E.S. Dodd

Different kλD regimes for nonlinear effects on Langmuir wavesa)

As Langmuir waves (LWs) are driven to large amplitude in plasma, they are affected by nonlinear mechanisms. A global understanding, based on simulations and experiments, has emerged that identifies various nonlinear regimes depending on the dimensionless parameter kλD, where k is the Langmuir wave number and λD is the electron Debye length. The nonlinear phenomena arise due to wave-wave and wave-particle coupling mechanisms, and this basic separation between fluid-like nonlinearities and kinetic nonlinearities depends on the degree to which electron and ion Landau damping, as well as electron trapping, play a role. Previous ionospheric heating experiments [Cheung et al. Phys. Plasmas 8, 802 (2001)] identified cavitation/collapse and Langmuir decay instability (LDI), predominantly wave-wave mechanisms, to be the principal nonlinear effects for driven LWs with kλD<0.1, in agreement with fluid simulations [DuBois et al. Phys. Plasmas 8, 791 (2001)]. In the present research, collective Thomson scattering meas...

Revised Knudsen-layer reduction of fusion reactivity

Recent work by Molvig et al. [Phys. Rev. Lett. 109, 095001 (2012)] examined how fusion reactivity may be reduced from losses of fast ions in finite assemblies of fuel. In this paper, this problem is revisited with the addition of an asymptotic boundary-layer treatment of ion kinetic losses. This boundary solution, reminiscent of the classical Milne problem from linear transport theory, obtains a free-streaming limit of fast ion losses near the boundary, where the diffusion approximation is invalid. Thermonuclear reaction rates have been obtained for the ion distribution functions predicted by this improved model. It is found that while Molvigs “Knudsen distribution function” bounds from above the magnitude of the reactivity reduction, this more accurate treatment leads to less dramatic losses of tail ions and associated reduction of thermonuclear reaction rates for finite fuel volumes.

Design of the opacity spectrometer for opacity measurements at the National Ignition Facility

Recent experiments at the Sandia National Laboratory Z facility have called into question models used in calculating opacity, of importance for modeling stellar interiors. An effort is being made to reproduce these results at the National Ignition Facility (NIF). These experiments require a new X-ray opacity spectrometer (OpSpec) spanning 540 eV-2100 eV with a resolving power E/ΔE > 700. The design of the OpSpec is presented. Photometric calculations based on expected opacity data are also presented. First use on NIF is expected in September 2016.

Conceptual design of initial opacity experiments on the national ignition facility

R. F. Heeter1,†, J. E. Bailey4, R. S. Craxton5, B. G. DeVolder2, E. S. Dodd2, E. M. Garcia5, E. J. Huffman3, C. A. Iglesias1, J. A. King3, J. L. Kline2, D. A. Liedahl1, P. W. McKenty5, Y. P. Opachich3, G. A. Rochau4, P. W. Ross3, M. B. Schneider1, M. E. Sherrill2, B. G. Wilson1, R. Zhang5 and T. S. Perry2 1Lawrence Livermore National Laboratory, 7000 East Ave., Livermore, CA 94550, USA 2Los Alamos National Laboratory, Los Alamos, NM 87545, USA 3National Security Technologies, Livermore, CA 94550, USA 4Sandia National Laboratories, Albuquerque, NM 87185, USA 5Univ. of Rochester Laboratory for Laser Energetics, Rochester, NY 14623, USA

Gas-filled hohlraum experiments at the National Ignition Facility

Experiments done at the National Ignition Facility laser [J. A. Paisner, E. M. Campbell, and W. Hogan, Fusion Technol. 26, 755 (1994)] using gas-filled hohlraums demonstrate a key ignition design feature, i.e., using plasma pressure from a gas fill to tamp the hohlraum-wall expansion for the duration of the laser pulse. Moreover, our understanding of hohlraum energetics and the ability to predict the hohlraum soft-x-ray drive has been validated in ignition-relevant conditions. Finally, the laser reflectivity from stimulated Raman scattering in the fill plasma, a key threat to hohlraum performance, is shown to be suppressed by choosing a design with a sufficiently high ratio of electron temperature to density.

Design and Fabrication of Opacity Targets for the National Ignition Facility

Abstract Accurate models for opacity of partially ionized atoms are important for modeling and understanding stellar interiors and other high-energy-density phenomena such as inertial confinement fusion. Lawrence Livermore National Laboratory is leading a multilaboratory effort to conduct experiments on the National Ignition Facility (NIF) to try to reproduce recent opacity tests at the Sandia National Laboratory Z-facility. Since 2015, the NIF effort has evolved several hohlraum designs that consist of multiple pieces joined together. The target also has three components attached to the main stalk over a long distance with high tolerances that have resulted in several design iterations. The target has made use of rapid prototyped features to attach a capsule and collimator under the hohlraum while avoiding interference with the beams. This paper discusses the evolution of the hohlraum and overall target design and the challenges involved with fabricating and assembling these targets.

