“Fusion energy has always been one of those sci-fi technologies you read about,” says Evan Leppink, a PhD student in nuclear science and engineering. He recalls the time before fusion became part of his daily hands-on experience at MIT’s Plasma Science and Fusion Center, where he was investigating a unique way to drive current in a tokamak plasma with radio frequency (RF) waves.
Now, an award from the US Department of Energy’s (DOE) Office of Science Graduate Student Research (SCGSR) Program supports his work with a 12-month residency at the DIII-D National Fusion Facility in San Diego, California.
Like all tokamaks, DIII-D generates hot plasma in an annular vacuum chamber wrapped with magnets. Because plasma follows magnetic field lines, tokamaks can contain the turbulent plasma fuel as it gets hotter and denser, keeping it away from the edges of the chamber where it could damage wall materials. A key part of the tokamak concept is that part of the magnetic field is generated by electric currents in the plasma itself, which helps to confine and stabilize the configuration. Researchers often transmit high-power RF waves in tokamaks to drive this current.
Leppink, led by his MIT advisor Steve Wukitch, will contribute to the research tracking the transmission of RF waves in DIII-D using a unique compact antenna on the tokamak’s center column. Typically, antennas in the tokamak are placed on the outer edge of the donut, farthest from the central hole (or pillar), mainly because access and installation is easier there. This is called the “low field side” because the magnetic field is lower there than at the center column, the “high field side.” This MIT-led experiment will mount an antenna on the high-field side for the first time. There is some theoretical evidence that placing the wave launcher there could improve power penetration and power propulsion efficiency. And since the plasma environment is less harsh on this side, the antenna will survive longer, a factor important for any future power-generating tokamak.
Leppink’s work on DIII-D focuses specifically on measuring the density of plasmas generated in the tokamak, for which he developed a ‘reflectometer’. This small antenna emits microwaves into the plasma, which are reflected back to the antenna to be measured. The time it takes for these microwaves to traverse the plasma provides information about the plasma density, allowing researchers to create detailed density profiles, data critical to injecting RF power into the plasma.
“Research shows that when we try to inject these waves into the plasma to drive the current, they can lose power as they travel through the periphery of the tokamak and can even have trouble getting into the core of the plasma.” to penetrate where we would most like to direct,” says Leppink. “My diagnostics will measure this fringing area on the high field side near the launch vehicle in great detail, giving us a way to directly verify calculations or compare actual results with simulation results.”
Although he focuses on his own research, Leppink has excelled in preparing other students for success in their studies and research. In 2021 he received the NSE Outstanding Teaching Assistant and Mentorship Award.
“The highlights of TA’ing for me were the times I could watch students go from grappling with a difficult topic to fully understanding, often with just a nudge in the right direction, and then allowing them to do the rest following your own intuition is the way,” he says.
The correct direction for Leppink is toward San Diego and RF current driver experiments on DIII-D. He is grateful for the support of the SCGSR, a program created to prepare graduate students like him for careers in science, technology, engineering, or mathematics important to the mission of the DOE Office of Science. It offers thesis research opportunities through extended residencies in the DOE’s national laboratories. He has made several trips to DIII-D, partly to install his reflectometer, and has been impressed by the size of the operation.
“It takes a while to break everything down and say, ‘Okay, well, here’s my part of the machine. That’s what I do.’ It can definitely be overwhelming at times. But I am blessed to be able to work on what has been the workhorse tokamak of the United States for the last several decades.”