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Auto-magnetizing (AutoMag) liners 1 have demonstrated strong precompressed axial magnetic field production (>100 T) and remarkable cylindrical implosion uniformity during experiments 2 on the Z accelerator. However, both axial field production and implosion uniformity require further optimization to support use of AutoMag targets in magnetized liner inertial fusion (MagLIF) experiments. Recent experimental study on the Mykonos accelerator has provided data on the initiation and evolution of dielectric flashover in AutoMag targets; these results have directly enabled advancement of magnetohydrodynamic (MHD) modeling protocols used to simulate AutoMag liner implosions. Using these modeling protocols, we executed three-dimensional MHD simulations focused on improving AutoMag target designs, specifically seeking to optimize axial magnetic field production and enhance cylindrical implosion uniformity for MagLIF. By eliminating the previously used driver current prepulse and reducing the helical gap widths in AutoMag liners, simulations indicate that the optimal 30-50 T range of precompressed axial magnetic field for MagLIF can be accomplished concurrently with improved cylindrical implosion uniformity, thereby enabling an optimally premagnetized magneto-inertial fusion implosion with high cylindrical uniformity.

Magnetized Liner Inertial Fusion (MagLIF) is a magneto-inertial fusion (MIF) concept being studied on the Z-machine at Sandia National Laboratories. MagLIF relies on quasi-adiabatic heating of a gaseous deuterium (DD) fuel and flux compression of a background axially oriented magnetic field to achieve fusion relevant plasma conditions. The magnetic flux per fuel radial extent determines the confinement of charged fusion products and is thus of fundamental interest in understanding MagLIF performance. It was recently shown that secondary DT neutron spectra and yields are sensitive to the magnetic field conditions within the fuel, and thus provide a means by which to characterize the magnetic confinement properties of the fuel. 1 , 2 , 3 We utilize an artificial neural network to surrogate the physics model of Refs. [1] , [2] , enabling Bayesian inference of the magnetic confinement parameter for a series of MagLIF experiments that systematically vary the laser preheat energy deposited in the target. This constitutes the first ever systematic experimental study of the magnetic confinement properties as a function of fundamental inputs on any neutron-producing MIF platform. We demonstrate that the fuel magnetization decreases with deposited preheat energy in a fashion consistent with Nernst advection of the magnetic field out of the hot fuel and diffusion into the target liner.

X-ray radiography has been used to diagnose a wide variety of experiments at the Z facility including inertial confinement fusion capsule implosions, the growth of the magneto-Rayleigh-Taylor instability in solid liners, and the development of helical structures in axially magnetized liner implosions. In these experiments, the Z Beamlet laser (1 kJ, 1 ns) was used to generate the x-ray source. An alternate x-ray source is desirable in experiments where the Z Beamlet laser is used for another purpose (e.g., preheating the fuel in magnetized liner inertial fusion experiments) or when multiple radiographic lines of sight are necessary.

Magnetized Liner Inertial Fusion (MagLIF) is a magneto-inertial fusion concept that relies on fuel magnetization, laser preheat, and a magnetically driven implosion to produce fusion conditions. In MagLIF, the target is a roughly 10 mm long, 5 mm diameter, 0.5 mm thick, cylindrical beryllium shell containing ~1 mg/cm 3 D 2 gas. An axial magnetic field on the order of 10 T is applied to the target, and several kJ of laser energy is deposited into the fuel. Up to 20 MA of current is driven axially through the beryllium target, causing it to implode over approximately 100 ns. The implosion produces a ~100-μm diameter, ~8-mm tall fuel column with a burn-averaged ion temperature of several keV, that generates 10 11 -10 13 DD neutrons.

The performance of Magnetized Liner Inertial Fusion (MagLIF) experiments is sensitive to the amount of laser energy coupled to the fuel during the preheat stage. 1 A significant source of energy loss in this process comes from the need to heat a laser entrance hole foil (LEH) located at the entrance to the target that is required to contain the gaseous fusion fuel. The energy lost to the LEH is a function of its thickness which can be reduced by cryogenically cooling the fuel, lowering the pressure required for a given fuel density. 2 To realize this, a cryogenically-cooled laser target platform was commissioned in the Pecos chamber that enables rapid testing of preheat configurations, 3 and a cryogenically-cooled MagLIF configuration was tested that symmetrically cools the liner from the top and bottom, minimizing temperature gradients across the target. These new capabilities were utilized to perform a cryogenically-cooled MagLIF experiment that demonstrated >2 kJ of preheat energy coupled to the fuel for the first time on Z.

Extended-MHD simulations of power flow along a pulsed-power transmission line are performed in a 2-D axisymmetric geometry, in particular looking at the influence of Hall physics for a transmission line coupled to the liner used in a magnetized liner inertial fusion experiment at Sandia National Labs. It was recently shown by the authors that, for a coaxial transmission line, when Hall physics is included, significantly more blow-off occurs from plasma initialized against the anode compared to the cathode. The mechanism of this blow-off was traced to electron ${\text{E}}\times {\text{B}}$ drift modeled by the Hall term. This result is also observed for the present simulations, and it is shown that the anode blow-off significantly delays the coupling of current to the liner. It is also found that Hall MHD and MHD results are sensitive to the treatment of density floors and the plasma–vacuum interface. Although MHD shows more sensitivity than Hall MHD, correct modeling of the transition from plasma to vacuum remains an unsolved problem that must be addressed in order to improve the predictive capability of fluid-based power flow simulations with regard to energy coupling.

