May 8, 2023

A ‘moving stream’: Zap Energy’s novel approach to stabilizing fusion

Zap Energy's fusion device generates a ring of purple-colored plasma that expands as it moves from the top of the computer-generated picture to the bottom.

A key theoretical breakthrough led Zap’s co-founders to finally discover an effective way to hold Z pinches stable.

Imagine taking a bowl of Jell-O and squeezing it using just rubber bands. It’s not hard to imagine the mess you’d make — and how little Jell-O you’d be left with. It’s a good analogy for trying to contain a superheated plasma in a fusion core.

“Plasma doesn’t want to stay nice and neat,” says Hannah Meek, a research engineer at Zap Energy. “It’s a lot like water; it always wants to go the path of least resistance.”

Plasma is a state of matter made up of charged particles that has properties of both a liquid and a gas. Keeping a plasma hot and dense enough to sustain fusion requires keeping it contained without touching it. This crucial hurdle has stymied attempts to create practical fusion energy devices for decades. Some devices use powerful magnets to try and suspend the plasma to keep it from fizzling out. Others rely on laser arrays to rapidly superheat fuel pellets that turn into plasmas. And while a number of these experiments have briefly created fusion energy, none have done so at levels that yield enough energy to be useful beyond scientific curiosity.

The plasma containment conundrum also doomed early experiments using the Z pinch technique, where magnetic fields created around an electrified plasma compress it tightly and create the conditions necessary for fusion. A Z pinch works well to compress and heat plasmas. But like the rubber bands and the Jell-O, it is susceptible to instabilities that let the plasma escape.

“These instabilities grow very quickly — on a nanosecond timescale,” says Brian A. Nelson, Co-Founder and Chief Technology Officer at Zap Energy. For decades, sustained fusion with a Z pinch was thought to be impossible.

That is, until groundbreaking research by Uri Shumlak, fellow Zap Co-Founder and Chief Scientist and his Lawrence Livermore National Laboratory collaborator Charles Hartman. In the late 1980s and early 1990s, Shumlak and Hartman pioneered what’s known as sheared-flow stabilization, an innovative approach that treats the plasma within a fusion core as a moving stream rather than a static cloud.

“The assumption was that plasmas were static, and had no velocity,” Shumlak says. “I asked my PhD advisor, ‘Why don’t people consider velocity?’ He replied, ‘That’s a good question – you should go find out.’”

Put to work inside a series of test devices, the sheared-flow stabilization Shumlak and Nelson ultimately developed has proven capable of keeping a superheated plasma stable, making it possible to create a Z-pinch fusion core.

Sheared-flow-stabilized Z-pinch plasmas are tested in Zap Energy’s fusion R&D devices, such as FuZE, shown here.

Creating Sheared-Flow Fusion with Zap Energy's Z Pinch

Inside a Zap core, a puff of neutral hydrogen gas is zapped with electric current, turning it into a washer-shaped disk of plasma pushed by a self-generated magnetic field. The plasma accelerates along the inner electrode — picture a ring sliding along a pole — until it hits the nose cone at the end of the tube. There, the moving plasma’s magnetic field begins to squeeze it tightly, forming a Z pinch. At the same time, the geometry of the nose cone also contributes to different parts of the plasma ring to begin moving at different speeds. This sets the sheared flow in motion.

"When you have the inside of your plasma moving at a slightly different velocity than the next layer outside, we call that a shear,” Nelson says. This difference in velocity creates a stabilizing effect that keeps the plasma in place.

Shumlak likens it to cars on a freeway. “If you have a lane that’s moving particularly slowly next to a lane that’s moving very quickly, it’s hard to change lanes,” he explains. “But if all the cars are traveling at about the same speed, then it’s easy to change lanes.”

Differences in flow within the plasma help keep it stable, which allows fusion reactions to generate energy.

Keeping the plasma “lanes” moving at different speeds creates a stable, steady, Z-pinched plasma stream — what fusion physicists call quiescence — where fusion reactions can happen. It also enables a fusion device that’s smaller and more elegant than other proposed designs.

“The Z pinch that we work on is only about 2 feet long, which is incredibly small in comparison to other designs,” says Meek, Zap’s research engineer. “Most importantly, we’re able to hold a plasma steady without magnets, so it’s lighter, more compact and faster to make changes to our devices. Not having magnets is a key difference that lets us scale up quickly and avoid many of the challenges you run into with a lot of other fusion approaches.”