We present an overview of the magneto-inertial fusion (MIF) concept Magnetized Liner Inertial Fusion (MagLIF) pursued at Sandia National Laboratories and review some of the most prominent results since the initial experiments in 2013. In MagLIF, a centimeter-scale beryllium tube or 'liner' is filled with a fusion fuel, axially pre-magnetized, laser pre-heated, and finally imploded using up to 20 MA from the Z machine. All of these elements are necessary to generate a thermonuclear plasma: laser preheating raises the initial temperature of the fuel, the electrical current implodes the liner and quasi-adiabatically compresses the fuel via the Lorentz force, and the axial magnetic field limits thermal conduction from the hot plasma to the cold liner walls during the implosion. MagLIF is the first MIF concept to demonstrate fusion relevant temperatures, significant fusion production (>1013 primary DD neutron yield), and magnetic trapping of charged fusion particles. On a 60 MA next-generation pulsed-power machine, two-dimensional simulations suggest that MagLIF has the potential to generate multi-MJ yields with significant self-heating, a long-term goal of the US Stockpile Stewardship Program. At currents exceeding 65 MA, the high gains required for fusion energy could be achievable.
Auto-magnetizing (AutoMag) liners are cylindrical tubes composed of discrete metallic helices encapsulated in insulating material; when driven with a ∼2 MA, ∼100-ns prepulse on the 20 MA, 100-ns rise time Z accelerator, AutoMag targets produced >150 T internal axial magnetic fields [Shipley et al., Phys. Plasmas 26, 052705 (2019)]. Once the current rise rate of the pulsed power driver reaches sufficient magnitude, the induced electric fields in the liner cause dielectric breakdown of the insulator material and, with sufficient current, the cylindrical target radially implodes. The dielectric breakdown process of the insulating material in AutoMag liners has been studied in experiments on the 500-900 kA, ∼100-ns rise time Mykonos accelerator. Multi-frame gated imaging enabled the first time-resolved observations of photoemission from dynamically evolving plasma distributions during the breakdown process in AutoMag targets. Using magnetohydrodynamic simulations, we calculate the induced electric field distribution and provide a detailed comparison to the experimental data. We find that breakdown in AutoMag targets does not primarily depend on the induced electric field in the gaps between conductive helices as previously thought. Finally, to better control the dielectric breakdown time, a 12-32 mJ, 170 ps ultraviolet (λ = 266 nm) laser was implemented to irradiate the outer surface of AutoMag targets to promote breakdown in a controlled manner at a lower internal axial field. The laser had an observable effect on the time of breakdown and subsequent plasma evolution, indicating that pulsed UV lasers can be used to control breakdown timing in AutoMag.
Magnetic implosion of cylindrical metallic shells (liners) is an effective method for compressing preheated, premagnetized fusion fuel to thermonuclear conditions [M. R. Gomez et al., Phys. Rev. Lett. 113, 155003 (2014)] but suffers from magneto-Rayleigh-Taylor instabilities (MRTI) that limit the attainable fuel pressure, density, and temperature. A novel method proposed by Schmit et al. [Phys. Rev. Lett. 117, 205001 (2016)] uses a helical magnetic drive field with a dynamic polarization at the outer surface of the liner during implosion, reducing (linear) MRTI growth by one to two orders of magnitude via a solid liner dynamic screw pinch (SLDSP) effect. Our work explores the design features necessary for successful experimental implementation of this concept. Whereas typical experiments employ purely azimuthal drive fields to implode initially solid liners, SLDSP experiments establish a helical drive field at the liner outer surface, resulting in enhanced average magnetic pressure per unit drive current, mild spatial nonuniformities in the magnetic drive pressure, and augmented static initial inductance in the pulsed-power drive circuit. Each of these topics has been addressed using transient magnetic and magnetohydrodynamic simulations; the results have led to a credible design space for SLDSP experiments on the Z Facility. We qualitatively assess the stabilizing effects of the SLDSP mechanism by comparing MRTI growth in a liner implosion simulation driven by an azimuthal magnetic field vs one driven with a helical magnetic field; the results indicate an apparent reduction in MRTI growth when a helical drive field is employed.
