Two centers, supported by the National Nuclear Security Administration, will explore physics relevant to modeling the performance of the aging nuclear weapons stockpile

Two Centers of Excellence for studying basic science surrounding how hot plasmas behave, funded by the National Nuclear Security Administration, have been awarded to University of Michigan researchers.

One center—which simulates extreme astrophysics, such as exploding stars—is renewed and expanded to $12.5 million over the next five years, having received $5 million in the previous funding cycle. Carolyn Kuranz, U-M associate professor of nuclear engineering and radiological sciences, has led the center since 2019.

The other center, which has been running since 2003, focuses on nuclear fusion and X-ray generation using pulsed power. The collaboration has been centered at Cornell University, but longtime leaders David Hammer and Bruce Kusse have decided to pass the torch. Ryan McBride, U-M professor of nuclear engineering and radiological sciences who has been with the center since beginning his Ph.D., was best positioned to take the reins. This center will receive $14.5 million over five years.

Both centers are part of the NNSA’s effort to ensure that the U.S. stockpile of nuclear weapons will perform as expected since the U.S. ceased testing more than 30 years ago. This means that as old components are replaced with new, which may not be a perfect match for the original part, detailed physics simulations are needed to understand how the device will work.

The centers explore hot plasmas: materials at such a high temperature that they take a step beyond the gas phase, becoming a soup of loose electrons and positively charged ions—the atoms that gave up those electrons. This is the stuff of stars, uranium and hydrogen bombs, jets spat out by black holes and more.

“There’s overlapping science between stockpile stewardship and astrophysical systems. They’re very extreme states with high pressures, densities and temperatures,” Kuranz said.

The center Kuranz leads, called the Center for High Energy Density Laboratory Astrophysics Research (CHEDAR), will explore four main aspects of plasmas and systems involving plasmas:

• How radiation moves through them. As a plasma’s density and temperature changes, X-rays may find it harder or easier to get through it. This in turn affects how the plasma behaves: whether it can release energy or capture it.
• How plasmas mix. Massive stars synthesize elements necessary for life, such as the iron in our blood. It is important to understand how these heavy materials are ejected from the cores of stars, where they are formed and mix with lighter elements during a supernova explosion.
• Magnetized systems. Full of roving electrons and ions, plasmas can support powerful magnetic fields. Magnetized plasma can shed light on how stars are born, how black holes swallow the material around them and how Earth’s magnetic field bows against the rain of plasma coming from the sun.
• How to measure plasmas. Plasmas in the lab form, evolve and dissipate roughly a million times faster than the blink of an eye. In that evolution, they can give away secrets of collapsing stars and the cores of exploding atomic bombs, but understanding what happens in all three dimensions requires multiple ways of measuring the plasma. The team will experiment with different kinds of 2D imaging to get a handle on how the plasma evolves in space and time.

Kuranz and her colleagues will run experiments with lasers and machines that generate powerful magnetic pulses and then use that data to refine physics models that describe the dynamics they measure. Laser facilities include the National Ignition Facility, famous for breakthroughs in nuclear fusion reactions, and U-M’s Zetawatt-Equivalent Ultrashort Pulse Laser System (ZEUS), which will soon be the most powerful laser in the U.S.

When laser pulses dump energy into gas or metal targets, the metal vaporizes to form a plasma. That plasma can then be manipulated with subsequent laser pulses, enabling the team to explore a variety of conditions that can occur in plasmas.

In contrast, the center McBride leads will focus on approaches using powerful magnetic pulses, including devices at U-M, Cornell and Sandia National Laboratories. The most common method that the Center for Magnetic Acceleration, Compression and Heating (MACH) will use is Z-pinch implosions, which rely on magnetic fields to crush plasmas in cylindrical form toward the central “Z” axis.

The MACH team will focus on:

• Achieving even compression. Typically, cylindrical compression isn’t perfectly cylindrical—fingers of plasma can burst out through weak points in the magnetic field. These losses can reduce the fusion efficiency, so the center will test ways to smooth out the compression.
• Preparing to build more powerful fusion machines. It takes a lot of electrical power to generate a magnetic field strong enough to crush a metal cylinder, and the high power can cause plasmas to form where they aren’t wanted. These plasmas can short out the system before the electrical power is delivered to the fusion target. The MACH team will explore the physics of power transmission in the presence of such plasmas.
• Exploring fundamental physics. Members of the center have a lot of latitude in what aspects of Z-pinch experiments they explore, with the possibility of creating plasma jets, driving compression in different ways, and producing exploding plasma flows. One relatively new development by center partners at Imperial College London is the ability to bring a portable Z-pinch machine to a synchrotron facility. The synchrotron produces extremely bright x-rays that can be used to probe the inner structures of the compressed Z-pinch plasma.

“The nice thing about the center is that it is extremely flexible,” McBride said. “It allows students to be creative and try out all kinds of new ideas. As long as our collaborators at the national labs are interested in the results, it’s fair game.”

The Ph.D. graduates that come through the NNSA-funded centers are in high demand at the national labs as a generation of researchers approaches retirement. Students coming through programs like these may also land jobs at the NNSA itself or at new start-up companies trying to bring nuclear fusion to market as a viable source of sustainable electricity.

Partner universities in CHEDAR are the University of Notre Dame; the University of California, Los Angeles; and Rice University. Partner universities in MACH are Cornell University; Imperial College London; the Weizmann Institute of Science in Israel; the University of California, San Diego; Princeton University; the Massachusetts Institute of Technology; the University of New Mexico; the University of Washington; and the University of Rochester, New York.

Both Centers will be collaborating with the NNSA labs: Los Alamos National Laboratory, Sandia National Laboratories and Lawrence Livermore National Laboratory.