The Z machine at Sandia National Laboratories generates intense X-ray sources in the 1-10 keV photon energy range by imploding gas-puff or wire array loads. Notably, argon gas-puff implosions on the Z machine have achieved argon K-shell yields exceeding 300 kJ [1]. These yields, along with measured K-shell powers and emission regions, have been successfully reproduced using the 2-D MHD MACH2+TCRE code. The simulated ratio of Ly-alpha to Healpha plus the intercombination lines from MACH2+TCRE closely matched experimental results after the time of peak X-ray power, though the simulation had higher line ratios before the peak. This line ratio indicates that the plasma simulation is too hot before the peak power. This discrepancy was attributed to 3-D effects or to the steady-state atomic level kinetics assumption inherent in the simulations [2]. This presentation investigates the impact of time-dependent atomiclevel kinetics using the NRL DZAPP code: a coupled 1-D MHD, non-LTE atomic kinetics, and radiation transport code that includes a transmission line for driving the load. Results from simulations employing both steady-state and time-dependent non-LTE level kinetics will be presented and compared with experimental line ratios to assess the significance of atomic level kinetics effects on spectral line features.

We report two magnetized liner inertial fusion (MagLIF) experiments that produced record thermonuclear D-D neutron yields of 2.11 & times;10(13 )and 2.33 & times;10(13). These yields are about a factor of two higher than previous MagLIF results. The experiments achieved ion temperatures of 3.0 and 3.3 keV and stagnation pressures of 1.6 and 1.3 Gbar. The inferred Lawson parameters were chi=0.2 and 0.1, which are the largest reported for MagLIF. The performance increase used a high-aspect-ratio beryllium liner with a dielectric coating and modest increases in preheat energy ( similar to 2.2 kJ), peak current (18.5 MA), and axial magnetic field (15 T). Three-dimensional HYDRA simulations are consistent with the measured liner dynamics and fusion outputs. These results indicate a pathway to higher-yield MagLIF designs using coated, high-aspect-ratio liners and improved input parameters. Simulations further suggest that adding an ice fuel layer could increase yield by up to a factor of 2.5 by reducing liner convergence, instability feedthrough, and mix.

Radiation-induced refractive index change in fiber optic materials presents challenges for photonic Doppler velocimetry and other high-resolution interferometric techniques at fusion research facilities such as the Z Machine (Sandia National Laboratories). Signal pollution, including phase shifts and artificial resonances, has been observed in many radiation-producing experiments yet remains largely unexplained. We use radiation, thermal, and mechanical simulation to show that these noise sources originate from a combination of the thermo-optic effect and cyclical photoelastic compression of the fiber core and cladding, dynamically altering the refractive index of the fiber. In contrast, thermal dilation and elongation of the waveguide are shown to be insignificant. By simulating the evolution of refractive index along the waveguide, the interferometric phase shifts observed in several experiments are successfully reproduced. Analytical approximations based on shock compression of silica also support the findings. The identification of these effects may allow for their mitigation in designing diagnostics for plasma-generating facilities.

We present evidence for strong radiative cooling in a pulsed-power-driven magnetic reconnection experiment. Two aluminum exploding wire arrays, driven by a 20 MA peak current, 300 ns rise time pulse from the Z machine (Sandia National Laboratories), generate strongly driven plasma flows (MA≈7) with anti-parallel magnetic fields, which form a reconnection layer (SL≈120) at the mid-plane. The net cooling rate far exceeds the Alfvénic transit rate (τcool−1/τA−1≫1), leading to strong cooling of the reconnection layer. We determine the advected magnetic field and flow velocity using inductive probes positioned in the inflow to the layer, and inflow ion density and temperature from analysis of visible emission spectroscopy. A sharp decrease in x-ray emission from the reconnection layer, measured using filtered diodes and time-gated x-ray imaging, provides evidence for strong cooling of the reconnection layer after its initial formation. X-ray images also show localized hotspots, regions of strong x-ray emission, with velocities comparable to the expected outflow velocity from the reconnection layer. These hotspots are consistent with plasmoids observed in 3D radiative resistive magnetohydrodynamic simulations of the experiment. X-ray spectroscopy further indicates that the hotspots have a temperature (170 eV) much higher than the bulk layer (≤75 eV) and inflow temperatures (about 2 eV) and that these hotspots generate the majority of the high-energy (>1 keV) emission.

