Session PO8: Magneto-Inertial Fusion I

Stagnation morphology in Magnetized Liner Inertial Fusion experiments

In Magnetized Liner Inertial Fusion (MagLIF) experiments on the Z facility, an axial current of 15-20 MA is driven through a thick metal cylinder containing axially-magnetized, laser-heated deuterium fuel. The cylinder implodes, further heating the fuel and amplifying the axial B-field. Instabilities, such as magneto-Rayleigh-Taylor, develop on the exterior of the liner and may feed through to the inner surface during the implosion. Monochromatic x-ray emission at stagnation shows the stagnation column is quasi-helical with axial variations in intensity. Recent experiments demonstrated that the stagnation emission structure changed with modifications to the target wall thickness. Additionally, applying a thick dielectric coating to the exterior of the target modified the stagnation column. A new version of the x-ray self-emission diagnostic has been developed to investigate stagnation with higher resolution. Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology {\&} Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-NA0003525   

Quantification of MagLIF stagnation morphology using the Mallat Scattering Transformation

The morphology of the stagnated plasma resulting from MagLIF is measured by imaging the self-emission x-rays coming from the multi-keV plasma. Equivalent diagnostic response can be derived from integrated rad-hydro simulations from programs such as Hydra and Gorgon. There have been only limited quantitative ways to compare the image morphology, that is the texture, of the simulations to that of the experiments, to compare one experiment to another, or to compare one simulation to another. We have developed a metric of image morphology based on the Mallat Scattering Transformation, a transformation that has proved to be effective at distinguishing textures, sounds, and written characters. This metric has demonstrated excellent performance in classifying an ensemble of synthetic stagnations images. A good regression of the scattering coefficients to the parameters used to generate the synthetic images was found. Finally, the metric has been used to quantitatively compare simulations to experimental self-emission images.   

Multi-Objective data analysis using Bayesian Inference for MagLIF experiments

The MagLIF concept [1] has recently demonstrated Gbar pressures and confinement of charged fusion products at stagnation [2,3]. We present a new analysis methodology that allows for integration of multiple diagnostics including nuclear, x-ray imaging, and x-ray power to determine the temperature, pressure, liner areal density, and mix fraction. A simplified hot-spot model is used with a Bayesian inference network to determine the most probable model parameters that describe the observations while simultaneously revealing the principal uncertainties in the analysis. [1] S.A. Slutz, et al., Phys. Plasmas \textbf{17}, 056303 (2010), [2] M.R. Gomez et al., Phys. Rev. Lett.113, 155003 (2014), [3] P.F. Schmit et al., Phys. Rev. Lett. 113, 155004 (2014)\\ \Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-NA-0003525.   

MagLIF Pre-Heat Optimization on the PECOS Surrogacy Platform

Sandia's MagLIF Program is using the PECOS target area as a platform to optimize the coupling of laser energy into the fuel. After developing laser pulse shapes that reduced SBS and improved energy deposition (presented last year), we will report on the effect on integrated experiments with Z. Despite encouraging results, questions remained about the equivalency of He, (PECOS studies), versus D2 (Z). Furthermore, simulations imply that the goal of at least 1 kJ in the fuel will require higher pressures, requiring a re-design of the gas delivery system. We will present recent results for backscatter measurements and energy deposition profiles in 60 psi and 90 psi deuterium fills and compare them to previously studied helium fills.   

A New Laser Preheat Protocol For Maglif

Previous Magnetized Liner Inertial Fusion experiments at Sandia National Labs have preheated the fuel with the unsmoothed 2$\omega$ Z-Beamlet Laser. A new low intensity laser configuration, using phase plate smoothing and a low-power pulse shape, improved laser propagation and reduced stimulated Brillouin scattering in offline laser experiments. This allows for more efficient use of laser energy and better spot reproducibility. The new laser protocol is estimated to couple at least 650 J to the fuel, sufficient to produce comparable neutron yields with the previous unsmoothed configuration. Mid-Z dopants were also fielded on the underside of the window. Observation of these dopants provided evidence of window material mixing into the fuel with both the unsmoothed and smoothed beam, consistent with MHD simulation predictions.   

Study of laser preheating dependence on laser wavelength and intensity for MagLIF

The magnetized liner inertial fusion (MagLIF) scheme requires preheating underdense fuel to 100's eV temperature by a TW-scale long pulse laser via collisional absorption. To better understand how laser preheat scales with laser wavelength and intensity as well as to provide data for code validation, we have conducted a well-characterized experiment on OMEGA to directly compare laser propagation, energy deposition and laser plasma instabilities (LPI) using 2$\omega $ (527 nm) and 3$\omega $ (351 nm) lasers with intensity in the range of (1-5)x10$^{\mathrm{14}}$ Wcm$^{\mathrm{-2}}$. The laser beam (1 - 1.5 ns square pulse) enters the gas-filled plastic liner though a 2-\textmu m thick polyimide window to heat an underdense Ar-doped deuterium gas with electron density of 5.5{\%} of critical density. Laser propagation and plasma temperature are diagnosed by time-resolved 2D x-ray images and Ar emission spectroscopy, respectively. LPI is monitored by backscattering and hard x-ray diagnostics. The 2$\omega $ beam propagation shows a noticeable larger lateral spread than the 3$\omega $ beam, indicating laser spray due to filamentation. LPI is observed to increase with laser intensity and the 2$\omega $ beam produces more hot electrons compared with the 3$\omega $ beam under similar conditions. Results will be compared with radiation hydrodynamic simulations.   

