From Fermi to the Z Machine

The year was 1991. The Soviet Union was in turmoil, yet still very much firmly planted behind the Iron Curtain. Even so, Hertz Scholar/Fellow and Los Alamos National Laboratory physicist Irvin Lindemuth just had to know what the Soviet fusion scientists had been up to all these years.

Just weeks after an August coup attempt nearly toppled Soviet President Mikhail Gorbachev, Lindemuth found himself aboard a small Aeroflot plane en route from Moscow to Sochi, on the Black Sea. He and his wife, Hedy were heading for a young student plasma physics school, where he had been invited to lecture. He looked out to the aircraft’s wing, emblazoned with four red letters CCCP, the Cyrillic acronym for the Union of Soviet Socialist Republics.

“Getting up in the clouds you could see nothing but that wing out there,” Lindemuth recalled. “We just wondered if anything were to happen in Russia, another coup attempt or something, would anybody even know where we were at? It was an eerie feeling to say the least.”

Eight years prior, Lindemuth, a Hertz Scholar and Hertz Fellow, published a landmark scientific paper on what would later become known as Magnetized Target Fusion (MTF). It was an approach to fusion intended to blend the best components of inertial confinement and magnetic confinement, the two vanguard approaches scientists had been working on for decades. While some of the basic principles of MTF had been discussed since the 1940s by scientific luminaries such as Enrico Fermi, it had never really taken root.

In his 1983 paper, published by Nuclear Fusion, Lindemuth theorized that by generating a magnetic field in the fuel of a fusion target (a shell containing tritium or deuterium) it would be easier to heat. Putting a magnetic field in the fuel inhibits thermal conduction, trapping the heat inside, keeping the fuel warm and requiring less power to implode it. Theoretically with this method, fusion could be produced at a fraction of the power and cost as laser-based approaches. At the time, the work went largely ignored.

However, in 1988, the US intelligence community became interested in a 1979 paper on an unconventional approach to fusion. Although the main author of the paper was unknown, the paper had been submitted for publication by academician Yulii Borisovich Khariton, known through classified circles as the “Soviet Oppenheimer,” who had overseen the development of the Soviet Union’s first nuclear weapons.

Although the fusion concept was discounted by many in the U.S., Lindemuth made the then-provocative conclusion that the concept made sense if it were based on MTF principles. A year later, while attending an international Megagauss conference in central Russia, Lindemuth had a brief conversation with a co-author of the paper, who was obviously not at liberty to discuss specifics.

“At that time they couldn’t even tell us they were from the Russian Los Alamos but we knew they were because of classified information that we had,” Lindemuth said. “We just started talking about things of mutual interest and they said maybe one day maybe we could work together.”

Then in 1990, a team of American scientists who had participated in the “joint verification experiments,” became the first Western visitors to the highly secretive All-Russian Scientific Research Institute of Experimental Physics (RFNC-VNIIEF). The lab, located in the gated town of Sarov (known then as Arzamas-16) was affectionately nicknamed the “Russian Los Alamos.” The visitors brought home a prospectus that gave the U.S. its best look ever at the institution previously known only through classified information.The prospectus confirmed that the Soviet scientists had been working on a fusion method called MAGO, essentially the same magnetic compression idea Lindemuth had written about years before.

Lindemuth’s prior acquaintance with Russian scientists put him on the plane from Moscow to Sochi in Sept. 1991, where he had worried about his fate in the event of another coup. On his return trip, Lindemuth was met at his Moscow hotel by the Soviet fusion scientists who had written the 1979 paper. They had a proposal; Lindemuth and Los Alamos scientists would be allowed to work together with the Soviet scientists on the magnetic compression projects.

But because of the rapid change enveloping the Soviet Union, Los Alamos never made a formal response to the proposal. By late 1991, the Berlin Wall had fallen, the Soviet Union was collapsing, and Russia had begun a remarkable transformation. The U.S. government was concerned about where the Russian scientists would go after the collapse. Would they go off to work for Saddam Hussein or take their nuclear secrets to America’s other adversaries?

