Changing things up at the DIII-D National Fusion Facility

July 28, 2022, 12:06PMNuclear News
The new TRFS provides for automated adjustment of the direction of the DIII-D primary magnetic field. (Photos: GA and PPPL)

The DIII-D National Fusion Facility now boasts a unique automated system that allows for a quick reversal of the direction of its magnetic field, expanding the range of possible fusion experiments while reducing downtime. General Atomics, which operates the DIII-D for the Department of Energy’s Office of Science, announced the new Toroidal Field Reversing Switch (TFRS) on July 26.

“Being able to reverse magnetic fields between plasma pulses without changing other parameters of an experiment is highly useful for validating the models used to predict the behavior of future devices,” said DIII-D special projects lead engineer Alex Nagy, who led the project. “But it needs to be done during the experiment, because any delays mean conditions inside the vessel can change.”

How it works: The toroidal magnetic field that confines the plasma is generated when a powerful electric current flows through the 24 large copper coils around the tokamak. When the direction of the current through the coils is reversed, the direction of the magnetic field is also reversed. DIII-D is unique among operating tokamaks in its ability to change the toroidal field direction—which can change key processes, such as the directions of drifts that carry particles through the plasma—without changing other parameters. That flexibility has provided key insights into the physics of handling and dissipation of high heat fluxes prevalent in fusion devices, according to GA, but it required an overnight operation with several staff members.

The TFRS turns that overnight operation of removing and repositioning several large copper plates into an automated process that takes about two minutes, using copper plates with motor-driven contact pins that move back and forth to configure the direction of current.

The benefits: Before the addition of the TFRS, the process required that experiments with the same magnetic field direction be scheduled in succession, even when they required very different plasma conditions and were performed by different teams. With the new capability, researchers can complete comprehensive experiments in a single day and improve their ability to maintain stable conditions.

Because experiments on DIII-D are conducted in a series of plasma pulses followed by a ten-minute “cool-down” period, the TFRS allows researchers to switch magnetic field direction back and forth during experiments with no delays.

This new flexibility will maximize the facility’s experimental time and expand the range of science that can be conducted. The TFRS is already helping provide key insights into the physics of handling and dissipation of the high heat fluxes that exist in fusion devices, according to GA.

Credit where due: The TFRS was designed by Nagy, a DIII-D researcher with the Princeton Plasma Physics Laboratory, who worked with Taylor Raines, an early-career PPPL engineer. The multi-finger contacts within the switch were supplied by Staubli International, which developed the TFRS in collaboration with Nagy and Raines. The team’s project was described in a recent article in the journal IEEE Transactions on Plasma Science.

“Upgrades like these keep DIII-D at the forefront of fusion science,” Nagy said. “We’re excited to see the work that can be done with this new capability.”

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