On the nature of dark spots arising in a tokamak plasma during a disruption

The sudden appearance of bright and dark spots against the background lithium emission from the central regions of the plasma column was observed during major plasma disruptions in the T-11M tokamak with a lithium limiter. The measurements were performed with the help of an AXUV photodiode array operating in the energy range 1–5000 eV. Such spots in the plasma core arise in the fast transient stage of a major disruption (during the onset of the positive pulse of the plasma current) and are rather narrowly localized over r (in particular, over the vertical axis). It is supposed that the observed dark spots are related to the development of magnetic islands induced in the plasma core by an outer MHD perturbation. This effect can be used as a tool for studying specific forms of MHD resonance.