Active disruption avoidance for H-mode density limits on TCV and ASDEX Upgrade

Disruptions represent the highest concern for next-step fusion devices based on the tokamak principle. Active disruption avoidance and off-normal event handling need to be developed in Plasma Control Systems (PCS) to predict and react when the plasma approaches dangerous operational boundaries. EUROfusion programme has put strong emphasis on disruption research, focusing on their mitigation and prevention, as well as on the study of relevant disruption paths, such as H-mode density limits. Future fusion power plants are foreseen to operate at high densities in the high confinement mode (H-mode). At densities close to the Greenwald limit, the plasma can exhibit a back transition from the H-mode to the low confinement mode (L-mode). In addition to confinement degradation, a radiation instability, the MARFE, can develop, showing a poloidally localized volume of "cold" and dense plasma. The onset of this edge radiative instability is a non-negligible issue during plasma current ramp down on devices like ITER and DEMO. After the installation of the baffles in the divertor, the development of the H-mode density limit on TCV has been found to be consistent with the one observed on ASDEX Upgrade (AUG). Similarly, to the framework implemented in this latter, a disruption avoidance tool to handle H-mode density limits has been ported to TCV and integrated in the real-time control system. Such an approach relies on a proximity measure with respect to the operational boundary defined in the H98y,2 vs edge normalized density state space, and allows identifying the energy confinement degradation with increasing density that is associated with approaching the density limit, MARFE formation and disruption. Main concepts and schemes for active avoidance of the density limits will be described with reference to the generic framework implemented in the two machines to handle off-normal events potentially leading to disruption.