Physicist Masaaki Yamada's pioneering career at PPPL
In 2015 Masaaki Yamada, distinguished laboratory research fellow at the U.S. Department of Energy's (DOE) Princeton Plasma Physics Laboratory (PPPL), won the James Clerk Maxwell Prize in Plasma Physics. The award, from the American Physical Society Division of Plasma Physics, honored Yamada's "fundamental experimental studies of magnetic reconnection relevant to space, astrophysical and fusion plasmas, and for pioneering contributions to the field of laboratory plasma astrophysics." The recognition culminated decades of dedicated study and experimentation, made even more special because it came from esteemed professionals in his field.
"Masaaki has a great talent for identifying the major problems, devising clever experiments, and extracting iron-clad data from those experiments," said PPPL director Stewart Prager.
But his journey to the Maxwell Prize did not happen quickly. It began many decades ago, when Yamada was a boy in Hokkaido's Kitami-city. "At that time, the rural areas in Japan did not have many physics teachers," he recalled, "so I studied by myself."
Yamada had always been interested in rockets — he read about rocketry pioneers like Robert Goddard and Werner von Braun and kept up to date on the American rocket program. The launch of Sputnik by the Soviet Union on Oct. 4, 1957 only deepened his interest. "I was a typical Sputnik kid," he noted.
And it was Sputnik that, in a sense, helped launch Yamada's career. The beach-ball-sized device proved that mankind could put an artificial satellite into orbit around the Earth. So, realizing that they might be falling behind, nations around the globe began scrambling to strengthen their science programs to compete with the Soviets. The Japanese educational system, too, began to stress science.
The precocious Yamada and his family realized that if he wanted to pursue his interest in science he would have to leave his hometown. So when he was 14, his parents sent him to high school in Tokyo. After graduation he enrolled in the University of Tokyo. "By that time I was very interested in doing rocket science," he recalls.
After reading a book by Soviet physicist Lev Landau about plasma, Yamada's interests changed. "In that book, Landau said that plasma is everything, that plasma solves all kinds of problems, including nuclear fusion," said Yamada. "I was very stimulated."
He approached Setsuo Ichimaru, a young professor at the university who was a "very exciting guy," and told him about his new interest. The professor, though, had just been recruited by the University of Illinois at Urbana-Champaign. So in the middle of his graduate studies, Yamada transferred, following his mentor to the United States. And he changed the focus of his studies from engineering to physics. Then, after working on some basic plasma experiments involving arc discharges, he applied for a post-doctoral position at PPPL, where Professor Thomas Stix hired him.
Yamada arrived at PPPL in 1973 and began working as a basic scientist, studying wave phenomena, Landau damping, and other fundamental processes. Five years later, he became involved in a movement to develop a kind of fusion machine that could outperform the tokamak. The doughnut-shaped tokamak had been shown to outperform the stellarator, an early fusion facility invented by Princeton University astrophysicist Lyman Spitzer, who founded the project that became PPPL.
Plasma physicists were learning that tokamaks could efficiently confine plasma within magnetic fields, but the machines were complicated and hard to build. "So," said Yamada, "I proposed to build a special tokamak that would have the shape of a sphere." He submitted his design, known alternately as a "spherical torus" or an "ultra-low-aspect-ratio tokamak," to the PPPL program committee in 1978, and the members liked it. Significantly, Harold Furth, the Laboratory's director, liked it, too.
Furth asked Yamada to meet him in his office, where he said that his idea was good, but not as pioneering as it could be. "Why don't you yank out the central conductor?" Furth wondered. Their conversation resulted in construction of the Spheromak 1 (S-1) in 1982, a machine with no central electromagnet. Unfortunately, while innovative, the machine had weaknesses.
"It was a simple idea for a reactor — a machine with simple geometry — but the confinement properties weren't good," Yamada said. In other words, S-1 was unable to keep the plasma within its magnetic borders. In addition, the entire plasma mass tended to tilt, becoming unstable. After several years of intensive research, the device was shut down in 1988.
The weaknesses of the spheromak led to a crucial realization. While studying the S-1's deficiencies, Yamada learned that the magnetic field interacted with the plasma in such a way that the field organized itself into a state of minimal magnetic energy. The self-organization occurred through a process known as magnetic reconnection, which takes place when magnetic field lines break apart and then snap together again.
During reconnection, the energy stored within the magnetic fields is converted to the kinetic energy of particles. To Yamada, reconnection seemed the key to understanding not only why the S-1 had not worked as expected, but also the behavior of magnetized plasmas in general. He also realized that reconnection was an important factor in plasma that occurs in outer space, including in the sun's corona, the Earth's magnetosphere, and the vast emptiness between stars and galaxies.