Laser heating and temperature distribution in MagLIF experiments

In a series of MagLIF laser heating experiments performed at Z, the deuterium gas fill was doped with a trace amount of argon for spectroscopy diagnostics. A germanium spherical crystal spectrometer was employed to observe time-integrated argon K-shell line emission spatially resolved along the axis of the gas cylindrical volume. Simultaneously, an x-ray imager was fielded to record narrow band images centered on selected argon lines. The He-like argon resonance and intercombination lines and their associated satellite transitions in Li-like Ar are observed in the spectra and were recorded with high spectral resolution power. No H-like argon Ly$\alpha $ line emission was observed. Also, narrow band images of the intercombination line were obtained. The spectrum is temperature sensitive and the availability of spatially resolved spectra and image data affords a two-objective analysis that produces the electron temperature distribution spatially resolved along the axial and radial directions. In turn, this information permits us to assess the spatial distributions of heat flow and inverse Bremsstrahlung absorption in the plasma.   

Experimental Observation of the Stratified Electrothermal Instability on Dielectric-Coated Thick Aluminum

The first direct observation of the stratified electrothermal instability on the surface of thick metal is reported. Aluminum rods coated with 70 um Parylene-N were driven to 1 MA in approximately 100 ns, with the metal thicker than the skin depth. The dielectric coating suppressed plasma formation, prolonging the observability of discrete azimuthally-correlated stratified structures perpendicular to the current. Assuming blackbody emission, radiometric calculations indicate strata are temperature perturbations that grow exponentially with rate 0.04/ns in 3000 - 10,000 K aluminum.   

Development of the striation and filament form of the electrothermal instability

Magnetically imploded liners have broad application to ICF, dynamic material property studies, and flux compression. An important consideration in liner performance is the electrothermal instability (ETI), an Ohmic heating instability that manifests in 2 ways: assuming vertical current flow, ETI forms hot, horizontal bands (striations) in metals, and vertical filaments in plasmas. Striations are especially relevant in that they can develop into density perturbations, which then couple to the dangerous magneto Rayleigh-Taylor (MRT) instability during liner acceleration. Recent visible emission images of Ohmically heated rods (Awe et al., IEEE Trans. Plasma Sci., 45, 584-589 (2017)) show evidence of both the striation and filament form of ETI, suggesting several questions: (1) can simulation qualitatively reproduce the data? (2) If so, what seeds the striation ETI, and how does it transition to filaments? (3) Does the striation develop into a strong density perturbation, important for MRT? In this work, we use analytic theory and 3D MHD simulation to study how isolated resistive inclusions, embedded in a perfectly smooth rod and communicating through current redistribution, can be used to address the above questions.   

Experimental Study of Magnetic Field Production and Dielectric Breakdown of Auto-Magnetizing Liners

AutoMag liners premagnetize the fuel in MagLIF targets and provide enhanced x-ray diagnostic access and increased current delivery without requiring external field coils [Slutz et al., Phys. Plasmas 24, 012704 (2017)]. AutoMag liners are composite liners made with discrete metallic helical conduction paths separated by insulating material. First, a low dI/dt “foot” current pulse (~1 MA in ~100 ns) premagnetizes the fuel. Next, a higher dI/dt pulse with larger induced electric field initiates breakdown on the composite liner’s surface, switching the current from helical to axial to implode the liner. Experiments on MYKONOS have tested the premagnetization and breakdown phases of AutoMag and demonstrate axial magnetic fields above 90 Tesla for a 550 kA peak current pulse. Electric fields of 17 MV/m have been generated before breakdown. AutoMag may enhance MagLIF performance by increasing the premagnetization strength significantly above 30 T, thus reducing thermal-conduction losses and mitigating anomalous diffusion of magnetic field out of hotter fuel regions, by, for example, the Nernst thermoelectric effect.   

Enhancing Neutron Yield in Cylindrical Implosions with an Applied Magnetic Field

Laser-driven MagLIF (magnetized liner inertial fusion) is being developed on the OMEGA laser; multiple experimental campaigns have been conducted that have examined yield dependence on magnetic field, preheat energy, and fill pressure. Magnetic fields were generated in the region of interest using coils driven by current from the magneto-inertial fusion electrical discharge system (MIFEDS). A variety of coil designs were used and current was varied to generate different levels of magnetic field without impeding the path of the 40 implosion beams. We demonstrate a large enhancement of neutron yield by applying an initial field of 10 T along the axis of a cylindrical implosion. The work presented herein was funded in part by the Advanced Research Projects Agency-Energy (ARPA-E), U.S. Department of Energy, under Award Number DE-AR0000568 and the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944.   

Fuel Areal-Density Measurements in Laser-Driven Magnetized Inertial Fusion from Secondary Neutrons

Laser-driven magnetized liner inertial fusion is being developed on the OMEGA laser to provide the first data at a significantly smaller scale than the Z pulsed-power machine in order to test scaling and to provide more shots with better diagnostic access than Z. In OMEGA experiments, a 0.6-mm-outer-diam plastic cylinder filled with 11 atm of D$_{\mathrm{2}}$ is placed in an axial magnetic field of $\sim $10 T, the D$_{\mathrm{2}}$ is preheated by a single beam along the axis, and then the cylinder is compressed by 40 beams. Secondary DT neutron yields provide a measurement of the areal density of the compressed D$_{\mathrm{2}}$ because the compressed fuel is much smaller than the mean free path and the Larmor radius of the T produced in D--D fusion. Measured secondary yields confirm theoretical predictions that preheating and magnetization reduce fuel compression. Higher fuel compression is found to consistently lead to lower neutron yields, which is not predicted by simulations. The information, data, or work presented herein was funded in part by the Advanced Research Projects Agency-Energy (ARPA-E), U.S. Department of Energy, under Award Number DE-AR0000568 and the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944.