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Technology

ITER-like disruption mitigation at KSTAR

27 Jan 2020 - Michael Lehnen, chairman of the ITER Disruption Mitigation System Task Force
KSTAR_SPI_Team_Jan2020_ed.jpg
Happy faces after the successful experiments—from left to right: Larry Baylor (ORNL, senior scientist of the shattered pellet injection development team), Uron Kruezi (ITER, leading the ITER disruption mitigation system design team), JayHyun Kim (NFRI, shattered pellet injection project leader at KSTAR), Jeffrey Herfindal (ORNL, disruption mitigation expert), and chairman of the ITER Disruption Mitigation System Task Force Michael Lehnen (ITER). Team members who are not in the picture: Nick Eidietis (General Atomics, leader of the experiments group of the ITER Disruption Mitigation System Task Force), SooHwan Park (NFRI, responsible engineer for injector installation).

Two weeks ago at the Korean tokamak KSTAR, the technology chosen for disruption mitigation at ITER—shattered pellet injection—was tested for the first time in an ITER-like dual-injection configuration.

KSTAR_SPIO_ed.jpg
One of the two injectors at KSTAR—the pellets are formed in the cryogenic chamber visible in the foreground and are then injected into the plasma through a flight tube of more than 10 m in length. The gas distribution panel is visible on the right. Vacuum components also ensure that propellant gas that is used to release and accelerate the pellets is held back to not enter the plasma before the pellet. A microwave cavity measures the velocity and mass of the injected pellet.
To lessen the impact of high-energy plasma disruptions that can lead to significant thermal loading of in-vessel components, an international task force is leading an extensive program to validate the design of an effective disruption mitigation system.

The design work of the ITER disruption mitigation system is driven by a team of engineers that was formed in 2019 at the ITER Organization, after the responsibility for the system was taken over from the US Domestic Agency. This team continues what has been developed at the Oak Ridge National Laboratory (ORNL) as a voluntary US contribution, progressing the design with high intensity in order to have it ready to integrate into port plug designs by 2022. In parallel, the task force is investing in optimizing key technologies as well as validating the functionality of the design. The first large-scale experimental project of the task force is the KSTAR shattered pellet injector project.

ITER's disruption mitigation strategy is based on the injection of cryogenic pellets from different toroidal locations into plasmas that are unstable and will disrupt. This method aims to decrease electromagnetic and thermal loads to machine components.

Just before they enter the plasma chamber, these pellets—which have a diameter of 28.5 mm and a length of more than 50 mm—are shattered into small fragments. This concept ensures that the frozen deuterium and neon is ablated and assimilated by the plasma to achieve the high densities that are required for disruption mitigation. The plasma thermal and magnetic energy will be dissipated through radiation rather than leading to large heat loads and electromagnetic forces.

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