Dynamic of Density Profiles in JET During Slow L-H Transition

In tokamaks, the H-mode is characterized by density and temperature pedestals that build up at the L-H transition. During JET 2012 campaign, discharges were carried out with a smooth increase of heating power to compare the L-H transition threshold in JET-C and JET-ILW. In these discharges, plasma passed through intermediate regimes between the L-mode and the H-mode: divertor oscillation and M-mode. The divertor oscillation (DO) is characterized by a periodic (typ. 10ms) oscillation of plasma radiation with an opposition of phase between the inner and outer side Da signal. Although reminiscent of the dithering L-H transitions, several features distinguish the divertor oscillations. The DO is an oscillation between two different plasma states: firstly the edge plasma density increases with low divertor radiation, secondly follows a sudden decrease of the plasma density with increased divertor radiation. During the high plasma density phase, the plasma stored energy increases slightly whereas dithering cycles are periodic L-H-L transitions at frequency an order of magnitude higher. Improvement of reflectometry data processing to reduce noise and radial jitter allows to follow the dynamic of the density profile during the DO. This study is preliminary; a very limited number of shots were processed. The observations lead us to propose a simple model to explain the periodic density oscillations.
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