Electromagnetics

ASD Scorpius

ASD Scorpius is a 125-meter-long linear induction accelerator that will serve as an important diagnostic tool for the nuclear stockpile. The accelerator will be used create multiple short, high-energy bursts of x-rays to take radiographic images of contained subcritical experiments with plutonium. The project—a collaboration between LLNL, Los Alamos National Laboratory, Sandia National Laboratories, and Nevada National Security Site (NNSS)—addresses a capability gap in fully understanding the late stages of a nuclear weapon implosion, when plutonium is under the most extreme conditions and pressure from explosively driven shocks.  

By using plutonium at a subcritical state rather than non-fissile surrogates, the accelerator will provide more accurate data for computer simulations modeling weapon behavior, enable accurate assessments of the enduring nuclear stockpile, and identify the effects of aging and manufacturing methods on nuclear weapons. This data will also inform future stockpile changes, including certification of modernized warheads and incorporation of enhanced safety features. 

Scorpius is powered by the Lab’s innovative solid-state pulsed-power technology, which make it possible to take pictures as fast as every 200 nanoseconds, an aperture speed of five million frames per second. The pulsers also provide unique flexibility in programming pulse width and spacing, as well as optimizing the number x-rays imaging an object as a function of time. As the project’s pulsed-power lead, LLNL is building and delivering all 984 pulsers to the NNSS, where the accelerator is being installed nearly 1000 feet underground.

Electromagnetics at NIF

The National Ignition Facility uses three different pulsed power systems to amplify beams’ energy by more than 20 trillion times to deliver the laser power required for fusion ignition and other experiments. The process begins with the world’s most energetic capacitor bank. The 192 Power Conditioning modules can store up to 400 megajoules of energy (roughly 400 sticks of dynamite) and release it in 400 millionths of a second to deliver 2 terawatts of peak power to the laser’s flashlamps as the laser beams are injected into the amplification system.

On a typical shot, when the beams pass through specially-made slabs of laser glass doped with neodymium, 7680 flashlamps convert around 330 megajoules of electrical energy from the capacitors into pulses emitted white light. The light excites the neodymium atoms, which release energy in the form of photons, greatly amplifying the beam’s power. The beams pass through the main amplifier four times, building up energy with each pass. The system provides 99% of NIF’s energy and power and determine the beam’s characteristics.

During and after the shot, electromagnetics technologies are used to image the target capsule and fuel during an implosion, providing data used to assess performance and inform future experiments. The innovative polar dilation x-ray imager functions like a slow-motion camera, recording, stretching, and magnifying the incoming electron signals to take highly-detailed pictures less than 10 picoseconds apart, or 200 billion frames per second. Other electromagnetics diagnostics include the Single Line of Sight Detector (SLOS), which uses flash x-ray imaging to take pictures inside of a target capsule as it evolves, and the advanced radiographic capability (ARC), which images the fuel and target capsule before and during peak compression to show the density and distribution of compressed fuel.

Sirius 1 Impedance-matched Marx Generator

First invented in 1924 and widely used for pulsed power accelerators since, a Marx generator is an electrical circuit that charges multiple capacitors in parallel and discharges them in series, stacking voltages to generate a pulse. The impedance-matched Marx generator (IMG) builds on this concept, amplifying power through triggered emission of radiation and stacking electromagnetic waves on top of each other in the same way a laser stacks photons. Invented by engineers at LLNL, the IMG is the first major innovation in Marx generator design in more than 90 years.  

IMGs only need one stage to compress stored energy and are designed so that the reflected electromagnetic waves generated cancel out. This maximizes power transfer, which significantly improves efficiency and reduces potential damage to components from voltage reversal, and means IMGs can be built with longer-lasting components. A full-power version of the Sirius 1 prototype would generate pulses that are 34 percent more powerful—and up to eight times faster—than a state-of-the-art conventional Marx generator while needing six times less stored energy. 

