The I-phase is an H-mode confinement regime of tokamaks characterized by limit cycle oscillations, the so-called LCOs or bursts. These bursts are the manifestation of a periodic flattening of the plasma edge pressure profile. The profile flattening is caused by increased radial transport, driven by a high-frequency plasma edge mode that periodically appears. This short-living mode is intrinsically connected to each burst. It vanishes once the profiles are fully flattened, and it reestablishes during profile recovery once critical gradients are reached and a new cycle begins. In this paper, we describe for the first time the unambiguous presence of the I-phase at the tokamak à configuration variable (TCV). As the I-phase confinement regime is found in the parameter regime between the L-mode and the fully developed H-mode, it is often confused with dithers between H-mode and L-mode. Therefore, we are highlighting the differences between these two phenomena. Furthermore, we show the two-dimensional dynamics of the I-phase mode and bursts and the associated filamentary transport, enabled by the outstanding capabilities of the 2D TCV Gas Puff Imaging diagnostics.
This paper describes the I-phase, an H-mode confinement regime in tokamaks characterized by limit cycle oscillations, or bursts. These bursts are caused by a high-frequency plasma edge mode that periodically appears, leading to increased radial transport and flattening of the plasma edge pressure profile. The paper highlights the differences between the I-phase and dithers between H-mode and L-mode, and presents the two-dimensional dynamics of the I-phase mode and bursts using TCV's Gas Puff Imaging diagnostics.