Magnetized Liner Inertial Fusion (MagLIF) is a magneto-inertial fusion concept, which is presently being studied on the Z Pulsed Power Facility. The concept utilizes an axial magnetic field and laser heating to produce fusion-relevant conditions at stagnation despite a peak magnetically driven implosion velocity of less than 100 km/s. Initial mymargin experiments demonstrated the viability of the concept but left open questions about the amount of laser energy coupled to the fuel and the role that mix played in the stagnation conditions. In this paper, simple methodologies for estimating the laser energy coupled to the fuel and determining the stagnation pressure and mix are presented. These tools enabled comparisons across many experiments to establish performance trends, as well as allow comparisons with 2-D magnetohydrodynamics simulations. The initial experiments were affected by low laser energy coupling (0.2–0.6 kJ), which resulted in reduced neutron yields (1– $2\times 10^{12}$ ). In addition, all early experiments utilized mid-Z (aluminum) fuel-facing components. Mixing from these components had a significant impact on stagnation and increased with laser energy. Lower neutron yields (1– $3\times 10^{11}$ ) were measured with higher laser coupling (0.8–1.2 kJ), which significantly deviated from the predicted scaling. When all fuel-facing components were made from a low-Z material (beryllium), neutron production increased ( $3.2\times 10^{12}$ ) and scaled as expected with laser energy; experimental yields were approximately 40% of simulated yields. In addition, roughly I4 yield scaling was observed in experiments, where the load current was varied from 16–18 MA. These results represent the first step in experimental demonstration of stagnation performance scaling with input parameters in MagLIF.

We present experimental results from the first systematic study of performance scaling with drive parameters for a magnetoinertial fusion concept. In magnetized liner inertial fusion experiments, the burn-averaged ion temperature doubles to 3.1 keV and the primary deuterium-deuterium neutron yield increases by more than an order of magnitude to 1.1×10^{13} (2 kJ deuterium-tritium equivalent) through a simultaneous increase in the applied magnetic field (from 10.4 to 15.9 T), laser preheat energy (from 0.46 to 1.2 kJ), and current coupling (from 16 to 20 MA). Individual parametric scans of the initial magnetic field and laser preheat energy show the expected trends, demonstrating the importance of magnetic insulation and the impact of the Nernst effect for this concept. A drive-current scan shows that present experiments operate close to the point where implosion stability is a limiting factor in performance, demonstrating the need to raise fuel pressure as drive current is increased. Simulations that capture these experimental trends indicate that another order of magnitude increase in yield on the Z facility is possible with additional increases of input parameters.

We present two-dimensional temperature measurements of magnetized and unmagnetized plasma experiments performed at Z relevant to the preheat stage in magnetized liner inertial fusion. The deuterium gas fill was doped with a trace amount of argon for spectroscopy purposes, and time-integrated spatially resolved spectra and narrow-band images were collected in both experiments. The spectrum and image data were included in two separate multiobjective analysis methods to extract the electron temperature spatial distribution T_{e}(r,z). The results indicate that the magnetic field increases T_{e}, the axial extent of the laser heating, and the magnitude of the radial temperature gradients. Comparisons with simulations reveal that the simulations overpredict the extent of the laser heating and underpredict the temperature. Temperature gradient scale lengths extracted from the measurements also permit an assessment of the importance of nonlocal heat transport.

Magnetized Liner Inertial Fusion (MagLIF) at Sandia National Laboratories involves a laser preheating stage where a few-ns laser pulse passes through a few-micron-thick plastic window to preheat gaseous fusion fuel contained within the MagLIF target. Interactions with this window reduce heating efficiency and mix window and target materials into the fuel. A recently proposed idea called “Laser Gate” involves removing the window well before the preheating laser is applied. In this article, we present experimental proof-of-principle results for a pulsed-power implementation of Laser Gate, where a thin current-carrying wire weakens the perimeter of the window, allowing the fuel pressure to push the window open and away from the preheating laser path. For this effort, transparent targets were fabricated and a test facility capable of studying this version of Laser Gate was developed. A 12-frame bright-field laser schlieren/shadowgraphy imaging system captured the window opening dynamics on microsecond timescales. The images reveal that the window remains largely intact as it opens and detaches from the target. A column of escaping pressurized gas appears to prevent the detached window from inadvertently moving into the preheating laser path.

We report on experimental measurements of how an externally imposed magnetic field affects plasma heating by kJ-class, nanosecond laser pulses. The experiments reported here took place in gas cells analogous to magnetized liner inertial fusion targets. We observed significant changes in laser propagation and energy deposition scale lengths when a 12T external magnetic field was imposed in the gas cell. We find evidence that the axial magnetic field reduces radial electron thermal transport, narrows the width of the heated plasma, and increases the axial plasma length. Reduced thermal conductivity increases radial thermal gradients. This enhances radial hydrodynamic expansion and subsequent thermal self-focusing. Our experiments and supporting 3D simulations in helium demonstrate that magnetization leads to higher thermal gradients, higher peak temperatures, more rapid blast wave development, and beam focusing with an applied field of 12T.

