Pellet-fueled I-mode plasmas in ASDEX Upgrade

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 $f_{\mathrm{GW}}$ , 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 $f_{\mathrm{GW}}$ 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 $f_{\mathrm{GW}}\lt0.4$ it rises with increasing $f_{\mathrm{GW}}$ , while for $f_{\mathrm{GW}} \gt 0.4$ the energy confinement time plateaus with increasing $f_{\mathrm{GW}}$ , or even decreases with $f_{\mathrm{GW}}$ for the pellet-fueled plasmas. Similarities with Ohmic and L-mode energy confinement time dependency on density are discussed.