A Trojan Horse for Fusion Disruptions

Thin-walled diamond shells carry payloads of boron dust; the dust mitigates destructive plasma disruptions in fusion confinement systems.


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    Credit: General Atomics (left) and R. Moyer, University of California, San Diego (right)

    Left: See an image of a 3.6-millimeter-diameter, 40-micrometers-thick, 30-milligram boron-filled diamond shell. Right: (a) Visible imaging composite showing (1) shell pellet hitting the boundary of the plasma, (2) continuing through the q=3/2 surface, and (3) ablating and releasing boron dust in the core. (b) Expanded view of (3), highlighting shell and dust. Injection velocity ≈ 230 meter/second.

The Science

To put the energy-producing power of a star to work, researchers create and contain plasma—the ultra-hot gas that makes up a star’s core—inside a bagel-shaped tokamak. Inside the tokamak, the plasma can go unstable and rapidly terminate, in a process called a “disruption.” A disruption can damage tokamak walls and other structures. Mitigating disruptions means injecting impurities into the plasma. The impurities radiate the plasma energy evenly around the tokamak as light. But how do you add impurities deeply into something so hot? A team at the DIII-D National Fusion Facility devised a way to inject impurities deep into the plasma using thin-walled diamond shells that carry a payload of boron dust.

The Impact

Disruption mitigation is of particular concern for future devices, such as ITER. Why? Their much more energetic plasma carries greater risks of damaging the surrounding tokamak structure. Safely mitigating disruptions in ITER will be essential for reliable operation. This work offers a proof-of-concept.

Summary

When instabilities cause magnetically confined tokamak plasmas to lose confinement (a “disruption”), the rapid, concentrated release of the plasma’s stored energy can damage the tokamak wall and surrounding structure. This energy release takes the form of heat that vaporizes or melts plasma-facing materials, large electrical currents that can produce damaging forces in the walls, and the formation of high-energy “runaway” electron beams that can cause intense localized damage.

Disruption mitigation is a major focus of current fusion research. The process is analogous in some ways to the airbags in a car: While they cannot prevent a collision, they can dissipate the impact energy in safe fashion. Likewise, plasma disruption mitigation methods seek to quench the stored energy in way that lessens the risk to the tokamak.

Mitigating disruptions is of particular concern for future devices, such as ITER, because their much more energetic plasma carries greater risks of damaging the surrounding tokamak structure. Safely mitigating disruptions in ITER will be essential for reliable operation.

Disruption mitigation involves injecting impurities into the plasma that then radiate the plasma energy evenly around the tokamak as light. One challenge is that methods such as massive gas injection have difficulty reaching the plasma core (where most of the plasma thermal energy is stored) before instabilities that release the plasma energy occur. What scientists need is a way to get impurities into the core, which they believe can provide “inside-out” radiative cooling of the plasma, as well as high impurity assimilation and suppression of runaway electron production.

A team at the DIII-D National Fusion Facility (DIII-D) tokamak have developed a method of impurity injection involving thin-walled diamond shells carrying a payload of boron dust. By encapsulating boron impurities in thin-walled diamond shells, the researchers have demonstrated inside-out disruption mitigation when they fire pellets into the plasma core. The shells gradually ablate as they travel through the plasma until they reach the plasma core and deposit their payload. Initial experiments have demonstrated that shell pellets fired into the core at around 200 meter/second can deposit boron dust impurities deep in the plasma core where they are most effective. The diamond shells gradually ablate in the plasma without causing a disruption before releasing the dust at the magnetic axis.

The large size and high temperatures of ITER or a future tokamak reactor will require more complex methods to inject shell pellets than initially demonstrated on DIII-D. Future work is aimed at creating more sophisticated shell designs that can carry larger payloads and penetrate reactor-class plasmas. 

Funding

This work was supported by the Department of Energy, Office of Science, Office of Fusion Energy Sciences, using the DIII-D National Fusion Facility, a DOE Office of Science user facility.

Publications

E.M. Hollmann, P.B. Parks, D. Shiraki, N. Alexander, N.W. Eidietis, C.J. Lasnier, and R.A. Moyer, “Demonstration of tokamak discharge shutdown with shell pellet payload impurity dispersal.” Physical Review Letters 122, 065001 (2019). [DOI: 10.1103/PhysRevLett.122.065001]

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