Thin polyimide (PI) windows are used to contain gases in a variety of targets including National Ignition Facility ignition targets. Magnetized liner inertial fusion targets shot on the Sandia National Laboratory Z-facility and on the University of Rochester OMEGA laser facility typically contain deuterium gas in the pressure range from a few to as many as 15 atm, with the window diameters ranging from a few tenths of a millimeter at OMEGA to several millimeters at the Z-facility. These pressures are generally higher, with larger plastic deformations, than previously investigated. We have fabricated and assembled PI windows and measured their deflections and burst pressures for these pressure and diameter ranges at room temperature. The results are dependent on PI formulation and the details of the window assembly geometry. We analyze the scaling behavior of these higher-pressure windows similarly to but with an extension of the analysis of Bhandarkar et al. [Fusion Sci. Technol., Vol. 70, p. 332]...

The 100-Gbar Laser Direct Drive program calls for ablator capsules with no defects larger than 0.5 μm in lateral dimension and fewer than ten defects with lateral dimensions between 0.1 and 0.5 μm....

At the Z Facility at Sandia National Laboratories, the magnetized liner inertial fusion (MagLIF) program aims to study the inertial confinement fusion in deuterium-filled gas cells by implementing a three-step process on the fuel: premagnetization, laser preheat, and Z-pinch compression. In the laser preheat stage, the Z-Beamlet laser focuses through a thin polyimide window to enter the gas cell and heat the fusion fuel. However, it is known that the presence of the few μm thick window reduces the amount of laser energy that enters the gas and causes window material to mix into the fuel. These effects are detrimental to achieving fusion; therefore, a windowless target is desired. The Lasergate concept is designed to accomplish this by “cutting” the window and allowing the interior gas pressure to push the window material out of the beam path just before the heating laser arrives. In this work, we present the proof-of-principle experiments to evaluate a laser-cutting approach to Lasergate and explore the subsequent window and gas dynamics. Further, an experimental comparison of gas preheat with and without Lasergate gives clear indications of an energy deposition advantage using the Lasergate concept, as well as other observed and hypothesized benefits. While Lasergate was conceived with MagLIF in mind, the method is applicable to any laser or diagnostic application requiring direct line of sight to the interior of gas cell targets.

High-gain magnetized liner inertial fusion (MagLIF) is a possible way to realize fusion. To investigate electrothermal effects on the transport of magnetic flux and thermal flux in MagLIF, this study has developed a resistive magnetohydrodynamics (MHD) module including an axial magnetic field using the code MULTI-IFE. MagLIF driven by a peak current of 60 MA releases 1080 MJ of fusion energy for a 1-cm-long liner, corresponding to an energy gain of approximately 180. The magnetic field is decompressed by electrothermal effects owing to the great temperature gradient in the fuel. This papers shows that a Nernst flux limiter of between 0.1 and 0.3 prevents the Nernst velocity from significantly decompressing the axial magnetic field and achieving a relative high yield. Compared with that of a simple gas target, the magnetic flux loss in the high-gain MagLIF target can be reduced from 70% to 30% owing to the magnetic insulation in the cryogenic deuterium-tritium. Most of the cryogenic DT layer in a high-gain MagLIF target is burned, resulting in a significant increase in the fusion yield.

Fuel magnetization in magneto-inertial fusion (MIF) experiments improves charged burn product confinement, reducing requirements on fuel areal density and pressure to achieve self-heating. By elongating the path length of 1.01 MeV tritons produced in a pure deuterium fusion plasma, magnetization enhances the probability for deuterium–tritium reactions producing 11.8−17.1 MeV neutrons. Nuclear diagnostics thus enable a sensitive probe of magnetization. Characterization of magnetization, including uncertainty quantification, is crucial for understanding the physics governing target performance in MIF platforms, such as magnetized liner inertial fusion (MagLIF) experiments conducted at Sandia National Laboratories, Z-facility. We demonstrate a deep-learned surrogate of a physics-based model of nuclear measurements. A single model evaluation is reduced from O(10–100) CPU hours on a high-performance computing cluster down to O(10) ms on a laptop. This enables a Bayesian inference of magnetization, rigorously accounting for uncertainties from surrogate modeling and noisy nuclear measurements. The approach is validated by testing on synthetic data and comparing with a previous study. We analyze a series of MagLIF experiments systematically varying preheat, resulting in the first ever systematic experimental study of magnetic confinement properties of the fuel plasma as a function of fundamental inputs on any neutron-producing MIF platform. We demonstrate that magnetization decreases from BR∼0.5 to BR∼0.2 MG cm as laser preheat energy deposited increases from Epreheat∼460 J to Epreheat∼1.4 kJ. This trend is consistent with 2D LASNEX simulations showing Nernst advection of the magnetic field out of the hot fuel and diffusion into the target liner.

The Magnetized Liner Inertial Fusion experimental campaign conducted at the University of Rochester’s Laboratory for Laser Energetics has evolved significantly since its start in 2014. Scientific r...

Z-beamlet experiments conducted at the PECOS test facility at Sandia National Laboratories (SNL) investigated the nonlinear processes in laser plasma interaction (or laser-plasma instabilities) that complicate the deposition of laser energy by enhanced absorption, backscatter, filamentation, and beam-spray that can occur in large-scale laser-heated gas cell targets. These targets and experiments were designed to provide better insight into the physics of the laser preheat stage of the Magnetized Liner Inertial Fusion scheme being tested on the SNL Z-machine. The experiments aim to understand the trade-offs between laser spot size, laser pulse shape, laser entrance hole window thickness, and fuel density for laser preheat. Gas cell target design evolution and fabrication adaptations to accommodate the evolving experiment and scientific requirements are described in this paper.