In the first auto-magnetizing liner implosion experiments on the Z Facility, precompressed internal axial fields near 150 T were measured and 7.2-keV radiography indicated a high level of cylindrical uniformity of the imploding liner's inner surface. An auto-magnetizing (AutoMag) liner is made of discrete metallic helical conductors encapsulated in insulating material. The liner generates internal axial magnetic field as a 1-2 MA, 100-200 ns current prepulse flows through the helical conductors. After the prepulse, the fast-rising main current pulse causes the insulating material between the metallic helices to break down ceasing axial field production. After breakdown, the helical liner, nonuniform in both density and electrical conductivity, implodes in 100 ns. In-flight radiography data demonstrate that while the inner wall maintains cylindrical uniformity, multiple new helically oriented structures are self-generated within the outer liner material layers during the implosion; this was not predicted by simulations. Furthermore, liner stagnation was delayed compared to simulation predictions. An analytical implosion model is compared with experimental data and preshot simulations to explore how changes in the premagnetization field strength and drive current affect the liner implosion trajectory. Both the measurement of >100 T internal axial field production and the demonstration of cylindrical uniformity of the imploding liner's inner wall are encouraging for promoting the use of AutoMag liners in future MagLIF experiments.
The Magnetized Liner Inertial Fusion concept (MagLIF) [Slutz et al., Phys. Plasmas 17, 056303 (2010)] is being studied on the Z facility at Sandia National Laboratories. Neutron yields greater than 1012 have been achieved with a drive current in the range of 17-18 MA and pure deuterium fuel [Gomez et al., Phys. Rev. Lett. 113, 155003 (2014)]. We show that 2D simulated yields are about twice the best yields obtained on Z and that a likely cause of this difference is the mix of material into the fuel. Mitigation strategies are presented. Previous numerical studies indicate that much larger yields (10-1000 MJ) should be possible with pulsed power machines producing larger drive currents (45-60 MA) than can be produced by the Z machine [Slutz et al., Phys. Plasmas 23, 022702 (2016)]. To test the accuracy of these 2D simulations, we present modifications to MagLIF experiments using the existing Z facility, for which 2D simulations predict a 100-fold enhancement of MagLIF fusion yields and considerable increases in burn temperatures. Experimental verification of these predictions would increase the credibility of predictions at higher drive currents.
Auto-magnetizing (AutoMag) liners [Slutz et al., Phys. Plasmas 24, 012704 (2017)] are designed to generate up to 100 T of axial magnetic field in the fuel for Magnetized Liner Inertial Fusion [Slutz et al., Phys. Plasmas 17, 056303 (2010)] without the need for external field coils. AutoMag liners (cylindrical tubes) are composed of discrete metallic helical conduction paths separated by electrically insulating material. Initially, helical current in the AutoMag liner produces internal axial magnetic field during a long (100 to 300 ns) current prepulse with an average current rise rate d I / d t = 5 k A / n s. After the cold fuel is magnetized, a rapidly rising current (200 k A / n s) generates a calculated electric field of 64 M V / m between the helices. Such field is sufficient to force dielectric breakdown of the insulating material after which liner current is reoriented from helical to predominantly axial which ceases the AutoMag axial magnetic field production mechanism and the z-pinch liner implodes. Proof of concept experiments have been executed on the Mykonos linear transformer driver to measure the axial field produced by a variety of AutoMag liners and to evaluate what physical processes drive dielectric breakdown. A range of field strengths have been generated in various cm-scale liners in agreement with magnetic transient simulations including a measured field above 90 T at I = 350 kA. By varying the helical pitch angle, insulator material, and insulator geometry, favorable liner designs have been identified for which breakdown occurs under predictable and reproducible field conditions.