Maria Gatu Johnson, MIT

PSFC research scientist Maria Gatu Johnson

Paul Rivenberg

Maria Gatu Johnson: Embracing the Challenges of Inertial Fusion

Paul Rivenberg  |  PSFC

Research scientist Maria Gatu Johnson is intrigued. The results of her recent experiment are very different from what she anticipated, giving rise to even more questions. But her smile suggests she’s enjoying every minute of her bewilderment. “It’s the fun of physics,” she says. Fortunately, she works at MIT’s Plasma Science and Fusion Center, in the field of high-energy-density physics, where her research on inertial fusion diagnostics provides her plenty of questions to puzzle over and resolve.

Since her childhood in Sweden, Gatu Johnson has always been attracted to solving the difficult problems. Her enjoyment of such challenges led her to pursue Physics at Uppsala University, over many competing interests. During her final year a class in Energy Physics pushed her toward a greater test of her skills — fusion — when professors encouraged her to do her final thesis project on JET, the world’s largest magnetic confinement fusion experiment, located in Oxfordshire, England.

The experience of working on this project gave her a new direction. She continued at Uppsala, focusing her Ph.D. thesis on helping to develop a neutron spectrometer for JET, a tool to measure properties and characteristics of the neutrons created during fusion.

“We really put it together, installed it on JET, took the first data, found out what was going wrong, discovered how to fix it and got some good physics out of the results. It was really exciting.”

Searching for a postdoctoral position, Gatu Johnson found something compellingly different at the MIT Plasma Science and Fusion Center. “This High-Energy-Density Physics (HEDP) group was working on inertial confinement instead of magnetic confinement fusion. But the tools for making measurements, the ‘diagnostics,’ are similar.”

In magnetic confinement fusion, magnetic fields confine plasma inside a vessel, where increasing heat and pressure create fusion reactions. Inertial confinement attempts the same goals by aiming a series of laser beams at a small pellet of fuel.

“In the magnetic confinement world all measurements are time-resolved: you try to find out how things evolve over time,” notes Gatu Johnson. “In the inertial confinement world everything happens over a few nanoseconds: it’s more of a time-integrated total measurement. But in both cases researchers are looking at the neutrons from the fusion reaction to determine what is going on in the plasma at the time of burn when the plasma starts to heat itself from its own fusion reactions. The goal of the diagnostic is the same.”

Gatu Johnson works with five senior research scientists, five graduate students and three  research specialists on a small linear accelerator, which she continually develops and upgrades. Housed in the recesses of the PSFC’s Nabisco Laboratory, the accelerator allows the team to develop, test and calibrate diagnostics that are later fielded on two larger facilities.

The OMEGA laser at the University of Rochester’s Laboratory for Laser Energetics (LLE) targets 60 symmetrically placed laser beams at a pellet of fuel less than 1 mm in diameter, imploding the capsule directly to create fusion reactions. The National Ignition Facility (NIF) at Lawrence Livermore National Laboratory uses an “indirect” approach, enclosing the fuel in a tiny gold cylinder with side entrances (a hohlraum), and aiming 192 laser beams through those ports, where laser light hitting the interior walls converts to x-rays, raising the temperature to 3 million degrees.

Although these two laboratories approach inertial confinement fusion differently, they both run a diagnostic tool that has become Gatu Johnson’s responsibility: the Magnetic Recoil Neutron Spectrometer (MRS).  And the goal is the same for both — to measure the neutron energy spectrum, calculating from that the number of fusion reactions created, the temperature of the plasma, and how well the fuel is compressed. The data from the spectrometer help guide the experiments to their goals. In the case of NIF, the diagnostic helped confirm that the energy created from fusion reactions was greater than the amount of energy delivered to the fusion fuel, a milestone on the path to realizing net fusion energy gain in a laboratory.

Gatu Johnson notes that at MIT, OMEGA and NIF she is “often the only woman in the room.” She remembers as a child in Sweden sitting around the dinner table, the only female child among two brothers and five cousins, and credits that in part for her drive to prove herself, as well as for her comfort around male colleagues.  She enthuses over the camaraderie among her teammates.

“ I really love just the day-to-day getting to work with all these people,” she says. “At the PSFC, whenever anyone has a project they need help with, together we sit down around the table and have discussions and come up with ideas. And it is the same working with people at our collaborating laboratories.”

In this supportive environment, Gatu Johnson is ready for the next challenge, helping to solve the world’s energy problems. 

“The end goal of our research is fantastic — trying to make fusion energy work,” she says.

“That’s the big puzzle that’s always in the back of my mind.”

Topics: High-energy-density physics