We have commissioned a new time-resolved, x-ray imaging diagnostic for the Z facility. The primary intended application is for diagnosing the stagnation behavior of Magnetized Liner Inertial Fusion (MagLIF) and similar targets. We have a variety of imaging systems at Z, both time-integrated and time-resolved, that provide valuable x-ray imaging information, but no system at Z up to this time provides a combined high-resolution imaging with multi-frame time resolution; this new diagnostic, called TRICXI for Time Resolved In-Chamber X-ray Imager, is meant to provide time-resolved spatial imaging with high resolution. The multi-frame camera consists of a microchannel plate camera. A key component to achieving the design goals is to place the instrument inside the Z vacuum chamber within 2 m of the load, which necessitates a considerable amount of x-ray shielding as well as a specially designed, independent vacuum system. A demonstration of the imaging capability for a series of MagLIF shots is presented. Predictions are given for resolution and relative image irradiance to guide experimenters in choosing the desired configuration for their experiments.

Axial magnetic field is one of the main parameters of magnetized liner inertial fusion (MagLIF), which is greatly different from other traditional inertial confinement fusion configurations. The introduce of axial magnetic field dramatically increases energy deposit efficiency of alpha particles, when initial Bz increases from 0 to 30 T, the ratio of deposited alpha energy rises from 7% to 53%. In the MagLIF process, the evolvement of magnetic flux in fuel can be roughly divided into three main stages: undisturbed, oscillation, and equilibrium. The distributions and evolution characteristic of axial magnetic field are both determined by the liner conductivity, fuel conductivity, and the fluid dynamics. The pressure imbalance between fuel and liner, caused by laser injection, is the source of fluid oscillation, which is an intrinsic disadvantage of laser preheating method. This fluid oscillation does not lead the magnetic flux to decrease monotonically in the fuel during implosion process, but oscillate repeatedly, even increase in a short time. Nernst effect plays a negative role in MagLIF process. As initial axial magnetic field decreases from 30 to 20 to 10 T, the Nernst effect causes magnetic flux loss to increase from 28% to 44% to 73% correspondingly, and the deposited alpha energy ratio drops from 44% to 27% to 4% respectively. So the initial magnetic field is supposed to be moderately high. The radial distribution of temperature in fuel should be as uniform as possible after preheating, which is helpful in reducing the influence of Nernst effect. Compared with Nernst effect, the end loss effect is much responsible for rapid drawdown of fusion yield. A large number of physical images are acquired and summarized through this work, which are helpful in understanding the process of magnetic flux compression and diffusion in MagLIF process. The simulation can act as a powerful tool and the simulation results can serve as a useful guidance for the future experimental designs.

Benefiting from laser preheat and magnetization, magnetized liner lnertial fusion (MagLIF) has a promising potential because theoretically it can dramatically lower the difficulties in realizing the controlled fusion. In this paper, the end loss effect caused by laser preheat in MagLIF process is chosen as an objective to explore its influences, and a one-dimensional and heuristic model of this effect is proposed based on the jet model of ideal fluid, in which the high-dimensional influences, such as geometric parameters and sausage instability, are taken into consideration. To complete the verification progress, the calculation results of one-dimensional MIST code and two-dimensional programs TriAngels and HDYRA are compared, and the application scopes of this heuristic model are discussed and summarized. Based on this model, the key parameters and influences of the end loss effect on the MagLIF implosion process and pre-heating effect are obtained. The calculation results show that the MagLIF load maintains a similar hydrodynamic evolution process in most of the implosion processes with different laser entrance radii, and experiences the same percentage of mass (～16%) lost during stagnation stage. With the same driving current, the fuel temperature will rise higher in the model with more mass losing, so the fusion yields do not change too much. The mass loss ratio seems to play a dominant role. It is recommended to design the laser entrance hole as small as possible in the experiment to increase the yield. The predictions obtained after considering the end loss effect lower the preheating temperature and fusion yield, but no change happens to the regularity trend. As the liner height increases, the preheating temperature, peak current, fuel internal energy, and fusion yield each still show a monotonically downward trend. Therefore, under the premise of fixed driving capability and laser output capability, it is suggested that the liner height in MagLIF load design should be as short as possible. The established heuristic model and conclusions are helpful in better understanding the physical mechanism in the process of MagLIF preheat and end loss.

Optimizing the performance of the Magnetized Liner Inertial Fusion (MagLIF) platform on the Z pulsed power facility requires coupling greater than 2 kJ of preheat energy to an underdense fuel in the presence of an applied axial magnetic field ranging from 10 to 30 T. Achieving the suggested optimal preheat energies has not been experimentally achieved so far. In this work, we explore the preheat design space for cryogenically cooled MagLIF targets, which represent a viable candidate for increasing preheat energies. Using 2D and 3D HYDRA MHD simulations, we first discuss the various physical effects that occur during laser preheat, such as laser energy deposition, self-focusing, and filamentation. After identifying the changes that different phase plates, gas-fill densities, and magnetic fields bring to the aforementioned physical effects, we, then, consider higher laser energies that are achievable with modest upgrades to the Z Beamlet laser. Finally, with a 6.0-kJ upgraded laser, 3D calculations suggest that it is possible to deliver 4.25 kJ into the MagLIF fuel, resulting in an expected deuterium neutron yield of Y DD ≃ 1.5 × 10 14, or roughly 50 kJ of DT equivalent yield, at 20-MA current drive. This represents a 10-fold increase in the currently achieved yields for MagLIF.

