Session NI03: Invited: High Energy Density Science I

Compressibility, Structure, and Melting of Platinum to 500 GPa

Many high-pressure experiments rely on calibration standards to inform how materials respond under compression.  Platinum is a common pressure standard used in both static and dynamic compression experiments.  Measurement of the crystallographic structure at various pressure–temperature conditions is critical to benchmarking standards such as platinum.  This work reports density and structural data from in situ x-ray diffraction and velocimetry of shocked and ramped platinum.  Different temperature states were accessed by varying the initial shock pressure before further compression along a quasi-isentrope.  The face-centered cubic (fcc) phase was measured up to 480(20) GPa under ramp compression.  Observations of liquid diffraction for platinum shocked to 500 GPa constrain the melting curve on the Hugoniot.  From these results, a melt curve is constructed incorporating previously reported melting measurements. Optical pyrometry provides a lower bound on the Hugoniot melting temperature.   

A computational and experimental exploration of the effects of strong magnetic fields on the deceleration-stage Rayleigh-Taylor instability related to high energy density systems

The Rayleigh-Taylor instability (RTI) has been identified as one of the largest inhibitors of successful high-gain inertial confinement fusion (ICF) experiments. It has been shown that the presence of a magnetic field both lowers the ignition threshold, increases fusion yield, and damps RTI growth. Thus, the development of an experimental platform from which deceleration-stage RTI can be studied with and without a magnetic field is crucial to understanding the disruptive nature and potential mitigation of RTI in ICF. A series of computational design studies identifies a viable design to study deceleration-stage RTI at the National Ignition Facility (NIF). The resulting experimental data from the novel design demonstrates that key physics is missing in the design model. A subsequent Omega-EP experimental design, intended to mirror the NIF platform, demonstrates the presence of a significant high-intensity-laser-generated hot-electron population. A computational study of the impact of hot-electron induced preheat on the NIF and Omega-EP targets illustrates the possibility for a preheat regime to render RTI growth unresolvable experimentally, as is seen in the NIF data. Additional Omega-EP experimental results illustrate the need for 3D rather than 2D computational models to be used in the design process of high-fidelity targets to study deceleration-stage RTI with and without a magnetic field. LA-UR-XX-XXXX   

Non-uniform Joule heating and plasma formation driven by machined 2D and 3D surface perturbations on dielectric coated and bare aluminum rods

The electrothermal instability (ETI) is a Joule heating-driven instability that instigates runaway heating on conductors driven to high current density, altering the 3D evolution of the target. Density perturbations formed through ETI can seed the magneto-Rayleigh-Taylor instability, which is acknowledged as one of the key factors limiting the conditions achievable in magnetically driven high energy density physics (HEDP) experiments. Understanding the formation and evolution of ETI is critical in developing predictive computational tools for HEDP experiments as well as strategies for mitigating its impact.

Most metals include complex distributions of imperfections (voids, resistive inclusions) which seed ETI. To simplify comparison with modeling and theory, our experiments examined growth of ETI from relatively void/inclusion free, 99.999% pure, diamond-turned aluminum rods. We then introduced a variety of deliberately machined and well-characterized perturbations into the target surface, including micron-scale quasi-hemispherical voids, or “engineered” defects (ED), and sinusoidal patterns of varying wavelength and amplitude. Larger diameter ED exhibited qualitatively different behavior from smaller diameters, an effect that we can accurately model. In this study, we evaluated the use of dielectric coatings to hydrodynamically tamp the target surface, effectively delaying surface plasma formation. ED were also machined into sinusoidally perturbed surfaces to understand the relative importance of surface roughness compared with voids, both of which can initiate non-uniform surface heating and plasma formation. We observed a transition from heating dominated by surface roughness to that by ED, consistent with theoretical predictions. These experiments constrain our computational tools, which will enable advances in magnetically driven HEDP target design.   

