,} Physics Today {\bf 57}, 35 (2004)]. This numerical study was performed in 3-D using the non-paraxial wave-based {\it LPSE} simulation code [J.F. Myatt {\it et al.,} J. Comp. Phys {\bf 399}, 108916 (2019)]. The mitigating effects are described in terms of the requirement that total angular momentum be conserved and the degree to which a difference in AM between crossing beams limits the acoustic wave response.   

Experimental Studies of LPI Mitigation Via Enhanced Laser Bandwidth at the Nike Laser

Experiments at the Nike laser facility have demonstrated that the laser output spectrum can be broadened from an intrinsic 1 THz bandwidth to almost 5 THz with stimulated rotational Raman scattering (SRRS) after the final amplifier. A near term objective of this research is to utilize this new capability to demonstrate reduced growth of laser plasma instabilities (LPI) due to the increased bandwidth. This poster will discuss the current experimental platform used to study backscattered light in long scale length plasmas. Comparison of these results to simulations using FASTrad3D [Gardner, Phys. Plasmas (1998)] and LPSE [Myatt, Phys. Plasmas (2017)] will be used to guide a new set of experiments to examine cross-beam energy transport (CBET) with enhanced laser bandwidth. The next experiments are being planned to use low-density foam targets and will incorporate the use of a grid image refractometer (GIR) diagnostic [Oh, Rev. Sci. Instru. (2015)] to determine plasma conditions during the peak of the interaction pulses.   

Using a Fourth Laser Wavelength on the NIF to Control the Flow-Induced Crossed Beam Energy Transfer

Controlling and understanding the many laser plasma interactions (LPI) in laser driven inertial confinement fusion (IFC) continues to be a major challenge. Crossed beam energy transfer (CBET) occurs as a result of plasma flows at the laser entrance holes and within the gold hohlraums used in Indirect drive ICF experiments on the National Ignition Facility (NIF). Of the 192 beamlines on the NIF, 2/3s are outer cone beamlines, depositing their energy near the laser entrances within the hohlraum. In these experiments, we use a 1.1 MJ high density carbon ablator platform to measure the transfer that occurs in ICF experiments between outer cone beamlines using a newly developed fourth laser oscillator on the NIF allowing 4 colors. In this series we infer the early time CBET by comparing hohlraum wall expansion and the late time CBET by comparing Stimulated Brillouin Backscatter. We will present data and analysis and compare with simulations results. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.