Discovery of a turbulent ExB mixing mechanism of the ohmic breakdown in a tokamak

Although the ohmic breakdown has generally been used to produce initial plasmas in a tokamak, its physical mechanism has been obscured due to complicated electromagnetic (EM) topologies. Previous researches have assumed the traditional Townsend theory is applicable to the ohmic breakdown. However, we found clear experimental evidence that the existing theories cannot explain. We developed a theoretical model and a particle simulation code BREAK [1] to systematically study the ohmic breakdown physics. As a result, we propose an entirely new type of breakdown mechanism, namely a turbulent ExB mixing avalanche [2], which successfully explains the experimental results of the KSTAR device. As the plasma responses to the externally driven EM fields, the self-electric fields produced by the plasma play crucial roles during the ohmic breakdown. They drastically decrease the plasma growth rate and cause dominant ExB transports to differ significantly from the Townsend theory. The comprehensive plasma dynamics in the complex EM topology sheds new light on a design strategy of robust breakdown scenarios in a tokamak fusion reactor.

References:

[1] Min-Gu Yoo, et al., Computer Physics Communications 221, 143–159 (2017).

[2] Min-Gu Yoo, et al., Nature Communications9, 3523 (2018)