March 22, 2023

A history of the Z pinch: Magnetic compression without magnets

A graphic illustration shows a brownish-red rod being compressed to reveal fissures within it, with circular blue lines surrounding the rod as a way to show the inwardly-pinching electromagnetism.

A lightning strike showcases the ‘pinch effect.’ The underlying physics are the foundation for how Zap Energy produces fusion.

In the late 1800s, a bolt of lightning struck a kerosene refinery in New South Wales, Australia, and left behind a mystery. The strike hit a lightning rod on the refinery’s roof, crumpling the hollow metal tube as if it had been crushed by the hand of a giant. The twisted rod would likely have been no more than a curiosity but for the plant’s manager, who sent it to two scientists at the University of Sydney.

The manager didn’t know it, but the rod would inspire some of the very first experiments in fusion.

A compressed and distorted piece of metal is at the foreground of the industrial landscape of a kerosene refinery.
A crushed lightning rod from a kerosene refinery was first used to describe the physics of a Z pinch in 1905. Credit: ©University of Sydney, Anyu Zhang

Examining the rod in 1905, James Pollock and Samuel Barraclough concluded that the rod had been crushed, not by the force of the strike itself, but by magnetism. Scientists had already noted that two wires carrying electric currents in the same direction would bend toward each other, as if irresistibly attracted. The explanation lies in the fundamental relationship between electricity and magnetism. Moving electrons, also known as an electric current, create a proportional magnetic field that wraps around the current.

Today, physics students learn to memorize how this force works with what’s called the “right-hand rule.” Point your right thumb in the direction of an electric current and curl your other four fingers inward. They’ll reveal the direction of the associated magnetic field.

Pollock and Barraclough’s breakthrough came from realizing that a hollow metal rod is the same thing as a circle of parallel wires, says Brian James, a retired professor of physics at the University of Sydney. “They are the first people to describe how you’ve got this compressive effect if there’s parallel current in a metal tube.”

The force they observed in the lightning rod was later named the “pinch effect,” for the way it squeezed matter inward. Decades later, it serves as the fundamental physics underlying Zap Energy’s approach to fusion.

From a metal rod to a fusion core

Fusion happens when the nuclei of two atoms hit each other with enough force that they join together, or fuse. Because atomic nuclei typically repel each other, fusion takes a lot of energy. To get atoms moving fast enough, most fusion experiments rely on heating a state of matter called a plasma to over a million degrees Fahrenheit.

One way to heat that plasma to the extreme temperatures necessary for fusion is to compress it with intense force. In 1957, an experiment in the United Kingdom called the Zero Energy Thermonuclear Assembly (ZETA) set out to do just that. Their fusion device relied on the same fundamental pinch technique that Pollock and Barraclough had discovered more than 50 years before.

A plasma is a state of matter that behaves like something between a gas and a liquid, and it’s filled with charged particles like ions and electrons. All those charged particles mean that you can run an electric current through a plasma just like you can in a copper wire. And following the right-hand rule, that current creates its own magnetic field — the stronger the current, the stronger the field.

Inside the ZETA core, researchers shocked hydrogen gas with an electric current to create a plasma. Then, by running more current through the plasma, they aimed to create a pinch effect — called a Z pinch in this case, for the axis along which the current flowed. If that current was strong enough, the pinch would heat the plasma to temperatures where atomic nuclei would begin fusing, throwing off energetic neutrons that could be captured to create electrical energy. Hopes were initially high.

“They thought it was going to be so easy,” James says. “You’ll put a big current through the gas, it will pinch it down to a really high temperature, you’ll get loads of neutrons.”

Not so fast: frustrations in fusion's development

The first results from the experiment seemed to confirm that ZETA was a success. Researchers measured large amounts of neutrons coming from the experiment, which was exactly what they’d been looking for to confirm fusion. In an article in the journal Nature in January 1958, and at a packed press conference that same month, researchers with the experiment announced they were fairly sure that they’d managed to create fusion.

The results were met with widespread excitement in the U.K. — “A Sun of Our Own” proclaimed the London Daily Sketch — where the supposed breakthrough was a welcome retort to the recent launch of the Sputnik satellite by the Soviets.

But the acclaim was short lived. As more data emerged, it turned out that the ZETA experiment’s neutrons hadn’t come from fusion at all, but from an entirely different mechanism related to instabilities in the plasma itself. The human-made Sun never existed.

Later work with Z-pinch devices also revealed key difficulties in maintaining fusion.

 “The problems they found very quickly were instabilities,” James says. “You’d pinch the [plasma] down, heat it up and it would get a kink in it, or a twist in it and hit the wall, and that would be the end of the discharge.”

Those instabilities eventually led the Z pinch to be abandoned for decades in favor of other fusion device designs, with the majority focused on tokamaks, which use massive superconducting magnets to attempt to confine the plasma. But those experiments proved to have their own problems, stalling attempts to create a working fusion core. 

A new path forward for the Z pinch emerges

In those decades, human energy needs have expanded dramatically, even as the climate has come under increasing pressure from the burning of fossil fuels. The need for an abundant, renewable energy source like fusion continues to grow as recent advancements in plasma physics and new materials and technologies have created new opportunities for fusion development.

“If humans are going to become an advanced civilization who continue to prosper on this planet — not just for the next few decades, but for the next few millennia and beyond — we need to get to fusion,” says Benj Conway, Zap Energy Cofounder and CEO.

A bevy of thick, insulated electrical wires converge into the crown-shaped steel head of a Zap Energy fusion device.
Zap’s Z pinches are driven by a powerful bank of capacitors capable of releasing electric current 20 times stronger than the average bolt of lightning.

Conway, an entrepreneur, investor and former diplomat, became aware of experiments by University of Washington professors Uri Shumlak and Brian Nelson, which demonstrated a method called “sheared-flow-stabilization" that could subdue instabilities in a Z-pinch plasma. The three formed Zap Energy to develop Z-pinch fusion as a commercial venture.

More than a century after Pollock and Barraclough described Z pinch in a lightning rod, one of fusion’s oldest ideas gained a new path forward.

Editor’s note: This post is the first of Zap Energy’s new blog, an inside look at the science, technology and people working to make fusion power a reality.