Micropinch Formation Dynamics in Hybrid X-pinches

An X-ray streak camera with a temporal resolution of 17 ps was used to investigate the micropinch formation dynamics in Cu and Ni hybrid X-pinches on a 400 kA peak current, 50 ns rise time pulsed-power machine. The focus was on L-shell radiation with energies below 1 keV before the X-pinch continuum burst which signals X-pinch micropinch formation to determine imploding plasma conditions just before stagnation. No Li-like copper lines were observed in Cu spectra prior to micropinch formation. Analysis of Ne-like copper lines pre-continuum indicates an average electron temperature of ~200 eV and 4.5x1028 𝑚−3 electron density. The spectra suggest that the electron temperature jumps to ~1 keV inferred from the continuum and the postcontinuum line emission. There was no sign of a rapid temperature change or a substantial surge in radiation emission during the 200 ps pre-continuum X-ray burst to support a major role in micropinch formation for the radiative collapse process. Two-dimensional extended MHD simulations coupled to a collisional-radiative spectral analysis code were used to help develop a new hypothesis that does not involve radiative collapse to explain micropinch formation. These simulations highlight the role of the rapid radial implosion and compression of high-temperature, low-density plasma, the axial outflow of the cold wire core, the dynamo term in the generalized ohms law, and the importance of dynamic pressure of the imploding Cu plasma in the final phase of the micropinch formation process.   

Advances in Thermonuclear Fusion Yield Produced in Sheared-Flow-Stabilized Z Pinches

The sheared-flow-stabilized (SFS) Z-pinch fusion concept, developed at the University of Washington with LLNL collaborators, is now on a path to commercialization at Zap Energy [Levitt et al., Phys. Plasmas 30, 9 (2023)]. Recent experiments corroborate predicted thermonuclear fusion reaction rates, as the discharge current is scaled toward higher gain performance.  Two SFS Z-pinch platforms, FuZE and FuZE-Q, are leveraged for parallel operations with different power supplies and configuration optimization strategies.  Experimental campaigns are underway to increase the triple product, pinch stability duration, and DD fusion neutron production. These efforts aim to scale the pinch current, plasma density, and plasma temperature to reach scientific breakeven.

In this presentation, we report recent results from both experimental platforms, demonstrating operation with thermonuclear plasmas.  Electron temperatures in excess of 2 keV are measured with Thomson scattering [Levitt et al., Phys. Rev. Lett. 132, 15 (2024)], placing a lower bound on ion temperature, which is also independently inferred from impurity radiation spectra to be greater than 2.5 keV.  Simultaneous Thomson electron density measurements result in pinch pressure profiles [Goyon et al., in review 2024] in agreement with predictions by analytical and computational models [Meier et al., Phys. Plasmas 28, 9 (2021)].  These devices produce DD neutron yields that scale strongly with plasma current (Yn~I^11) [ Levitt et al., Phys. Plasmas 30, 9 (2023)] up to 10^10, consistent with adiabatic scaling of a SFS Z pinch [Shumlak, J. Appl. Phys. 127, 20 (2020)], and with 2D MHD modeling results. Measurements of neutron spatial isotropy place stringent limits on the possible contribution of beam-target fusion events and suggest that roughly 90% of the observed yield originates from thermalized deuterium plasma.  These recent advances in performance constitute a factor of 10^5 improvement in fusion yield since 2021.   

Improved current scaling and neutron yield for MJ-Class dense plasma focus (DPF) via simulation-guided design.

.The Dense Plasma Focus (DPF) is a well-studied source of short (<100 ns) pulses of ions, neutrons, and x-rays. DPF-based platforms are attractive because they exhibit high yield to energy conversion efficiency and can typically be constructed at a lower cost than traditional accelerators for equivalent output. The scaling of yield versus input current and energy is also favorable, but saturates at the MA and MJ level. [Auluck Phys. Plasmas 2023]. New results on the MegaJOuLe Neutron Imaging Radiography DPF (MJOLNIR) device demonstrate that design changes, led by experimentally validated simulations, have extended neutron yield scaling by two-orders-of-magnitude. In particular, state-of-the-art Particle-In-Cell simulations of the full ~6µs discharge were pivotal in showing an electrode shape redesign would lead to significant plasma temperature increase. The new design produced a record neutron yield of 1.2 × 10^12 with 3.6 MA peak current and 1.3 MJ of stored energy, the highest deuterium-deuterium yield yet reported in the literature. Experimental data are compared to simulations to highlight the two main mechanisms contributing to neutron production: thermonuclear yield from compressional heating of the plasma and beam-target events during subsequent pinch disruption. An extensive suite of diagnostics characterizes the implosion dynamics including: fast framing camera images, interferometry, neutron time-of-flight and neutron source spatial imaging. Finally, we describe a semi-analytical model developed to predict performance for varying discharge and electrode parameters, identifying the contribution of each neutron generation mechanism as well as potential applications of DPFs for plasma material interaction and nuclear resonance transmission analysis. Prepared by LLNL under Contract DE-AC52-07NA27344.