Characteristics of dithering cycles during the L–I–H transition on Experimental Advanced Superconducting Tokamak (EAST)

• Edge density fluctuation level is periodically suppressed during dithering phase.
• Occurrence of dithering depends on plasma configuration, but has no clear dependence on heating power.
• Dithering event is at least 2 cm inside the separatrix and extends into the SOL.
• Both stored energy and density increase during the dithering phase and the increasing rates decrease with the dithering length.

An intermediate phase (labeled I phase) with dithering cycles between the L-mode and H-mode has been observed and experimentally characterized on Experimental Advanced Superconducting Tokamak (EAST). A typical characteristics of the I phase is that the DαDα signal, edge density fluctuation level and edge radiation show several kHz periodical oscillation. The analysis shows that the dithering event is at least 2 cm inside the separatrix and extends into the scrape-off layer (SOL) region. It is found that this dithering occurs in plasma with double null (DN) or upper single null (USN) configuration and cannot be observed in plasma with lower single null (LSN) configuration where the ion B×∇BB×∇B drift direction is ‘unfavorable’, i.e. away from the X-point, in this device. The dithering cycle length (ΔtditherΔtdither) has no clear dependence on the heating power. Both stored energy and density increase during the dithering phase and the increasing rates decrease with ΔtditherΔtdither. The evolution of density profiles during the L–I–H transition is analyzed and presented.