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.
Timing spread between the thirty-six Saturn modules affects peak electrical power delivered to the Bremsstrahlung diode and can affect vacuum power flow and impedance behavior of the load. To reduce the module spread, a new megavolt gas-insulated closing switch was developed employing design techniques developed for the Z-machine laser triggered switches while retaining Saturn’s simpler electrical triggering. Two modules were temporarily outfitted with the new switches and used separately into local resistive loads (instead of the usual Saturn electron beam load). A reliable operating point and switch time jitter at that point were the goals of the experiments. The target switch reliability is less than one pre-fire in one thousand switch-shots, and a timing standard deviation of 4 nanoseconds. The switches were able to meet both requirements but the number of tests at the chosen point are limited.
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 1.1×1013 (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.
The Saturn X-ray generator is a 2.5 megavolt, 10 megampere electrical driver at Sandia National Laboratories. Saturn has been in operation for more than 30 years. Work is underway to identify key areas of the machine, improvement of which would benefit operational efficiency and reproducibility of the system. Saturn is used to create high-dose, short-pulse intense ionizing radiation environments for testing electronic and mechanical systems. Saturn has 36 identical modules driving a common electron beam bremsstrahlung load. Each module utilizes a microsecond Marx generator, a megavolt gas switch, and untriggered water switches in a largely conventional pulse-forming system. Achieving predictable and reliable radiation exposure is critical for users of the facility. Saturn has endured decades of continual use with minimal opportunities for research, improvements, or significant preventive maintenance. Because of degradation in components and limited attention to electrical performance, the facility has declined both in the number of useful tests per year and their repeatability. The Saturn system resides in a cylindrical tank 33m in diameter, and stores 5.6 MJ at the nominal operating Marx charge voltage. The system today is essentially identical to that described by Bloomquist in 1987. [1] Advances in technology for large pulsed power systems affords opportunities to improve the performance and more efficiently utilize the energy stored. Increased efficiency can improve reliability and reduce maintenance. The goals for the Saturn improvement work are increased shot rate, reduced X-ray dose shot-To-shot dose fluctuation, and reduced required maintenance. Major redesign with alternate pulsed power technology is considered outside the scope of this effort. More X-ray dose, larger exposure area, and lower X-ray endpoint energy are also important considerations but also deemed outside the scope of the present project due to schedule and resource constraints. The first considerations, described here, are improving the present design with better components.
Herein we describe the design, simulation and performance of a 118-GW linear transformer driver (LTD) cavity at Sandia National Laboratories. The cavity consists of 20 to 24 'Bricks'. Each brick is comprised of two 80 nF, 100 kV capacitors connected electrically in series with a custom, 200 kV, three-electrode, field-distortion gas switch. The brick capacitors are bi-polar charged to a total of 200 kV. Typical brick circuit parameters are 40 nF (two 80 nF capacitors in series) and 160 nH inductance. Over the course of over 10,000 shots the cavity generated a peak electrical current and power of 1.19 MA and 118 GW.
Here we present details of the design, simulation, and performance of a 100-GW linear transformer driver (LTD) cavity at Sandia National Laboratories. The cavity consists of 20 “bricks.” Each brick is comprised of two 80 nF, 100 kV capacitors connected electrically in series with a custom, 200 kV, three-electrode, field-distortion gas switch. The brick capacitors are bipolar charged to ±100 kV for a total switch voltage of 200 kV. Typical brick circuit parameters are 40 nF capacitance (two 80 nF capacitors in series) and 160 nH inductance. The switch electrodes are fabricated from a WCu alloy and are operated with breathable air. Over the course of 6,556 shots the cavity generated a peak electrical current and power of 1.03 MA (±1.8%) and 106 GW (±3.1%). Experimental results are consistent (to within uncertainties) with circuit simulations for normal operation, and expected failure modes including prefire and late-fire events. New features of this development that are reported here in detail include: (1) 100 ns, 1 MA, 100-GW output from a 2.2 m diameter LTD into a 0.1 Ω load, (2) high-impedance solid charging resistors that are optimized for this application, and (3) evaluation of maintenance-free trigger circuits using capacitive coupling and inductive isolation.
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.