Computers model frozen fuel in blazing fusion reactor

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The pellet and the plasma

Samtaney’s fusion refueling simulation ran on computer clusters at Princeton and on supercomputers at the National Energy Research Scientific Computing Center at the Berkeley lab.

As the frozen hydrogen isotope pellet enters the plasma, its outer layers ablate, turning to gas.  A pocket of high-pressure gas forms and the pellet moves across the magnetic lines of force containing the plasma.

Samtaney’s simulation aligned with experimental findings that show launching pellets from outside the torus – where the magnetic field is relatively weak – makes them stall, so the fuel disperses at the plasma edge. Pellets injected from inside the torus – where the magnetic field is strongest – end up at the center of the plasma, right where they’re wanted.

Samtaney thinks something more is going on.  He believes the pellet ablates quickly at first, but the gas cloud shields it and slows ablation.  As the gas becomes ionized and the ablated cloud moves away, “the pellet is exposed again and starts ablating faster” until another gas cloud forms and ablation slows again, Samtaney adds.

“It’s a fluctuation thing, and I think also the reason the fuel is pushed toward the core,” Samtaney says.  He hopes his next simulation will give clues to whether he’s right.

Future research also will focus on simulating the pellet with greater resolution.

Samtaney also is working with Roman Samulyak, a researcher at the Department of Energy’s Brookhaven National Laboratory, and Paul Parks of General Atomics, a private research company.

Samulyak and Parks developed a detailed simulation of the area immediately around the pellet.  The researchers want to merge this highly localized simulation with Samtaney’s global model.

“They have important parts of physics that are missing in my calculations, and I have important parts of physics that are missing in their calculations,” Samtaney says.

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