Real-Time Pellet Ablation Radiation Monitoring for Refined Plasma Core Density Control in ASDEX Upgrade

Reliable core density control is a key requirement in a future fusion reactor. In e.g. EU-DEMO, the designated solution relies entirely on pellets, millimeter-sized bodies formed from solid hydrogen fuel. In a tokamak device, fueling pellets would have to be injected from the torus inboard side, and hence, transferred through a curved guiding system. This will result in a considerable size distribution of pellets, and in some cases, their complete loss.In EU-DEMO, unrecognized missed-out pellets are predicted as a potential cause of disruptions and even are regarded as safety importance class events. At ASDEX Upgrade, efforts are under way to develop methods that recognize and counteract the effects of missed-out pellets. They rely on pellet arrival recognition and analyzing in real time the intense radiation emitted during pellet ablation. For any pellet launch request, either its arrival is confirmed or a missed-out case is stated.In a first demonstration, a corrective actuation was implemented by the immediate substitution of missed-out pellets. Rather than characterizing the pellet size just as 0 (missed-out) or 1 (confirmed), we envisage commissioning an estimator for the arriving pellet mass, mapping the total amount of emitted ablation radiation to the pellet size range 0 to 1, with 1 being the maximum possible size. This mapping requires not only the numerical integration of the ablation radiation signal in real time, but also consideration of the potential impact of several pellet and plasma parameters.Data from this investigation indicate an empirical power function comprised of the relevant pellet and plasma parameters is capable of a tentative control approach. Moreover, these data can be useful for gaining deeper insight into the underlying physics of the pellet fueling process.