I-mode studies at ASDEX upgrade

Abstract

The H-mode threshold power, Pthres, is well-known to be low when the ion ÑB drift is towards the X-point, while it is about 2 times higher when directed away from the X-point, the socalled unfavourable configuration. In addition, Pthres is also about 2 times higher in hydrogen compared to deuterium. These high Pthres cases provide conditions to study L-modes over a wider range of heating power. Previous investigation in ASDEX Upgrade, [1, 2], revealed in these scenarios that the power degradation of the confinement time was much weaker than tE µ P−0.72 predicted by the scaling and these plasmas were named "Improved L-modes". The weak power degradation is caused by an increase of the edge temperatures with a concomitant steepening of the gradients, similar to the H-mode pedestal, whereas the density profile almost does not change [3]. Therefore, these plasmas exhibit an edge transport barrier for heat but L-mode particle transport. Recently, such plasmas, re-baptised "I-modes", have been extensively studied in Alcator C-Mod [4, 5]. The same features in heat transport have been observed and the lack of transport reduction in the particle channel is attributed to fluctuations occurring at the pedestal top. Motivated by these studies and by the extended experimental and diagnostic possibilities at ASDEX Upgrade, we also studied the I-mode.