In 1988, Yamada took a sabbatical trip to Switzerland where he spent time skiing and pondering what to do next. The answer came to him: why not build a machine that could address magnetic reconnection directly? That gave rise to the Magnetic Reconnection Experiment (MRX). Beginning operation in 1995, MRX resembles a large steel barrel connected to a mass of tubes and wires. Two sets of inductive coils inside the barrel, known as "flux cores," create plasmas whose magnetic field lines separate and reconnect in a predetermined manner. Scientists can thus study reconnection in a controlled environment, rather than waiting to observe the phenomenon in space.
One of the goals for MRX was to test the Sweet-Parker theory, which had long been assumed to model magnetic reconnection correctly. Not everyone agreed that the Sweet-Parker theory needed investigating. PPPL theoretical physicist Russell Kulsrud wondered why anyone would build MRX, since reconnection seemed to be fully understood.
Yamada's response was simple: "I don't believe the theory."
Before Yamada could begin to test the theory, he had to find funding for both construction and operation of the MRX. He got help from former PPPL director Ron Davidson; together they approached the Navy, which agreed to provide some money. Davidson also found funds in the Laboratory's budget that could support MRX. Additional money came from NASA, the National Science Foundation, and DOE.
Their financial support paid off. "The machine ended up being very productive," said Yamada. After performing experiments, Yamada and his team found that while the Sweet-Parker theory accurately described magnetic reconnection in some situations, in others it did not.
For instance, magnetic reconnection was widely suspected to be the reason why the sun's atmosphere was vastly hotter than the sun's surface. The surface typically measures around 5,000 degrees Celsius, while the atmosphere, or corona, can reach temperatures over 1 million degrees Celsius. Physicists thought that magnetic reconnection was occurring in the atmosphere and releasing energy that heated the surrounding plasma.
MRX experiments showed that reconnection events that happened far above the sun's surface were occurring more quickly than the Sweet-Parker theory predicted. Kulsrud, who had been skeptical about the need for MRX, got excited. "He started coming to my office every day, contributing significantly to my research," Yamada said. Hantao Ji, now a Princeton University professor of astrophysical sciences, joined the MRX team in 1995 and has been an important part of the project.
MRX eventually produced so much data that Yamada's group was able to publish a paper in the prestigious Physical Review Letters (PRL) every year. To date, MRX data has been the basis for more than 15 PRL papers and two Nature papers, among others.
MRX has produced more than just information. Since its inception, more than a half dozen graduate students have earned doctorates performing research using MRX. In addition, several post-docs have received training using the machine.
Yamada's research with MRX has made him a pioneer. "In this new field, which we call 'laboratory astrophysics,'" said Philip Efthimion, who heads the Plasma Science & Technology Department, "Yamada has tackled extremely challenging astrophysical problems, where it's been difficult to make progress just by making observations from satellites and ground-based observatories.
"Sophisticated modeling in a controlled laboratory experiment like MRX allows you to look at the fine details of the physics and gives you a better connection to the astrophysical measurements," Efthimion said.
MRX has been operating for 20 years, and Yamada has been seeking funds to upgrade the machine. New funding has now arrived. It will pay for the construction of FLARE, which will be twice as big as MRX and will let researchers create reconnection events that more closely resemble those that occur in outer space. FLARE is being built on the Princeton University campus and is scheduled to be brought to PPPL and completed in 2017, with Hantao Ji as principal investigator.
Yamada continues to be passionate about mentoring students. Recently he has become involved in summer exchange programs for Princeton students with the University of Tokyo. He also became president of the non-profit University of Tokyo's alumni association in New York City, which provides funding for 20 exchange students at the University of Tokyo and major U.S. universities.
Overall, Yamada contributes a great deal of his success to PPPL's intellectually stimulating environment. "I appreciate the strong support I have had at PPPL throughout my career," Yamada said. "I wouldn't have been able to do what I've done without it."
Though Yamada is intensely involved in physics and education, he has other interests that take him outside the laboratory. He enjoys golf, for instance, and relishes traveling internationally with his wife. These days he has been trying to get back into painting. "I used to paint when I was a kid," Yamada said.
It's been a long time since Yamada was running through the fields of Kitami-city, dreaming about rockets. But though he never followed in the engineering footsteps of the people who helped launch Sputnik and Mercury and Apollo, his research over the decades has taken him to space nonetheless. And plasma physics has benefitted immeasurably from the journey.
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