Description
Tearing modes are large-scale magnetohydrodynamic instabilities in tokamaks that lead to the formation of helical structures known as magnetic islands. Magnetic islands containing accumulated impurities are called radiative tearing modes. Impurity accumulation inside an island is expected to increase the growth rate of a tearing mode. Impurities radiate and cool the island interior, which in turn increases the local resistivity. As a result, the current inside the island is reduced, causing the island to grow [1]. There is a special case in which the growth rate of a tearing mode is dominated by radiation from impurities, which can theoretically lead to explosive growth of the mode and subsequent disruption [2]. To date, only a single direct observation has been reported of a tearing mode with impurity accumulation inside the O-point [3]; several other studies provide indirect or interpretative evidence [2, 4, 5, 6].
The present work discusses experimental evidence and demonstrates, for the first time, simultaneous measurements of temperature, density, and tungsten concentration in radiative tearing modes. A simultaneously hollow electron temperature profile and peaked electron density profile in a q > 1 tearing mode is reported for the first time. The evolution and formation of radiative (2,1) TMs are illustrated using four exemplary discharges. We report characteristic time scales and peaking factors for temperature and density at the island O-point. Finally, we provide a physical explanation for the observed behaviour of radiative TMs based on magnetohydrodynamic and particle-transport theories.
[1] Rebut, P. H., and M. Hugon 1985 Plasma Phys. and Contr. Nucl. Fusion Res., IAEA 1985 2: 197.
[2] F. Salzedas et al. 2002 Phys. Rev. Lett. 88(7):075002.
[3] X. Feng et al. 2023 Nucl. Fusion 64(1):016006.
[4] W. Suttrop et al. 1997 Nucl. Fusion 37(1):119.
[5] L. Delgado-Aparicio et al. 2011 Nucl. Fusion 45(9):1815.
[6] L. Xu et al. 2017 Nucl. Fusion 57(12):126002.