The magnetized liner inertial fusion (MagLIF) scheme relies on coupling laser energy into an underdense fuel raising the fuel adiabat at the start of the implosion. To deposit energy into the fuel, the laser must first penetrate a laser entrance hole (LEH) foil which can be a significant energy sink and introduce mix. In this paper, we report on experiments investigating laser energy coupling into MagLIF-relevant gas cell targets with LEH foil thicknesses varying from 0.5 μm to 3 μm. Two-dimensional (2D) axisymmetric simulations match the experimental results well for 0.5 μm and 1 μm thick LEH foils but exhibit whole-beam self-focusing and excessive penetration of the laser into the gas for 2 μm and 3 μm thick LEH foils. Better agreement for the 2 μm-thick foil is found when using a different thermal conductivity model in 2D simulations, while only 3D Cartesian simulations come close to matching the 3 μm-thick foil experiments. The study suggests that simulations may over-predict the tendency for the laser to self-focus during MagLIF preheat when thicker LEH foils are used. This effect is pronounced with 2D simulations where the azimuthally symmetric density channel effectively self-focuses the rays that are forced to traverse the center of the plasma. The extra degree of freedom in 3D simulations significantly reduces this effect. The experiments and simulations also suggest that, in this study, the amount of energy coupled into the gas is highly correlated with the laser propagation length regardless of the LEH foil thickness.

Laser heating of a gas in a cylindrical liner held in by a thin foil window is a critical process in the MagLIF (magnetized liner inertial fusion) concept [S. A. Slutz and R. A. Vesey, Phys. Rev. Lett. 108, 025003 (2012)]. Window burnthrough and gas heating for OMEGA scale MagLIF cylinders as a function of time have been determined using spectrally integrated soft x-ray diagnostics. Window laser absorption is classified in terms of the emitted x-rays from the window plasma as a function of laser energy and shows that the laser energy absorbed is weakly dependent on incident intensity. Radiation–hydrodynamic simulations overestimate the amount of laser energy absorbed by the window as evidenced by the increase in x-ray radiation across several photon energy bands compared to experiments. Gas temperatures inferred from soft x-ray emission from the front 1 mm of the liner are shown to evolve in time in a similar manner to simulation predictions. Soft x-ray emission from the gas within the region of the liner that is normally imploded is shown to meet the 100 eV requirements set by the initial point design for laser-driven MagLIF.

Cylindrical foil liners, with foil thicknesses on the order of 400 nm, are often used in university-scale Z-pinch experiments (∼1 MA in 100 ns) to study physics relevant to inertial confinement fusion efforts on larger-scale facilities (e.g., the magnetized liner inertial fusion effort on the 25-MA Z facility at Sandia National Laboratories). The use of ultrathin foil liners typically requires a central support rod to maintain the structural integrity of the liner target assembly prior to implosion. The radius of this support rod sets a limit on the maximum convergence ratio achievable for the implosion. In recent experiments with a support rod and a pre-imposed axial magnetic field, helical instability structures in the imploding foil plasma were found to persist as the foil plasma stagnated on the rod and subsequently expanded away from the rod [Yager-Elorriaga et al., Phys. Plasmas 25(5), 056307 (2018)]. We have now used the 3D extended magnetohydrodynamics simulation code PERSEUS (which includes Hall physics) [C. E. Seyler and M. R. Martin, Phys. Plasmas 18(1), 012703 (2011)] to study these experiments. The results suggest that it is the support rod that is responsible for the helical structures persisting beyond stagnation. Furthermore, we find that as the radius of the support rod decreases (i.e., as the convergence ratio increases), the integrity and persistence of the helical modes diminish. In the limit with no support rod, we find that the structure of the final stagnation column is governed by the structure of the central precursor plasma column. These simulation results and their comparisons to experiment are presented.

The Magnetized Liner Inertial Fusion (MagLIF) experimental platform [M. R. Gomez et al., Phys. Rev. Lett. 113, 155003 (2014)] represents the most successful demonstration of magneto-inertial fusion (MIF) techniques to date in pursuit of ignition and significant fusion yields. The pressing question remains regarding how to scale MIF concepts like MagLIF to more powerful pulsed-power drivers while avoiding significant changes in physical regimes that could adversely impact performance. In this work, we propose a conservative approach for scaling general MIF implosions, including MagLIF. Underpinning our scaling approach is a theoretical framework describing the evolution of the trajectory and thickness of a thin-walled, cylindrical, current-driven shell imploding on preheated, adiabatic fuel. By imposing that scaled implosions remain self-similar, we obtain a set of scaling rules expressing key target design parameters and performance metrics as functions of the maximum driver current I max. We identify several scaling paths offering unique, complementary benefits and trade-offs in terms of physics risks and driver requirements. Remarkably, when scaling present-day experiments to higher coupled energies, these paths are predicted to preserve or reduce the majority of known performance-degrading effects, including hydrodynamic instabilities, impurity mix, fuel energy losses, and laser-plasma interactions, with notable exceptions clearly delineated. In the absence of α heating, our scaling paths exhibit neutron yield per-unit-length scaling as Y ∝ [ I max 3 , I max 4.14 ] and ignition parameter scaling as χ ∝ [ I max , I max 2.14 ]. By considering the specific physics risks unique to each scaling path, we provide a roadmap for future investigations to evaluate different scaling options through detailed numerical studies and scaling-focused experiments on present-day facilities. Overall, these results highlight the potential of MIF as a key component of the national ignition effort.

