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Hohenberger, M.

Publications and source records attributed to Hohenberger, M..

44 records · Page 3
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Hot-spot mix in large-scale HDC implosions at NIF

Mix of high-Z material from the capsule into the fuel can severely degrade the performance of inertial fusion implosions. On the Hybrid B campaign, testing the largest high-density-carbon capsules yet fielded at the National Ignition Facility, several shots show signatures of high levels of hot-spot mix. We attribute a ~40% yield degradation on these shots to the hot-spot mix, comparable to the level of degradation from large $P_2$ asymmetries observed on some shots. Additionally, a range of instability growth factors and diamond crystallinity were tested and they do not determine the level of mix for these implosions, which is instead set by the capsule quality.

Fusion experiments↗
DOE OSTI2020

Hotspot parameter scaling with velocity and yield for high-adiabat layered implosions at the National Ignition Facility

Here, in this paper, we present a study on hotspot parameters in indirect-drive, inertially confined fusion implosions as they proceed through the self-heating regime. The implosions with increasing nuclear yield reach the burning-plasma regime, hotspot ignition, and finally propagating burn and ignition. These implosions span a wide range of alpha heating from a yield amplification of 1.7–2.5. We show that the hotspot parameters are explicitly dependent on both yield and velocity and that by fitting to both of these quantities the hotspot parameters can be fit with a single power law in velocity. The yield scaling also enables the hotspot parameters extrapolation to higher yields. This is important as various degradation mechanisms can occur on a given implosion at fixed implosion velocity which can have a large impact on both yield and the hotspot parameters. The yield scaling also enables the experimental dependence of the hotspot parameters on yield amplification to be determined. The implosions reported have resulted in the highest yield (1.73×10 16 ±2.6%), yield amplification, pressure, and implosion velocity yet reported at the National Ignition Facility.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2020

Symmetric fielding of the largest diamond capsule implosions on the NIF

We present results for the largest diamond capsule implosions driven symmetrically on the National Ignition Facility (NIF) (inner radius of ~1050 μm) without the use of cross beam transfer in cylindrical Hohlraums. We show that the methodology of designing Hohlraum parameters in a semi-empirical way using an extensive database resulted in a round implosion. In addition, we show that the radiation flux symmetry is well controlled during the foot of the pulse and that swings in P2 symmetry between the inflight dense shell and hot spot are within ±4 μm and that swings around peak compression are also within the symmetry specification of ±4 μm. We observed a stronger dependence of symmetry on the capsule scale than previously observed and also observed enhanced inner beam propagation for experiments using a gas fill density of 0.3 mg/cm 3 and 1000 μm inner radius capsules. We have observed sufficient symmetry and mass remaining at near full NIF power and energy, up to 480 TW and 1.9 MJ, with little laser–plasma interactions (low laser backscattered light) and predict that this design could support extended NIF energy of up to 2.1 MJ.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2020

Hot-electron generation at direct-drive ignition-relevant plasma conditions at the National Ignition Facility

Laser–plasma interaction instabilities can be detrimental for direct-drive inertial confinement fusion by generating high-energy electrons that preheat the target. Moreover, an experimental platform has been developed and fielded on the National Ignition Facility to investigate hot-electron production from laser–plasma instabilities at direct-drive ignition-relevant conditions. The radiation-hydrodynamic code DRACO has been used to design planar-target experiments that generate plasma and interaction conditions comparable to direct-drive ignition designs: I L ~10 15 W/cm 2 , T e > 3 keV, and density-gradient scale lengths of L n ~ 600 μ m in the quarter-critical density region. The hot-electron properties were inferred by comparing the experimentally observed hard x-ray spectra to Monte Carlo simulations of hard x-ray emission from hot electrons depositing energy in the target. Hot-electron temperatures of ~40 keV to 60 keV and the fraction of laser energy converted to hot electrons of ~0.5% to 5% were inferred in plastic targets for laser intensities at the quarter-critical density surface of (~4 to 14) × 10 14 W/cm 2 . The use of silicon ablators was found to mitigate the hot-electron preheat by increasing the threshold laser intensity for hot-electron generation from ~3.5 × 10 14 W/cm 2 in plastic to ~6 × 10 14 W/cm 2 in silicon. The overall hot-electron production is further reduced in silicon ablators when the intensity threshold is exceeded.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2020

Hotspot conditions achieved in inertial confinement fusion experiments on the National Ignition Facility

We describe the overall performance of the major indirect-drive inertial confinement fusion campaigns executed at the National Ignition Facility. With respect to the proximity to ignition, we can describe the performance of current experiments both in terms of no-burn ignition metrics (metrics based on the hydrodynamic performance of targets in the absence of alpha-particle heating) and in terms of the thermodynamic properties of the hotspot and dense fuel at stagnation—in particular, the hotspot pressure, temperature, and areal density. We describe a simple 1D isobaric model to derive these quantities from experimental observables and examine where current experiments lie with respect to the conditions required for ignition.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2020

X-ray diffraction at the National Ignition Facility

We report details of an experimental platform implemented at the National Ignition Facility to obtain in situ powder diffraction data from solids dynamically compressed to extreme pressures. Thin samples are sandwiched between tamper layers and ramp compressed using a gradual increase in the drive-laser irradiance. Pressure history in the sample is determined using high-precision velocimetry measurements. Up to two independently timed pulses of x rays are produced at or near the time of peak pressure by laser illumination of thin metal foils. The quasi-monochromatic x-ray pulses have a mean wavelength selectable between 0.6 Å and 1.9 Å depending on the foil material. The diffracted signal is recorded on image plates with a typical 2 θ x-ray scattering angle uncertainty of about 0.2° and resolution of about 1°. Analytic expressions are reported for systematic corrections to 2 θ due to finite pinhole size and sample offset. A new variant of a nonlinear background subtraction algorithm is described, which has been used to observe diffraction lines at signal-to-background ratios as low as a few percent. Variations in system response over the detector area are compensated in order to obtain accurate line intensities; this system response calculation includes a new analytic approximation for image-plate sensitivity as a function of photon energy and incident angle. This experimental platform has been used up to 2 TPa (20 Mbar) to determine the crystal structure, measure the density, and evaluate the strain-induced texturing of a variety of compressed samples spanning periods 2–7 on the periodic table.

47 OTHER INSTRUMENTATION↗