Theoretical physicist Fatima Ebrahimi has for the first time produced computer simulations of the efficiency of a novel technique for starting up doughnut-shaped fusion facilities called tokamaks. The pioneering technique, known as coaxial helicity injection (CHI), could eliminate the large central solenoid coil that is needed to make “a star in a jar” in tokamaks today.

Solenoid coils run down the center of a tokamak and produce circular magnetic fields. These fields combine with those created by other tokamak coils to cage the hot, charged plasma gas that fuels fusion reactions.

Dispensing with solenoids could be a key step toward developing fusion energy. Future tokamaks — especially compact spherical tokamaks like the National Spherical Torus Experiment-Upgrade (NSTX-U) at PPPL — may simply lack space for these coils.

The CHI technique could essentially replace the solenoid. During CHI, researchers inject looping field lines into a tokamak’s vacuum vessel through openings in the vessel floor. The lines expand to fill the vessel, like a balloon inflating with air, until a process called magnetic reconnection snaps the loops closed. The closed lines then induce current in the plasma — just as solenoids do — to help keep the superhot gas confined.

Ebrahimi’s simulations showed that narrowing part of the looping field lines inside the tokamak could cause 70 percent of the lines to close, compared with just 20 percent-to-30 percent without such narrowing. “For the first time, we see a large volume of closure during computer simulations,” she said. The more field lines that close, the greater the current flowing through the plasma, and the stronger the magnetic fields that confine the plasma.

“The findings help us figure out how we can get maximum start-up current in spherical tokamaks,” Ebrahimi said. “That is a direct application of the research. But now we also have insight into some basic physical phenomena, such as the physics behind the process of reconnection that causes the lines to close.”

The simulations could strongly boost the advancement of fusion energy. “Can we create and sustain a big enough magnetic bubble in a tokamak to support a strong electric current without a solenoid?” asks Ebrahimi. “The findings indicate that 'yes, we can do it.’”