Pellet-fueled I-mode plasmas in ASDEX Upgrade

Pellet-fueled I-mode plasmas in ASDEX Upgrade

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This letter reports on the efforts carried out at the ASDEX Upgrade tokamak to integrate I-mode plasmas with pellet fueling and michael harris sunglasses 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 here 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.