Research news from the division Plasma Edge and Wall
Intermittent turbulent fluctuations in ASDEX Upgrade I-mode plasmas
The I-mode confinement regime is characterized by steep edge profile gradients in the electron and ion temperatures, but not in the density. Hence, energy transport behaves like in H-mode while particle confinement is L-mode like. Most probably connected to this peculiar behavior is a quasi-coherent feature in the fluctuation characteristics called the weakly coherent mode (WCM), which resides in the plasma edge. It is unclear what the underlying instability of the WCM is and how density and temperature fluctuations can be decoupled.
Scientists from different research institutes in Germany, France and Portugal have recently joined forces to look in more detail into the I-mode on the ASDEX Upgrade tokamak. The main diagnostics used were Doppler and conventional reflectometry. In this effort, strong fluctuation amplitude bursts have been found in the I-mode edge plasma. The structures appear only in I-mode, and they become stronger with increasing confinement quality. They have higher amplitudes than the L-mode fluctuations (see figure) and are strongly intermittent. The central result is the demonstration that the density fluctuation bursts are linked to the WCM. This is a hint that they might play a role in inhibiting the density profile to steepen in I-mode. These results can be looked up in one of the "most read" recent articles in Nuclear Fusion, which is also highlighted by a "LabTalk".
T. Happel et al., Nucl. Fusion 56, 064004 (2016)