The use of the recombination continuum for x-ray radiation production in Z pinches has been discussed as an option 1 for generating high yields in the warm photon energy range, ħω > 13 keV. The free-bound continuum emission is an inherently weaker radiation production mechanism than K-shell line emission. But it is challenging, if at all possible, to heat stagnated Z-pinch plasmas with atomic numbers Z A >36 to temperatures ~10 keV needed for efficient K-shell line emission. Experiments with argon gas-puffs 2,3 on Decade Quad and pre-refurbishment Z produced up to 10 to 20% of the total K-shell yield in high-energy continuum above the K-edge, 4.4 keV. As the atomic number of the load material increases from Z A =18 (argon) to Z A =26 and 29 (iron and copper, respectively), the scaling factor determining the ratio of the recombination continuum to line K-shell yield increases by a factor of 2 to 3. This indicates a possibility of using wire-array Z-pinch implosions on next-generation pulsed power facilities to produce significant continuum yields in warm photons. Such an option is feasible, provided that stripping substantial fractions of iron and copper ions at stagnation to an H-like state can be demonstrated. We analyze K-shell continuum yields measured in recent Z experiments with argon gas-puff and stainless-steel wire-array loads, with a view of scaling present-day results to higher driver currents.

On pulsed fusion experiments, the neutron time of flight (nToF) diagnostic provides critical information on the fusion neutron energy spectrum. This work presents an analysis technique that uses two collinear nToF detectors, potentially to measure nuclear bang time and directional flow velocities. Two collinear detectors may be sufficient to disambiguate the contributions of nuclear bang time and directional flow velocities to the first moment of the neutron energy spectrum, providing an independent measurement of nuclear bang time. Preliminary results from measured nToF traces on the National Ignition Facility and additional applications of this technique are presented.

On December 5, 2022, an indirect drive fusion implosion on the National Ignition Facility (NIF) achieved a target gain G_{target} of 1.5. This is the first laboratory demonstration of exceeding "scientific breakeven" (or G_{target}>1) where 2.05 MJ of 351 nm laser light produced 3.1 MJ of total fusion yield, a result which significantly exceeds the Lawson criterion for fusion ignition as reported in a previous NIF implosion [H. Abu-Shawareb et al. (Indirect Drive ICF Collaboration), Phys. Rev. Lett. 129, 075001 (2022)PRLTAO0031-900710.1103/PhysRevLett.129.075001]. This achievement is the culmination of more than five decades of research and gives proof that laboratory fusion, based on fundamental physics principles, is possible. This Letter reports on the target, laser, design, and experimental advancements that led to this result.

Measuring the energy distribution (spectrum) of nonthermal radiation is a valuable diagnostic for Z-pinch plasmas. However, the spatial origin and transport of these X-rays are challenging to study in Z-pinches due to broad charge state distributions and spatial gradients of nonthermal emitters. Knowledge of this information can refine our understanding of plasma radiation, enhance the microphysics of multiphysics simulations, and constrain future Z-pinch experiments and diagnostics. In this work, nonthermal iron Ka X-rays are modeled in a Magnetized Liner Inertial Fusion (MagLIF) plasma produced on Sandia National Laboratories' Z-machine using a novel Monte Carlo radiation transport code. The code employs an ancillary screened-hydrogenic atomic data package to self-consistently calculate transport of thermal photons from a fusion core into a beryllium liner shell with 114 ppm iron impurities. Iron fluorescence production in the liner shell is analyzed with spatial statistics, providing novel insight into the origin of nonthermal radiation over a broad region of the plasma shell. Spatial analysis indicates an average radius of fluorescence production that is less than the radial midpoint of the liner plasma, suggesting enhanced photoabsorption near the boundary between the fusion core and liner plasma. Results include an approximated average iron ionization, average bulk thermal electron temperature, and iron K-shell fluorescence yield. A radial temperature profile from deposition of transport photons as well as an emergent transmission spectrum with escaped iron Ka and K ss intensities are constructed. Spectroscopic utility of the code is validated against experimental MagLIF data, showing good agreement among spectroscopic features.

X-ray imaging indicates magnetized liner inertial fusion (MagLIF) stagnation columns have a complicated quasi-helical structure with significant variations in x-ray brightness along the column. In this work, we describe MagLIF experiments aimed at controlling these stagnation structures by varying the initial liner geometry and composition. First, by varying the initial aspect ratio of the liner, we demonstrate a change in the stagnation structures that is consistent with helical magneto Rayleigh–Taylor (MRT) instabilities feedthrough from the outer-to-inner surfaces of the liner. Second, to minimize the seed for such instabilities, we incorporate a dielectric coating on the outer surface of the beryllium liner, which has previously been shown to reduce the growth of the electrothermal instability, a likely seed for MRT growth. Using this coating, we achieve a stagnation column with significantly reduced helical structure and axial variation in x-ray brightness. We discuss how this coating changes the evolution of structures through stagnation along with the spatial uniformity of neutron production. Finally, we show that these more uniform stagnations also result in improved reproducibility in stagnation temperatures and primary DD neutron yield.