Study of the ion kinetic effects in ICF run-away burn using a quasi-1D hybrid model

The loss of fuel ions in the Gamow peak and other kinetic effects related to the α particles during ignition, run-away burn, and disassembly stages of an inertial confinement fusion D-T capsule are investigated with a quasi-1D hybrid volume ignition model that includes kinetic ions, fluid electrons, Planckian radiation photons, and a metallic pusher. The fuel ion loss due to the Knudsen effect at the fuel-pusher interface is accounted for by a local-loss model by Molvig et al. [Phys. Rev. Lett. 109, 095001 (2012)] with an albedo model for ions returning from the pusher wall. The tail refilling and relaxation of the fuel ion distribution are captured with a nonlinear Fokker-Planck solver. Alpha heating of the fuel ions is modeled kinetically while simple models for finite alpha range and electron heating are used. This dynamical model is benchmarked with a 3 T hydrodynamic burn model employing similar assumptions. For an energetic pusher (∼40 kJ) that compresses the fuel to an areal density of ∼1.07g/cm2 a...

“Bloch wave” modification of stimulated Raman by stimulated Brillouin scattering

Using the reduced-description particle-in-cell (RPIC) method, we study the coupling of backward stimulated Raman scattering (BSRS) and backward stimulated Brillouin scattering (BSBS) in regimes where the reflectivity involves the nonlinear behavior of particles trapped in the daughter plasma waves. The temporal envelope of a Langmuir wave (LW) obeys a Schrodinger equation where the potential is the periodic electron density fluctuation resulting from an ion-acoustic wave (IAW). The BSRS-driven LWs in this case have a Bloch wave structure and a modified dispersion due to the BSBS-driven spatially periodic IAW, which includes frequency band gaps at kLW∼kIAW/2∼k0 (kLW, kIAW, and k0 are the wave number of the LW, IAW, and incident pump electromagnetic wave, respectively). This band structure and the associated Bloch wave harmonic components are distinctly observed in RPIC calculations of the electron density fluctuation spectra and this structure may be observable in Thomson scatter. Bloch wave components gro...

Hohlraum modeling for opacity experiments on the National Ignition Facility

This paper discusses the modeling of experiments that measure iron opacity in local thermodynamic equilibrium (LTE) using laser-driven hohlraums at the National Ignition Facility (NIF). A previous set of experiments fielded at Sandias Z facility [Bailey et al., Nature 517, 56 (2015)] have shown up to factors of two discrepancies between the theory and experiment, casting doubt on the validity of the opacity models. The purpose of the new experiments is to make corroborating measurements at the same densities and temperatures, with the initial measurements made at a temperature of 160 eV and an electron density of 0.7 × 1022 cm−3. The X-ray hot spots of a laser-driven hohlraum are not in LTE, and the iron must be shielded from a direct line-of-sight to obtain the data [Perry et al., Phys. Rev. B 54, 5617 (1996)]. This shielding is provided either with the internal structure (e.g., baffles) or external wall shapes that divide the hohlraum into a laser-heated portion and an LTE portion. In contrast, most inertial confinement fusion hohlraums are simple cylinders lacking complex gold walls, and the design codes are not typically applied to targets like those for the opacity experiments. We will discuss the initial basis for the modeling using LASNEX, and the subsequent modeling of five different hohlraum geometries that have been fielded on the NIF to date. This includes a comparison of calculated and measured radiation temperatures.This paper discusses the modeling of experiments that measure iron opacity in local thermodynamic equilibrium (LTE) using laser-driven hohlraums at the National Ignition Facility (NIF). A previous set of experiments fielded at Sandias Z facility [Bailey et al., Nature 517, 56 (2015)] have shown up to factors of two discrepancies between the theory and experiment, casting doubt on the validity of the opacity models. The purpose of the new experiments is to make corroborating measurements at the same densities and temperatures, with the initial measurements made at a temperature of 160 eV and an electron density of 0.7 × 1022 cm−3. The X-ray hot spots of a laser-driven hohlraum are not in LTE, and the iron must be shielded from a direct line-of-sight to obtain the data [Perry et al., Phys. Rev. B 54, 5617 (1996)]. This shielding is provided either with the internal structure (e.g., baffles) or external wall shapes that divide the hohlraum into a laser-heated portion and an LTE portion. In contrast, most in...

Spectroscopic characterization of ultrashort laser driven targets incorporating both Boltzmann and particle-in-cell models

A model that solves simultaneously both the electron and atomic kinetics was used to generate synthetic X-ray spectra to characterize high intensity ultrashort-laser-driven target experiments. A particle-in-cell simulation was used to model the laser interaction for both cluster and foil targets and provided the initial electron energy distribution function (EEDF) for a Boltzmann solver. Previously reported success in the spectroscopic characterization of an irradiated Ar cluster target has motivated the authors to apply this technique in a feasibility study to assess the possibility of recording time resolved spectra of a 10 micron Ti foil target irradiated by a 500 fs, I= 1.0 × 1018W/cm2 short-pulse laser. Though this model suggests that both Ar cluster and Ti foil plasmas are held in a highly non-equilibrium state for both the EEDF and the ion level populations for several picoseconds, the spectral line features of the foil experiment was shown to evolve too quickly to be seen by current ultrafast time resolved spectrometers.