Physicists from around the world gathered at the University of California, Irvine this past summer for a symposium in honor of Wei-li Lee, a senior physicist at the U.S. Department of Energy's (DOE) Princeton Plasma Physics Laboratory (PPPL). The week-long event, held from July 18-22, focused on gyrokinetic simulation — a technique Lee invented in the 1980s to model the behavior of particles within plasma, the ultrahot gas composed of electrons and atomic nuclei that fuels fusion reactions. Approximately 15 papers related to Wei-li Lee and the symposium are planned to appear in a special issue of Physics of Plasmas in mid-2017.
PPPL senior physicist Wei-li Lee. (Photo by Elle Starkman)
"Dr. Lee has enormous scientific vision and persistence," said Scott Parker, a physics professor at the University of Colorado, Boulder and one of the symposium's organizers. "He has been a role model and mentor to many leaders in the field today."
The simulations rely on a branch of plasma physics called gyrokinetics that helps scientists understand how charged particles spiral around and move along and between magnetic field lines within fusion plasmas. Gyrokinetics averages out fast oscillations in the motion of the particles spiraling around the field lines, simplifying the calculations needed for computers to simulate the plasma's behavior.
The more that physicists understand the mechanisms underlying the motion of particles, or "transport," between field lines, the better they can design doughnut-shaped facilities called tokamaks so that transport can be reduced. Such motion cools down fusion reactions, making them less efficient.
Lee's development of gyrokinetic simulations birthed a new subfield within the plasma physics world. "But I didn't do it alone," Lee said. "I was influenced by two PPPL scientists: Ed Freeman, who created gyrokinetics, and John Dawson, who pioneered simulation. And over the years, various people, including myself, have modified and improved the equations I devised." Lee's equations reformulate gyrokinetics so it can be fitted into particle simulations.
The resulting simulations are still in use throughout the world. "The gyrokinetic simulation that Wei-li pioneered is undoubtedly the most important breakthrough in fusion simulations, with hundreds of active researchers around the world currently developing and applying this powerful tool," said Zhihong Lin, a physics professor at the University of California, Irvine, co-sponsor of the symposium, and one of Lee's former students. Scientists plan to use the simulations to analyze the plasmas within ITER, the multinational fusion facility being built in the south of France to demonstrate the feasibility of fusion power. "But there is still some physics we have to put into the codes," Lee notes.
Physicists still debate how best to solve gyrokinetic simulation equations. One method, called particle simulation, is the most efficient in present-day supercomputers. It models the particle interactions within plasmas by using a much smaller number of particles than exist in real plasmas and giving each particle three degrees of freedom to move.
Because there is less communication between the particles in these simulations, computers can solve the equations more easily. But the small number of particles means that the simulations are relatively "noisy:" there is not enough information for the computer program to model the plasma as precisely as it could otherwise.
A separate technique called "continuum simulation" solves the same equations in five dimensions, not three, and reduces the amount of noise through a process called "coarse graining." But the greater amount of information in continuum simulations means that computers have to take more time and processing power to run them. "In continuum simulations, there are simply too many neighbors talking to one another, which can become a serious problem for modern parallel computers," Lee said.
The debate over the best method to solve the equations now includes a new idea: combining particle and continuum simulations. It's clear that modeling an entire plasma using just a continuum simulation would strain the capabilities of current supercomputers. "It would just be too much," Lee noted. "The volume of the simulated plasma using a five-dimensional grid would be too big. Fortunately, the two simulation methods complement each other at this stage of fusion research."
Lee earned a bachelor's degree from National Taiwan University, a master's from Duke University, and a Ph.D. from Northwestern University, where he studied mechanical engineering and aeronautical science. His advisor was Jacque Denavit, a plasma physicist who pioneered the continuum simulation code and told Lee that plasma physics would be a worthy subject for him to pursue.
So Lee applied for a position at PPPL, where he could work with simulation pioneer Dawson. His application was rejected, however, because of a lack of funding. So, he took a position at Fermilab, the DOE national laboratory outside Chicago, where he studied particle codes and beam physics.
Lee spent several years at Fermilab before learning that PPPL was building the Tokamak Fusion Test Reactor (TFTR), a machine that operated from 1982 to 1997 and produced the highest temperatures ever created in a laboratory. He reapplied for a position and was hired by Paul Rutherford, the head of PPPL's theory department. Unfortunately, Dawson, the scientist Lee had greatly wanted to work with, had already left the Lab for UCLA.
Lee spent 41 years at PPPL and retired in 2015. During his career he focused on gyrokinetic equations and finding ways to integrate them into simulation codes, as well as using those codes to investigate microturbulence in tokamaks. Recently, he has been trying to derive magnetohydrodynamic equations by taking a "gyrokinetic point of view" and calculating the radial electric field at the tokamak's edge. His research continues in his capacity as a senior physicist, a recently created position for retired researchers at PPPL. "I'm grateful that the Lab let me keep my office, and I'm happy that I can still come into my office and interact with my colleagues," said Lee. "I feel like my job isn't finished yet, but I also need some time for myself and my family, especially my grandchildren."
PPPL, on Princeton University's Forrestal Campus in Plainsboro, N.J., is devoted to creating new knowledge about the physics of plasmas — ultra-hot, charged gases — and to developing practical solutions for the creation of fusion energy. The Laboratory is managed by the University for the U.S. Department of Energy’s Office of Science, which is the largest single supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time.