October 3, 2016
“My first day of work was the first plasma of C-Mod.”
Principal research scientist Amanda Hubbard remembers her arrival at MIT in the fall of 1991, just as the Plasma Fusion Center’s Alcator C-Mod tokamak was ready to start creating plasmas, the fuel for fusion energy. It has been a fruitful collaboration. Twenty-five years later she is still learning from the device about plasma processes, and investigating a new mode of operation that could influence how magnetic fusion is approached in the future.
She was hired specifically to work on C-Mod’s electron-cyclotron emission (ECE) diagnostics, which provide a way of tracking the plasma temperature, an important parameter in a machine whose goal is to create plasmas dense and hot enough to produce fusion. Lately she has focused on the edge of the plasma nearest the outer wall of the donut-shaped vacuum chamber. MIT researchers have shown that the outer few centimeters of this edge govern the behavior of the entire plasma. She and her colleagues are experimenting with this edge plasma to better understand a new mode of operation: I-mode.
The standard operating regime for magnetic fusion experiments is called H-mode, a high confinement regime that reduces the plasma turbulence and allows high temperatures and densities at the outside edge, creating good conditions for maintaining fusion reactions.
“You can think of the H-mode or the I-mode as being an insulation layer at the edge of your plasma. If you have good insulation, then you’ll end up with a big temperature difference between the outside and the inside. You want the heat you are creating to stay in your living room. But, if you have poor insulation you’ll have to crank up the heat and it will just keep pouring out.”
One drawback of H-mode is that impurities, which inevitably enter the plasma from its interaction with the chamber walls, are slow to exit, diminishing the results.
“H-mode has high particle confinement, low particle transport, so once an impurity is in it tends to stay in for a longer time, and levels often increase over time. That would dilute the fusion fuel and quench the reaction. In I-mode the transport of impurities is quite fast. The impurities leave more quickly, but the heat is well contained.”
Hubbard and her colleagues also discovered that, unlike H-mode, the power to reach I-mode does not increase much with magnetic field, opening up the possibility of experimenting with fields far greater than the typical 5.5 Tesla. In fact, the regime is particularly well suited to high-field reactor concepts such as those being pursued by MIT.
“In the last couple of years we've been pushing it up to the maximum field on C-Mod — eight Tesla. This is two to three times any other tokamak in the world.”
She reveals that in the final week of I-mode experiments on C-Mod, using very high magnetic fields resulted in temperatures of over 90 million degrees C, some of the best results yet. She is including this data in her talk at the 26th IAEA Fusion Energy Conference in Kyoto, Japan, this month.
Although the C-Mod experiment is ending just short of its 25th anniversary, Hubbard will continue analyzing the data about I-mode and collaborating with other laboratories. She feels this mode of operation could be very attractive to metal-walled tokamaks around the world, which may not be able to use large amounts of power without impurities increasing and performance decreasing: tokamaks like JET in England, ASDEX Upgrade in Germany, WEST in France, and ITER, the next step tokamak being constructed in the south of France, for which she is helping to design the ECE diagnostics. The Plasma Science and Fusion Center also hopes to pursue this high-field research at MIT on a proposed Advanced Divertor Experiment (ADX).
Hubbard credits the quarter century success of the C-Mod project to the people she works with.
“I think the thing that is particularly nice here is the team atmosphere. We are big enough to do work that is really respected and recognized internationally; small enough to have a lot of flexibility and creativity, and relatively little bureaucracy, which I think has allowed us to be more innovative. And our technical operations team is absolutely fantastic.”
Like members of this team, she enjoys the hands-on working environment around the tokamak. Her future collaborations may not require daily tending to an on-site machine like C-Mod, but they will offer new experiments requiring her expertise — and she is always up for an experiment. “You get a real rush when you try something and it works out,” she says. “Or even when it doesn’t.”