Magnetized Liner Inertial Fusion experiments have been performed at the Z facility at Sandia National Laboratories. These experiments use deuterium fuel, which produces 2.45 MeV neutrons on reaching thermonuclear conditions. To study the spatial structure of neutron production, the one-dimensional imager of neutrons diagnostic was fielded to record axial resolved neutron images. In this diagnostic, neutrons passing through a rolled edge aperture form an image on a CR-39-based solid state nuclear track detector. Here, we present a modified generalized expectation–maximization algorithm to reconstruct an axial neutron emission profile of the stagnated fusion plasma. We validate the approach by comparing the reconstructed neutron emission profile to an x-ray emission profile provided by a time-integrated pinhole camera.

We present the first experimental study of plasmoid formation in a magnetic reconnection layer undergoing rapid radiative cooling, a regime relevant to extreme astrophysical plasmas. Two exploding aluminum wire arrays, driven by the Z machine, generate a reconnection layer (S_{L}≈120) in which the cooling rate far exceeds the hydrodynamic transit rate (τ_{hydro}/τ_{cool}>100). The reconnection layer generates a transient burst of >1  keV x-ray emission, consistent with the formation and subsequent rapid cooling of the layer. Time-gated x-ray images show fast-moving (up to 50  km s^{-1}) hotspots in the layer, consistent with the presence of plasmoids in 3D resistive magnetohydrodynamic simulations. X-ray spectroscopy shows that these hotspots generate the majority of Al K-shell emission (around 1.6 keV) prior to the onset of cooling, and exhibit temperatures (170 eV) much greater than that of the plasma inflows and the rest of the reconnection layer, thus providing insight into the generation of high-energy radiation in radiatively cooled reconnection events.

In magnetized liner inertial fusion (MagLIF), a cylindrical liner filled with fusion fuel is imploded with the goal of producing a one-dimensional plasma column at thermonuclear conditions. However, structures attributed to three-dimensional effects are observed in self-emission x-ray images. Despite this, the impact of many experimental inputs on the column morphology has not been characterized. We demonstrate the use of a linear regression analysis to explore correlations between morphology and a wide variety of experimental inputs across 57 MagLIF experiments. Results indicate the possibility of several unexplored effects. For example, we demonstrate that increasing the initial magnetic field correlates with improved stability. Although intuitively expected, this has never been quantitatively assessed in integrated MagLIF experiments. We also demonstrate that azimuthal drive asymmetries resulting from the geometry of the “current return can” appear to measurably impact the morphology. In conjunction with several counterintuitive null results, we expect the observed correlations will encourage further experimental, theoretical, and simulation-based studies. Finally, we note that the method used in this work is general and may be applied to explore not only correlations between input conditions and morphology but also with other experimentally measured quantities.

We report on a series of Magnetized Liner Inertial Fusion (MagLIF) experiments conducted on the Z pulsed power facility that utilized high aspect ratio (ratio of outer radius to wall thickness) liners with dielectric coatings and low-mix laser preheat configurations. The liners consisted of an aspect ratio of 10.6 beryllium tube coated with 75 μm of epoxy on the outside that have been shown to maintain a better implosion stability than uncoated beryllium and have demonstrated consistent stagnation performances in previous experiments [Ampleford et al., Phys. Plasmas 31, 022703 (2024)]. Two-dimensional HYDRA simulations were used to design three different “co-injection” laser configurations, whereby a second laser is used to provide an early prepulse before the main pulse, to reduce LEH foil mix while increasing the fuel density and coupled energy. The laser preheat energy for each configuration was constrained using dedicated laser experiments before being applied to the integrated MagLIF experiments on Z. The DD neutron yield for experiments using co-injection preheat configurations is found to increase with the specific preheat energy in line with simulations. The highest neutron yield achieved in this study of 1.1 × 1013 matches the highest reported in a MagLIF experiment to date and is a factor 3.5 times higher than similar experiments using preheat with no phase plate smoothing. We attempt to assess the effects of mix and morphology to explain the improved performance; however, neither factor is found to be conclusive within the uncertainty of the measurements.

