USING neutrons to “look through” fast-moving experiments requires an intensely bright neutron source originating from a very small spot. One tool that suits this scientific role is the dense plasma focus (DPF). The DPF uses a strong magnetic field to compress a plasma into a discharge called a “pinch” that creates a short, intense pulse of x rays, neutrons, and directed ion beams.
Introduced in the 1960s, the relatively compact size and uncomplicated design of the devices made them attractive as the foundation for fusion power plants, although that application was eventually discounted after decades of research. More recently, the DPF has been considered as a neutron source for radiography and other national security applications. An important complement to x-ray imaging, fast neutron imaging enables researchers to examine the lighter elements in objects of study. Unlike current commercial and industrial neutron imaging sources, the DPF generates an incredibly bright but very short flash (less than 100 nanoseconds) that is well-suited for taking still-frame pictures of highly dynamic processes.
The inability to model the DPF plasma with sufficient fidelity, however, has limited researchers’ detailed understanding of the physics needed to develop methods that produce a consistent, reliable, and intense neutron pulse. “Decades ago, the DPF was essentially a black box operated by rules of thumb,” says Livermore scientist Alex Povilus.
After a multiyear Livermore research program, the DPF’s time to shine may have finally arrived. A team led by researcher Andréa Schmidt has been using hundreds of computer simulations to understand—physically—what occurs in a DPF discharge. With this knowledge, Schmidt’s team has developed a deeper understanding of the effect of different DPF design aspects—pulsed-power capacitor bank characteristics, anode length and shape, operating voltage, device pressure—on its neutron pulse. Now, a prototype DPF design and ongoing modifications are informed by modeling predictions. The ability to guide experimental design using simulations and then challenge the simulations with experimental results has enabled the team to make remarkable progress in DPF design. The team’s ultimate goal is to create a new flash neutron imaging capability for the nuclear weapons enterprise.