Anomaly Detection of Radiative Collapse in Impurity Seeding Experiment Using Imaging Bolometer and Autoencoder

Radiative collapse in neon (Ne) seeded plasmas has been detected using a two-dimensional radiation measurement (InfraRed imaging Video Bolometer, IRVB) and an AutoEncoder (AE) on the Large Helical Device (LHD). In divertor detachment for fusion reactors, while the divertor heat load is mitigated using impurity injection, excessive impurity injection induces radiative collapse. Therefore, it is important to detect the precursor of the radiative collapse for the control of the impurity injection amount to maintain stable divertor detachment. In this study, 1219 images measured with the IRVB were used for training. By using the AE model, in the discharges with moderate Ne pulses, radiative collapse was successfully detected as an increase of abnormality earlier than the Ne pulse just before the radiative collapse. Moreover, by investigating the anomalous radiation structure, it was found to remain at the edge plasma region when radiation increased without radiative collapse, whereas the anomalous radiation structure progressed toward the core plasma when radiative collapse occurred.