In December, Siegfried “Sig” Hecker, director at Los Alamos, attended a Russian-focused meeting in Washington. When it was clear the Soviet Union might collapse, then-Energy Secretary James Watkins expressed concern about the fate of the Soviet laboratories.Hecker responded: “Why don’t we just go and ask them?”

“Fortuitously, there I was in a position to go to Russia, not just for technical talks but to seek out the leaders of the Russian laboratories and tell them ‘our directors want you to come to Los Alamos and Livermore and we’ll come back over there,’” Lindemuth said. “From our scientific perspective, they had gadgets that we didn’t have, and we were interested in them and wanted to learn more about the stuff they’d been doing. It was just scientific curiosity and the belief that the tools they had could help us reach our own goals.”

And from a global security perspective, Lindemuth said, the United States simply had to know the fate of the Soviet nuclear weapons design labs and their scientists.

“It’s hard to describe what it was like to go over there,” Lindemuth said. “Instead of just being a technical visit, our visit turned out to be a political visit, where we were tasked to go over there and find out as much as we could about what was happening over there.”

The ensuing exchanges of directors and scientists began a collaboration between Los Alamos and its Russian counterpart that would last the next 20-plus years. A series of experiments began in 1993 in high magnetic fields the American scientists hadn’t been able to generate before. For the Russian scientists, it was their first chance to become part of the global scientific community.

“They wanted to tell people what they did and never had the opportunity before, so this was a big thing for them,” Lindemuth said. “In a bigger picture it was the scientific collaboration that convinced the Russians that the Americans from Los Alamos were for real and would work with them on more sensitive issues.”

Lindemuth discovered that about the same time he’d published his landmark paper on magnetized fuel targets, the fusion scientists in Sarov had come up with essentially the same idea. The work led Lindemuth to conclude he was on the right path.

“I don’t think there would be a lot of activities in MTF right now if the Russian collaboration hadn’t stirred it up,” Lindemuth said. ‘I think everybody that’s involved learned about it, at least in part, from people that were involved in the Russian activities.”

From MTF to MagLIF

Stimulated by the fledgling US-Russian MAGO collaboration, Lindemuth and Dick Siemon, head of Los Alamos’ fusion energy program, had given talks, workshops and published papers on MTF, and by the late 1990s, word had gotten around, even if many fusion scientists were skeptical. Their work, along with the ongoing Russian collaboration, stirred up interest in MTF at a time when some in the scientific community were looking for alternatives to the conventional approaches that required large lasers.

“There’s always been optimism about fusion, both inertial confinement and magnetic confinement,” Lindemuth said. “People were starting to realize it was going to be a lot harder problem than they thought it was, so at least some people were starting to look at other options, and MTF reared its head.”

Meanwhile, at Sandia National Laboratory in New Mexico, Hertz Fellow Stephen Slutz was becoming frustrated by his research in pulse power fusion. He had pursued many different approaches involving electron and ion beams. Enter Irv Lindemuth, who by then had become an advocate of magnetized target fusion.

“I started thinking about alternatives,” Slutz said. “I honestly think Irv was very important to me in considering this. He came several times to Sandia to talk about MTF. I knew about it. I worked on the radiation driven approach, and people would ask me why don’t you put a magnetic field in the target? The idea isn’t new, but a magnetic field in a spherical target is a problem. It’s not a natural geometry.”

Sandia had a unique capability not found anywhere else, at the Z Pulsed Power Facility, touted as the world’s most powerful facility of its kind. The “Z machine” could implode objects with something called a zeta pinch, or simply “z pinch,” which uses an electric current outside the plasma fuel to generate a magnetic field that compresses the fuel.