In magnetically driven cylindrical liner implosions, azimuthally correlated structures form due to the magneto-Rayleigh-Taylor instability. When an axial magnetic field is applied to the system, helical structures have been observed despite the azimuthal magnetic field at the time of observation being several orders of magnitude greater than the applied axial magnetic field (~kT vs ~10 T). Magnetized Liner Inertial Fusion (MagLIF) is a promising fusion concept that relies on both an applied axial magnetic field and a liner implosion to reach fusion conditions. Theory and 2D simulations suggest that MagLIF performance scales to interesting regimes at higher currents and applied magnetic fields; however, implosion instabilities may limit performance. Efforts to understand the formation and scaling of helical structures in MagLIF-like implosions are underway on the Z facility.

Helium or neopentane can be used as surrogate gas fill for deuterium (D2) or deuterium-tritium (DT) in laser-plasma interaction studies. Surrogates are convenient to avoid flammability hazards or the integration of cryogenics in an experiment. To test the degree of equivalency between deuterium and helium, experiments were conducted in the Pecos target chamber at Sandia National Laboratories. Observables such as laser propagation and signatures of laser-plasma instabilities (LPI) were recorded for multiple laser and target configurations. It was found that some observables can differ significantly despite the apparent similarity of the gases with respect to molecular charge and weight. While a qualitative behaviour of the interaction may very well be studied by finding a suitable compromise of laser absorption, electron density, and LPI cross sections, a quantitative investigation of expected values for deuterium fills at high laser intensities is not likely to succeed with surrogate gases.

A variety of spherical crystal x-ray imager (SCXI) diagnostics have been developed and fielded on Magnetized Liner Inertial Fusion (MagLIF) experiments at the Sandia National Laboratories Z-facility. These different imaging modalities provide detailed insight into different physical phenomena such as mix of liner material into the hot fuel, cold liner emission, or reduce impact of liner opacity. However, several practical considerations ranging from the lack of a consistent spatial fiducial for registration to different point-spread-functions and tuning crystals or using filters to highlight specific spectral regions make it difficult to develop broadly applicable metrics to compare experiments across our stagnation image database without making significant unverified assumptions. We leverage experimental data for a model-free assessment of sensitivities to instrumentation-based features for any specified image metric. In particular, we utilize a database of historical and recent MagLIF data including $N_{\text{scans}} = 139$ image plate scans gathered across $N_{\text{exp}} = 67$ different experiments to assess the impact of a variety of features in the experimental observations arising from uncertainties in registration as well as discrepancies in signal-to-noise ratio and instrument resolution. We choose a wavelet-based image metric known as the Mallat Scattering Transform for the study and highlight how alternate metric choices could also be studied. In particular, we demonstrate a capability to understand and mitigate the impact of signal-to-noise, image registration, and resolution difference between images. This is achieved by utilizing multiple scans of the same image plate, sampling random translations and rotations, and applying instrument specific point-spread-functions found by ray tracing to high-resolution datasets, augmenting our data in an effectively model-free fashion.

Pulsed power generators create high-energy-density conditions by rapidly delivering an immense pulse of electrical current to a compact imploding load. Accurately measuring the shape and amplitude of this load current pulse is essential to understanding the behavior of all pulsed power experiments. At the Z Pulsed Power Facility, the closest-in load current measurements are provided by velocimetry techniques such as VISAR (velocity interferometer system for any reflector) and PDV (photonic Doppler velocimetry). Here, fiber-coupled interferometers measure the velocity history of an exploding metallic flyer plate that is embedded in the vertical walls of the current return can. The flyer plate is driven outward by the magnetic pressure from the load current such that magnetohydrodynamic modeling can be used to determine the load current waveform from the measured velocity history. In this paper, we present the first load current velocimetry measurements to be made from the horizontal top flyer plate that carries current radially inward from the return can to the load. These spatially resolved measurements, which span R = 5- 9 mm, are enabled by a transformative new velocimetry diagnostic-a line-imaging velocity interferometer called Z Line VISAR (ZLV)-whose optical performance overcomes the measurement challenges presented by the steep velocity gradients encountered on the top flyer plate. To validate ZLV's capabilities, a 14-MA, 100-ns experiment was conducted to losslessly couple current up the return can and radially inward across the top flyer plate. Comparisons between the ZLV data obtained from this experiment and two-dimensional magnetohydrodynamic simulations driven with the current measured on the return can indicate that the current delivery across the top flyer plate is indeed lossless to within the few-percent uncertainty of the ZLV data. Given that the current coupling is lossless, the experimental results are used to demonstrate that one-dimensional current unfold techniques can be applied to generate a radially resolved load current map from the ZLV velocity data. This analysis provides a template for how to use the ZLV diagnostic to determine the efficacy of current delivery in future experiments where losses may occur in close proximity to the load.