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Key, M H

Publications and source records attributed to Key, M H.

16 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Hydrodynamic instabilities and mix studies on NIF: predictions, observations, and a path forward

The goals of the Mix Campaign are to determine how mix affects performance, locate the "mix cliff", locate the source of the mix, and develop mitigation methods that allow performance to be increased. We have used several different drive pulse shapes and capsule designs in the Mix Campaign, to understand sensitivity to drive peak power, level of coast, rise time to peak power, adiabat, and dopant level in the capsule. Ablator material mixing into the hot spot has been shown conclusively with x-ray spectroscopy. The observed neutron yield drops steeply when the hot spot mix mass becomes too large. The mix appears to be driven by ablation- front Rayleigh-Taylor instabilities. A high foot, higher adiabat drive has a more stable ablation front and has allowed the mix mass in the hot spot to be reduced significantly. We found two recent high foot shots achieved neutron yields > 10 15 and measured neutron yield over clean 1D simulation (YOC) > 50%, which was one of the central goals of the Mix Campaign.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2013

Comparisons of angularly and spectrally resolved Bremsstrahlung measurements to two-dimensional multi-stage simulations of short-pulse laser-plasma interactions

A 2-D multi-stage simulation model incorporating realistic laser conditions and a fully resolved electron distribution handoff has been developed and compared to angularly and spectrally resolved Bremsstrahlung measurements from high-Z planar targets. For near-normal incidence and 0.5-1 × 10{sup 20} W/cm{sup 2} intensity, particle-in-cell (PIC) simulations predict the existence of a high energy electron component consistently directed away from the laser axis, in contrast with previous expectations for oblique irradiation. Measurements of the angular distribution are consistent with a high energy component when directed along the PIC predicted direction, as opposed to between the target normal and laser axis as previously measured.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

Temporally resolved characterization of shock-heated foam target with Al absorption spectroscopy for fast electron transport study

The CH foam plasma produced by a laser-driven shock wave has been characterized by a temporally resolved Al 1s-2p absorption spectroscopy technique. A 200 mg/cm{sup 3} foam target with Al dopant was developed for this experiment, which used an OMEGA EP [D. D. Meyerhofer et al., J. Phys.: Conf. Ser. 244, 032010 (2010)] long pulse beam with an energy of 1.2 kJ and 3.5 ns pulselength. The plasma temperatures were inferred with the accuracy of 5 eV from the fits to the measurements using an atomic physics code. The results show that the inferred temperature is sustained at 40-45 eV between 6 and 7 ns and decreases to 25 eV at 8 ns. 2-D radiation hydrodynamic simulations show a good agreement with the measurements. Application of the shock-heated foam plasma platform toward fast electron transport experiments is discussed.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

Fast-ignition transport studies: Realistic electron source, integrated particle-in-cell and hydrodynamic modeling, imposed magnetic fields

Transport modeling of idealized, cone-guided fast ignition targets indicates the severe challenge posed by fast-electron source divergence. The hybrid particle-in-cell (PIC) code Zuma is run in tandem with the radiation-hydrodynamics code Hydra to model fast-electron propagation, fuel heating, and thermonuclear burn. The fast electron source is based on a 3D explicit-PIC laser-plasma simulation with the PSC code. This shows a quasi two-temperature energy spectrum and a divergent angle spectrum (average velocity-space polar angle of 52 Degree-Sign ). Transport simulations with the PIC-based divergence do not ignite for >1 MJ of fast-electron energy, for a modest (70 {mu}m) standoff distance from fast-electron injection to the dense fuel. However, artificially collimating the source gives an ignition energy of 132 kJ. To mitigate the divergence, we consider imposed axial magnetic fields. Uniform fields {approx}50 MG are sufficient to recover the artificially collimated ignition energy. Experiments at the Omega laser facility have generated fields of this magnitude by imploding a capsule in seed fields of 50-100 kG. Such imploded fields will likely be more compressed in the transport region than in the laser absorption region. When fast electrons encounter increasing field strength, magnetic mirroring can reflect a substantial fraction of them and reduce coupling to the fuel. A hollow magnetic pipe, which peaks at a finite radius, is presented as one field configuration which circumvents mirroring.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

A novel zirconium K{alpha} imager for high energy density physics research

We report on the development and characterization of a zirconium K{alpha} imager for high energy density physics research. The imager consists of a spherically bent quartz crystal operating at 15.7 keV photon energy. We compare the performance of the imager in terms of integrated reflectivity (R{sub int}) and temperature dependent collection efficiency ({eta}{sub Te}) to that of the widely used Cu K{alpha} imager. Our collisional-radiative simulations show that the new imager can be reliably used up to 250 eV plasma temperature. Monte Carlo simulations show that for a 25 {mu}m thick tracer layer of zirconium, the contribution to K{alpha} production from photo-pumping is only 2%. We present, for the first time, 2D spatially resolved images of zirconium plasmas generated by a high intensity short pulse laser interacting with Zr solid targets.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2011

Fast electron temperature and conversion efficiency measurements in laser-irradiated foil targets using a bremsstrahlung x-ray detector

Measurements of fast electron temperature and conversion efficiencies using bremsstrahlung x-rays emitted from laser-produced, fast electrons are presented. Experiments were carried out using the Titan laser (150 J, 1.5 ps) at the Lawrence Livermore National Laboratory. The maximum intensity was 2 x 10{sup 20} W/cm{sup 2} on 250 {mu}m thick silver foil targets. The emission of bremsstrahlung x-rays from the fast electrons in the target was measured using a filter-stack based detector. The conversion efficiency of laser energy into fast electrons and the electron temperature were studied as a function of incident laser energy. Several models of the electron divergence angle were investigated, and the effect of the assumed divergence angle on the inferred conversion efficiency was quantified. This allows for upper and lower bounds on the conversion efficiency to be established for a range of possible divergence angles. The value for upper bound is 60% (from a 75 deg. divergence angle model) and for the lower bound is 25% (from a 0 deg. divergence angle model).

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2010

Single-shot divergence measurements of a laser-generated relativistic electron beam

The relativistic electron transport induced by an ultraintense picosecond laser is experimentally investigated using an x-ray two-dimensional imaging system. Previous studies of the electron beam divergence [R. B. Stephens et al. Phys. Rev. E 69, 066414 (2004), for instance] were based on an x-ray imaging of a fluorescence layer buried at different depths in the target along the propagation axis. This technique required several shots to be able to deduce the divergence of the beam. Other experiments produced single-shot images in a one-dimensional geometry. The present paper describes a new target design producing a single-shot, two-dimensional image of the electrons propagating in the target. Several characteristics of the electron beam are extracted and discussed and Monte Carlo simulations provide a good understanding of the observed beam shape. The proposed design has proven to be efficient, reliable, and promising for further similar studies.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