Looking for a way to use the Z machine, Slutz started thinking seriously about MTF in 2008, a slightly different version he called magnetized liner inertial fusion, or MagLIF. The approach was initially classified, so Slutz couldn’t discuss it openly for about a year. He eventually published a paper on MagLIF in 2010. It described different geometries and magnetic field configurations than Lindemuth had proposed for MTF, but was essentially the same idea, putting a magnetic field in the fusion target to reduce thermal losses and using a second magnetic field, instead of a laser, to crush a fuel-filled metal tube (called a liner) to produce fusion energy.

Imagine, Slutz said, a vertical pipe, where the electrical current from the Z machine is driven vertically, producing a magnetic field around the pipe. This field produces a pressure that drives the pipe inward. The current, and thus the field around the outside of the pipe, is fired for about 100 nanoseconds, which is an extremely long time in conventional fusion science timeframe. On an even longer time-scale a set of Helmholtz-like coils is fired about 3 milliseconds before the Z machine so the magnetic field can diffuse into the conductive metal tube. Then, as the tube starts to implode, a laser heats the fuel and z compresses the tube, a bit like rubber bands crushing a soda can.

“When I first started talking about it, everybody said what about these ends (of the tube) because they’re open,” Slutz said. “It took a while for people to accept that if the tube is long enough, it’s a tolerable loss. We lose both fuel and energy out the ends, but when you start imploding this thing, the area at the two ends is very small compared to the area of the tube, so it’s not a dominant loss”

Slutz’s published papers on MagLIF caught Lindemuth’s attention, and although he had retired from Los Alamos, Lindemuth started doing his own computer experiments, dusting off the old computer code he’d written in 1983. With Slutz’s help, Lindemuth reviewed his old research and began writing again about how MTF might work.

“When I read their papers I felt almost like a zealot who just converted someone to their religion; it was really exciting to see their results,” LIndemuth said. “I’d probably put in my mind that other magnetic field approaches would work better than what Steve claimed would work, but when I saw it worked I thought I’ve got to convince myself that it would work.”

To test the idea at Sandia, the laboratory needed a new device to create the uniform, high magnetic fields within the fuel, which was commissioned in 2013.The first MagLIF experiments on the Z machine began the following year. So far, Slutz said, the results have been promising. The reaction does in fact produce fusion, but needs to produce more, he said.

“We’ve fired quite a number of shots now, and we demonstrated that if you don’t use magnetic fields or the laser, you get almost no yields, and when you fire them both you change the yield by approximately 1,000 times,” Slutz said. “We’ve demonstrated that it works, now we’re trying to demonstrate that we know how it works.”

While Slutz is confident the experiments in the Z machine have demonstrated that magnetizing fuel will be a successful method to fusion, he’s busy working out the flaws, namely why the system doesn’t produce more yields. He cautioned there’s still a long way to go to reach the Holy Grail of fusion energy, the “breakeven” point where the energy yield is greater than the energy put in.

“My feeling is that this is just the beginning,” Slutz said. “We just know it won’t work if you don’t put a magnetic field in, and it does when you do, so we know it has a positive effect. But it’s going to take a lot of time before we have a really solid quantitative understanding exactly how it all works.”

Plainly, Slutz said, he needs a bigger machine. Reaching breakeven would require at least four times the pulse power energy the Z machine is capable of. Meanwhile, he’s looking at ways to increase the fuel density and preheat energy, and considering adding a layer separating the fuel from the liner to help deflect the blast wave caused by the laser.

“If we did these things within range of what we’re capable of we should be able to increase the yield by a factor of 10 or even 100,” Slutz said. “If we could do that before I retire I would be happy. I think that would demonstrate enough understanding that it would be sensible to talk about a new machine.”