Prior to implosion in Magnetized Liner Inertial Fusion (MagLIF), the fuel is heated to temperatures on the order of several hundred eV with a multi-kJ, multi-ns laser pulse. We present two laser heated plasma experiments, relevant to the MagLIF preheat stage, performed at Z with beryllium liners filled with deuterium and a trace amount of argon. In one experiment, there is no magnetic field and, in the other, the liner and fuel are magnetized with an 8.5 T axial magnetic field. The recorded time integrated, spatially resolved spectra of the Ar K-shell emission are sensitive to electron temperature Te. Individual analysis of the spatially resolved spectra produces electron temperature distributions Te(z) that are resolved along the axis of laser propagation. In the experiment with magnetic field, the plasma reaches higher temperatures and the heated region extends deeper within the liner than in the unmagnetized case. Radiation magnetohydrodynamics simulations of the experiments are presented and post-processed. A comparison of the results from experimental and simulated data reveals that the simulations underpredict Te in both cases but the differences are larger in the case with magnetic field.

The morphology of the stagnated plasma resulting from Magnetized Liner Inertial Fusion (MagLIF) is measured by imaging the self-emission x-rays coming from the multi-keV plasma, and the evolution of the imploding liner is measured by radiographs. Equivalent diagnostic response can be derived from integrated rad-MHD 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 simulations and experiments. We have developed a metric of image morphology based on the Mallat Scattering Transformation (MST), a transformation that has proved to be effective at distinguishing textures, sounds, and written characters. This metric has demonstrated excellent performance in classifying ensembles of synthetic stagnation images. We use this metric to quantitatively compare simulations to experimental images, cross experimental images, and to estimate the parameters of the images with uncertainty via a linear regression of the synthetic images to the parameters used to generate them. This coordinate space has proved very adept at doing a sophisticated relative background subtraction in the MST space. This was needed to compare the experimental self emission images to the rad-MHD simulation images. We have also developed theory that connects the transformation to the causal dynamics of physical systems. This has been done from the classical kinetic perspective and from the field theory perspective, where the MST is the generalized Green's function, or S-matrix of the field theory in the scale basis. From both perspectives the first order MST is the current state of the system, and the second order MST are the transition rates from one state to another. An efficient, GPU accelerated, Python implementation of the MST was developed. Future applications are discussed., Comment: 56 pages, 24 figures, Sandia National Laboratories Technical Report

Magnetized inertial fusion experiments are approaching regimes where the radial transport is dominated by collisions between magnetized ions, providing an opportunity to exploit effects usually associated with steady-state magnetic fusion. In particular, the low-density hotspot characteristic of magnetized liner inertial fusion results in diamagnetic and thermal frictions which can demix thermalized ash from fuel, accelerating the fusion reaction. For reactor regimes in which there is substantial burnup of the fuel, increases in the fusion energy yield on the order of 5% are possible., Comment: 5 pages, 4 figures

The one-dimensional imager of neutrons (ODIN) at the Sandia Z facility consists of a 10-cm block of tungsten with rolled edges, creating a slit imager with slit widths of either 250, 500, or 750 μm. Designed with a 1-m neutron imaging line of sight, we achieve about 4:1 magnification and 500-μm axial spatial resolution. The baseline inertial confinement fusion concept at Sandia is magnetized liner inertial fusion, which nominally creates a 1-cm line source of neutrons. ODIN was designed to determine the size, shape, and location of the neutron producing region, furthering the understanding of compression quality along the cylindrical axis of magnetized liner implosions. Challenges include discriminating neutrons from hard x-rays and gammas with adequate signal-to-noise in the 2 × 1012 deuterium-deuterium (DD) neutron yield range, as well as understanding the point spread function of the imager to various imaging detectors (namely, CR-39). Modeling efforts were conducted with MCNP6.1 to determine neutron response functions for varying configurations in a clean DD neutron environment (without x-rays or gammas). Configuration alterations that will be shown include rolled-edge slit orientation and slit width, affecting the resolution and response function. Finally, the experiment to determine CR-39 neutron sensitivity, with and without a high density polyethylene (n, p) converter, an edge spread function, and resolution will be discussed.The one-dimensional imager of neutrons (ODIN) at the Sandia Z facility consists of a 10-cm block of tungsten with rolled edges, creating a slit imager with slit widths of either 250, 500, or 750 μm. Designed with a 1-m neutron imaging line of sight, we achieve about 4:1 magnification and 500-μm axial spatial resolution. The baseline inertial confinement fusion concept at Sandia is magnetized liner inertial fusion, which nominally creates a 1-cm line source of neutrons. ODIN was designed to determine the size, shape, and location of the neutron producing region, furthering the understanding of compression quality along the cylindrical axis of magnetized liner implosions. Challenges include discriminating neutrons from hard x-rays and gammas with adequate signal-to-noise in the 2 × 1012 deuterium-deuterium (DD) neutron yield range, as well as understanding the point spread function of the imager to various imaging detectors (namely, CR-39). Modeling efforts were conducted with MCNP6.1 to determine neutron r...