Sirius 1 is the first step toward greater things. The team has developed conceptual designs for an IMG-based accelerator they believe could generate as much as 10 GJ of fusion energy, as well as next-generation machines optimized for achieving pressures of over 10 megabars with precise programmable pulse shapes for dynamic-material-physics experiments. These machines could be built for a fraction of the size and cost of conventional pulsed power machines. Other applications may include advanced propulsion, lightning-electromagnetic pulse (EMPs) studies, and combined radiation-effects-environments research. 

Megajoule Neutron Imaging Radiography Experiment (MJOLNIR)

MJOLNIR is a prototype flash neutron imaging platform that uses dense plasma focus (DPF) as a source of neutrons. Since neutrons are better scattered by lighter hydrogenous materials that x-rays might pass through, neutron imaging would be an important complement to x-ray radiography and help provide a clearer picture of what’s happening inside dynamic events. 

Using neutrons to radiograph fast-moving experiments requires a short, bright neutron burst originating from a very small source, in this case the DPF. The DPF head consists of an anode and a cathode in a coaxial geometry, separated by an insulator and located inside a pressure vessel filled with a low-density deuterium gas. During a discharge, an intense electrical current runs through the gas, which ionizes it into a plasma. The current is pushed off the insulator by its own magnetic field until it reaches the tip of the electrode, where it compresses or “pinches” the plasma. This pinching generates an extremely bright but brief flash of neutrons that allow for rapid-fire still-frame images. 

The project has leveraged and advanced state-of-the-art simulation tools that have led to a better understanding of DPF design, behavior, and physics and a renewed interest in the technology. Since operations began in 2018, the team has been able to produce 30 times more neutrons per shot, already achieving record yields at 1012, with the goal of producing 1013 to take high-resolution neutron pictures in 50 billionths of a second. 

The ultimate goal is to create a new flash neutron imaging capability for the nuclear weapons enterprise, but MJOLNIR may also provide a high repetition-rate platform for developing diagnostics, and in the future could be engineered for portable neutron imaging and other applications in the field. DPF-based neutron imaging may also find applications in medicine and the transportation sector for viewing fuel flow through engines. 

High Explosive Pulsed Power 

High-explosive pulsed power (HEPP) devices amplify electrical power by detonating explosives to generate ultrahigh magnetic fields and extremely intense current and energy pulses. Over the years, LLNL has developed a family of magnetic flux compression generators (FCGs) capable of generating electric currents greater than 100 million amperes (more than 6 million times the capacity of a typical 120 volt wall outlet). These platforms are used to support equation-of-state studies, materials science experiments, and high energy density science applications relevant to the Lab’s stockpile stewardship mission. 

FCGs function as explosively-driven electrical amplifiers, using kinetic energy to convert chemical energy (of explosives) into electrical energy—an energy conversion process similar to a gas-powered electric generator, except that FCGs are destroyed during operation. When provided an initial “seed” electrical energy, the devices amplify power in tens-to-hundreds of microseconds (ms, one millionth of a second) to generate magnetic energies and currents that exceed available large-scale capacitor bank systems, in a fraction of the size.  

LLNL’s high-energy HEPP devices typically use a two-stage configuration. In the Lab’s largest platform—which uses around 900 lb of explosives—the first stage consists of a high-gain helical generator, which amplifies a starting current of ~100 kiloamperes (kA, ~1000x the capacity of a wall outlet) to ~20 megaamperes (MA, 200x the starting current) in less than 0.0002 seconds (200 ms). The second-stage coaxial generator amplifies that current to 100 MA or more in less than 30 ms.  

FCGs are challenging to model because they involve physics that needs to be resolved on much smaller scales than size of the device. LLNL’s high-performance computing environment, state-of-the-art multiphysics codes like ARES and ALE3D, and longstanding expertise address these challenges and are therefore key to device and experiment design. 

MORE COVERAGE:

“3D Magneto-Hydrodynamic Modeling of an Overstressed Helical Magnetic Flux Compression Generator”