The Hertz connection

Now approaching 40 years working at Sandia, Slutz didn’t start out wanting to be on the forefront of fusion energy. As a Hertz Fellow, he attended the California Institute of Technology (Caltech), where he earned his PhD in astrophysics. His interest in fusion came “indirectly” through the Hertz Foundation, and Lawrence Livermore astrophysicist and Hertz Director Emeritus Lowell Wood, who interviewed him for the Fellowship and offered him a summer job at Livermore to work on x-ray lasers. During that summer, he was introduced to laser-driven inertial fusion primarily through discussions at lunch where LLNL physicist John Nuckolls and Steven Bodner, head of laser fusion at the Naval Research Laboratory.

“It had a profound effect, that introduction to inertial fusion at Livermore. It was because of Lowell that I got that job,” Slutz said.

At Sandia, Slutz would work with Mark Herrmann, another Hertz Fellow who is currently the head of Livermore’s National Ignition Facility. Herrmann, Slutz said, pushed him to think about MTF from different angles. He also credits Lindemuth with providing a basis for his research.

Lindemuth, who retired from Los Alamos in 2003, continued to make trips to Russia, meeting with high-ranking Russian scientists and DOE officials to discuss possible collaborations. In 2013, the U.S. and Russian governments signed an historic agreement to have scientists from Los Alamos, Livermore and Sandia work with Russian scientists on fusion and other projects. The collaborations ended abruptly in 2016, when Russia invaded the Ukraine. The Americans, in response, barred their scientists from working with the Russians. The work is currently in limbo.

Today, Lindemuth resides in Tucson, Ariz., is an avid golfer, and occasionally lectures on fusion at UC San Diego’s Center for Energy Research. He looks back fondly on his Hertz Scholarship, which allowed him, a minister’s son raised in Pennsylvania, to attend Lehigh University, where he got his BS in electrical engineering. As a Hertz Fellow, he studied applied physics and computer coding, earning his PhD in Engineering-Applied Science from the University of California Davis/Livermore.

He was interviewed for the Fellowship by one of his graduate school advisors, former Lawrence Livermore Lab director and legendary physicist Edward Teller, which opened the door to a job at LLNL, where he worked on thermonuclear fusion. He remained at Livermore for several years, moving to Los Alamos in 1978, where he grew his interest into pulse power and MTF for the next 25 years.

“The Hertz made my career possible. Period,” Lindemuth said. “I don’t know what kind of college education I would’ve gotten without it. Hertz’s scholarship was generous enough that it wasn’t hard for my parents to get me through college. Then, somehow I heard about Hertz Fellowship and started thinking maybe it would be good to get more education. I’m sure if I hadn’t gotten the Fellowship I might still be working in the aircraft industry as an electrical engineer.”

Regarding the promise of MTF, Lindemuth and Slutz are true believers in the approach and the impact it could have on the world. MTF-related projects based on Lindemuth’s work are ongoing at ARPA-E, as well as efforts at several private startup companies, Los Alamos and the University of Washington. Above all else however, Lindemuth said Slutz’s research at Sandia is the “most exciting thing going on.”

“Certainly in the U.S. what Steve’s kicked off at Sandia is pretty fantastic stuff,” Lindemuth said. “It’s in its relative infancy, but when you look at the results Steve has gotten and how quickly he’s got them, there’s every reason to be excited. If I had money to invest in fusion, I’d invest in MTF.”

Interestingly, Slutz’s interest lies not in fusion energy, but in fusion-propelled rockets, which could fly many times faster than conventional chemical rockets. As he nears retirement, Slutz said he’d like to continue working on MagLIF at Sandia part-time, with a focus on solving the laser heating and blast wave challenges.

“Fusion energy would be great, but a fusion rocket is more motivating to me, and the reason is economic,” Slutz said. “By nature of how difficult it is, fusion will be difficult to make cheaply. I’d like to see energy, but the niche of making a fusion rocket that goes faster than a chemical rocket can go has no competitors.”

“It’s been quite a long period of development and things move slowly,” Slutz added. “You have to have a lot of patience. When you think nothing’s happening, then something interesting happens.”