The fundamental physics of the magnetic field distribution in a plasma implosion with a preembedded magnetic field is investigated within a gas-puff $Z$ pinch. Time and space resolved spectroscopy of the polarized Zeeman effect, applied for the first time, reveals the impact of a preembedded axial field on the evolution of the current distribution driven by a pulsed-power generator. The measurements show that the azimuthal magnetic field in the imploding plasma, even in the presence of a weak axial magnetic field, is substantially smaller than expected from the ratio of the driving current to the plasma radius. Much of the current flows at large radii through a slowly imploding, low-density plasma. Previously unpredicted observations in higher-power imploding-magnetized-plasma experiments, including recent, unexplained structures observed in the magnetized liner inertial fusion experiment, may be explained by the present discovery. The development of a force-free current configuration is suggested to explain this phenomenon.

Amplification of the transverse scattered component of stimulated Brillouin scattering (SBS) can contribute to optical damage in the large aperture optics of multi-kJ lasers. Because increased laser bandwidth from optical phase modulation (PM) can suppress SBS, high energy laser amplifiers are injected with PM light. Phase modulation distributes the single-frequency spectrum of a master oscillator laser among individual PM sidebands, so a sufficiently high modulation index β can maintain the fluence for all spectral components below the SBS threshold. To avoid injection of single frequency light in the event of a PM failure, a high-speed PM failsafe system (PMFS) must be employed. Because PM is easily converted to AM, essentially all PM failsafes detect AM, with the one described here employing a novel configuration where optical heterodyne detection converts PM to AM, followed by passive AM power detection. Although the PMFS is currently configured for continuous monitoring, it can also detect PM for pulse durations ≥2 ns and could be modified to accommodate shorter pulses. This PMFS was deployed on the Z-Beamlet Laser (ZBL) at Sandia National Laboratories, as required by an energy upgrade to support programs at Sandia's Z Facility such as magnetized liner inertial fusion. Depending on the origin of a PM failure, the PMFS responds in as little as 7 ns. In the event of an instantaneous failure during initiation of a laser shot, this response time translates to a 30-50 ns margin of safety by blocking a pulse from leaving ZBL's regenerative amplifier, which prevents injection of single frequency light into the main amplification chain. The performance of the PMFS, without the need for operator interaction, conforms to the principles of engineered safety.

Magnetized Liner Inertial Fusion (MagLIF) is a magneto-inertial fusion concept currently being investigated on Sandia's Z facility. In MagLIF an axial magnetic field of 10–30 T is applied to the cold fusion fuel, which is contained in a cm-scale beryllium can, called a liner. The fuel is then heated by a few-kJ, TW-class laser to around 100 eV, The current from the Z machine flows axially through the liner, causing it to implode. The magnetized, laser-heated fuel is compressed and heated to multi-keV temperatures.

In Magnetized Liner Inertial Fusion (MagLIF), pressurized fuel inside of a cylindrical metal tube (or “liner”) is preheated with a laser pulse. The laser enters the pressurized fuel region through a thin laser entrance window (LEW). The LEW contains the pressurized fuel inside of the liner until the few-ns preheating laser pulse is applied, which ablates the LEW and preheats the fuel. Energy losses are thought to occur at the laser entrance window (LEW) as a result of laser-plasma interactions (LPI). Additionally, simulations are presently unable to reliably model the LPI losses.1 To reduce energy losses and computational uncertainties, the LEW could be weakened and removed very early in time, well before the preheating laser pulse arrives at the LEW.2 This general concept of removing the LEW very early in time is referred to as “Laser Gate.”2 One proposed implementation of Laser Gate, which we are working on at the University of Michigan, is to break the LEW very early in time by driving an electrical current through a thin wire that is wrapped around the perimeter of the LEW.2 The electrical current heats and melts the perimeter of the LEW, allowing the fuel pressure to push open the LEW. As the LEW is broken, it opens away from the contained fuel and out of the laser path. Before a significant amount of fuel has time to escape the liner, the preheating laser pulse is applied. Doing this successfully should reduce fuel-window mixing and LPI in MagLIF. For our experiments at UM, the pulsed electrical current is driven through the thin wire by a 13-kV mini-pulser. Additionally, a laser backlighting system is being developed to image the dynamics of the LEW as it opens. We will report on our first experimental tests of this implementation of Laser Gate.

We discuss the use of the PERSEUS extended-MHD simulation code for high-energy-density (HED) plasmas in modeling power flow in coaxial transmission lines and in the Magnetized Liner Inertial Fusion (MagLIF) experiment at Sandia National Labs. By formulating the fluid equations as a relaxation system in which the current is semi-implicitly time-advanced using the Generalized Ohm's Law (GOL), PERSEUS enables modeling of two-fluid phenomena in dense plasmas without the need to resolve the smallest electron length and time scales. F or both coaxial transmission lines and a simple MagLIF load and feed section, we examine how power coupling is affected by the presence of electrode plasma, and the differences in power flow dynamics resulting from the Hall term in the GOL.

