Improved confinement mode induced by the MARFE on the HT-7 superconducting tokamak

Abstract

In the HT-7 superconducting tokamak, the onset of a multifaceted asymmetric radiation from the edge (MARFE) usually occurs in the early ohmic discharges of each experimental campaign before wall conditioning. The occurrence and location of a MARFE is identified by different diagnostic systems. An improved confinement mode plasma which was induced by the MARFE is observed, and the global particle confinement time increases 1.9 times. The relaxation time between the MARFE event trigger and the L-H transition is about 1.4 ms, the following L-H transition time is 1.9 ms, and the improved confinement mode phase is maintained for about 40 ms. The MARFE cools the plasma edge, and the electron density profile is observed to become more narrow and peaked. The occurrence of a MARFE is strongly correlated with Zeff but not with the density, and it always occurs at Zeff = 3-8 ohmic discharges. In the case of a good wall condition (Zeff = 1-2), the onset of MARFEs has not been observed before reaching the Greenwald density limit.