Pellet-fueled I-mode plasmas in ASDEX Upgrade

Abstract

This letter reports on the efforts carried out at the ASDEX Upgrade tokamak to integrate I-mode plasmas with pellet fueling and to increase the I-mode Greenwald fraction fGW, two important requirements for any DEMO operational scenario. For the first time, stationary I-mode plasmas have been achieved with pellet fueling and the core Greenwald fraction has been increased up to 0.8. Larger fGW were not achieved due to technical constraints rather than to a physics-based limit. Pellet-fueled I-mode plasmas exhibit enhanced core and edge density, while core and edge temperature are reduced. Nonetheless, edge normalized gradients remain I-mode-like, namely shallow for the density and steep for the temperature. The I-mode energy confinement time is found to obey two distinct density dependencies: for fGW < 0.4 it rises with increasing fGW , while for fGW > 0.4 the energy confinement time plateaus with increasing fGW , or even decreases with fGW for the pellet-fueled plasmas. Similarities with Ohmic and L-mode energy confinement time dependency on density are discussed.

Overview of advances in ASDEX Upgrade plasma control to support critical physics research for ITER and beyond

“…AUG allows the plasma β to be controlled using either ECRH or NBI. This allows studies of the I-mode, an ELM-free regime characterised by the presence of a temperature pedestal and the absence of a density pedestal, which exists only within a narrow β window and can only be stabilized by feedback control of the plasma β [53,54]. Typically, NBI is used for this purpose.…”