Auto-magnetizing liners (AutoMag [1], [2]) are cylindrical tubes made of metallic helical conductors separated by electrically insulating material. In the first stage of AutoMag, helical current flows in the AutoMag liner and produces a strong internal axial field (20 to 100 T) during a 5 kA/ns current prepulse that lasts 100 to 300 ns. In the second stage, the rapidly-rising main current pulse (200 kA/ns) induces a strong electric field in the liner that causes the insulating material to undergo dielectric breakdown. Lastly, after breakdown, the liner current reorients to be primarily axial and the liner implodes radially via the Lorentz force. AutoMag is designed to eliminate the need for the external coil system [3] that is used to premagnetize fusion fuel in Magnetized Liner Inertial Fusion (MagLIF [4]) which impedes diagnostic access and necessitates use of a high inductance power feed that reduces current delivery to the liner. Experiments have been successfully executed on the Mykonos accelerator [5] to evaluate the magnetization and breakdown stages of AutoMag. Microscopic magnetic field probes (microBdots) measured magnetic field inside of the liner and an iCCD imager and fast photodiodes made spatially-resolved and temporally-resolved measurements, respectively, of optical plasma emission in the load region. Fields near 1 MG were measured in multiple experiments and plasma emission indicative of breakdown was observed for select liner designs. These experiments, the first of their kind, represent an advance in magneto-inertial fusion that will enable the design of a new class of MagLIF targets capable of reaching higher current delivered to the liner (better fuel compression) and higher magnetization fields inside of the fusion fuel (better thermal insulation). Success of the experiments on Mykonos has resulted in the design of Z Machine experiments (planned Spring 2018) to study the implosion stage of AutoMag.

We discuss the possible impacts of a new magnetized liner inertial fusion concept on magneto-inertial fusion approaches to fusion energy. Experiments in the last 1.5 years have already shown direct evidence of magnetic flux compression, a highly magnetized fusing fuel, significant compressional heating, a compressed cylindrical fusing plasma, and significant fusion yield. While these exciting results demonstrate several key principles behind magneto-inertial fusion, more work in the coming years will be needed to demonstrate that such targets can scale to ignition and high yield. We argue that justifying significant investment in pulsed inertial fusion energy beyond target development should require well-understood, significant fusion yields to be demonstrated in single-shot experiments. We also caution that even once target ideas and fusion power plants have been demonstrated, historical trends suggest it would still be decades before fusion could materially impact worldwide energy production.

A cryogenically cooled hardware platform has been developed and commissioned on the Z Facility at Sandia National Laboratories in support of the Magnetized Liner Inertial Fusion (MagLIF) Program. MagLIF is a magneto-inertial fusion concept that employs a magnetically imploded metallic tube (liner) to compress and inertially confine premagnetized and preheated fusion fuel. The fuel is preheated using a ∼2 kJ laser that must pass through a ∼1.5-3.5-μm-thick polyimide "window" at the target's laser entrance hole (LEH). As the terawatt-class laser interacts with the dense window, laser plasma instabilities (LPIs) can develop, which reduce the preheat energy delivered to the fuel, initiate fuel contamination, and degrade target performance. Cryogenically cooled targets increase the parameter space accessible to MagLIF target designs by allowing nearly 10 times thinner windows to be used for any accessible gas density. Thinner LEH windows reduce the deleterious effects of difficult to model LPIs. The Z Facility's cryogenic infrastructure has been significantly altered to enable compatibility with the premagnetization and fuel preheat stages of MagLIF. The MagLIF cryostat brings the liquid helium coolant directly to the target via an electrically resistive conduit. This design maximizes cooling power while allowing rapid diffusion of the axial magnetic field supplied by external Helmholtz-like coils. A variety of techniques have been developed to mitigate the accumulation of ice from vacuum chamber contaminants on the cooled LEH window, as even a few hundred nanometers of ice would impact laser energy coupling to the fuel region. The MagLIF cryostat has demonstrated compatibility with the premagnetization and preheat stages of MagLIF and the ability to cool targets to liquid deuterium temperatures in approximately 5 min.

Understanding plasma shock behavior can inform the design of fusion reactor and X-ray source concepts such as the Staged Z-pinch [1], Magnetized Liner Inertial Fusion (MagLIF) [2], and inertial confinement fusion (ICF) [3]. The cylindrical or spherical geometry of such concepts obscures imploding shocks from diagnostic view and complicates analysis of observations. A simpler, planar geometry eases diagnosis and comprehension of the physics underlying the shocks.

Magnetized liner inertial fusion (MagLIF) [1, 2] is a pulsed-power driven approach to inertial confinement fusion. Electrothermal instabilities (ETI) are thought to seed Magneto-Rayleigh Taylor (MRT), sausage mode, and kink mode instabilities in the imploding liner of MagLIF [3]. Understanding ETI may provide a way to improve fusion performance in MagLIF through instability mitigation. A single-capacitor pulsed power device was built with a low peak current of 4 kA and a long risetime of 600 ns to lengthen the transition time from the solid phase to the vapor phase. We have studied ETI growth rates on this facility. These growth rates have good agreement with ETI theory [4]. Preliminary results have shown the value of this facility and a need to investigate ETI further. We report on recent modifications and improvements to the facility and plans for future ETI studies.