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Visible imaging of (a) shell pellet hitting low-field-side boundary of plasma, (b) continuing through plasma toward core, (c) ablating and releasing boron dust in core. (d) Expanded view of (c), highlighting shell and dust. (e) Plasma cross-section with red dot indicating pellet location at time of dust release in (c). Injection velocity ≈ 230 m/s.
Source: General Atomics

Fusion offers the potential of near limitless energy by heating a gas trapped in a magnetic field to incredibly high temperatures where atoms are so energetic they fuse together when they collide.

The catch is, though, if that hot gas, called plasma, breaks free from the magnetic field, it must be safely put back in place to avoid damaging the fusion device. This problem has been one of the great challenges of magnetically confined fusion.

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During these disruptions, the rapid release of the energy in the plasma can damage the fusion device: Intense heat can vaporize or melt the walls, large electrical currents can generate damaging forces, and high-energy “runaway” electron beams can cause intense localized damage.

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Making disruptions less disruptive involves injecting material into the plasma that evenly radiates away plasma energy. One challenge is the material has difficulty reaching the middle of the plasma before a disruption occurs. Researchers hope that getting material into the middle can provide “inside-out” cooling of the plasma, preventing the disruption and the production of runaway electrons.

Researchers at the DIII-D National Fusion Facility have demonstrated a new technique to achieve this “inside-out” cooling before a disruption occurs.

A thin-walled diamond-shelled pellet carries a payload of boron dust deep into the plasma. Experiments show shell pellets fired into the core at around 450 miles per hour can deposit boron dust deep in the plasma where it is most effective. The diamond shells gradually disintegrate in the plasma before releasing the dust near the center of the plasma.

The new approach transforms prospects for fusion energy by potentially solving three major problems — efficiently radiating away the plasma’s heat, reducing forces by the plasma on the fusion device, and preventing the formation of energetic electron beams.

“Shell pellets offer the potential of dealing with all three facets of the challenge, eliminating risk of device harm,” said DIII-D Science Director, Richard Buttery.

Future work is aimed at creating more sophisticated shell designs that can carry larger payloads and penetrate reactor-class plasmas.

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