Review of contributions of image observations using visible cameras to advancements in sustaining long-pulse discharges in LHD

The contributions of image observations to the advancement of Ion Cyclotron Range of Frequencies (ICRF)-heated long-pulse discharges in the Large Helical Device (LHD) are reviewed. The plasma discharges were monitored using over 25 visible cameras, three fast-framing cameras, and various advanced plasma diagnostics, which revealed that most long-pulse discharges were interrupted by the following four events: termination of ICRF plasma heating due to arcing events in antennas, uncontrollable plasma density rise by outgassing from divertor plates, iron influx from plasma-facing components in the vacuum vessel, and carbon influx originating from the divertor regions. Image observations played a crucial role in mitigating the above four events that restricted the duration of long-pulse discharges by implementing appropriate countermeasures such as enhancing the cooling efficiency of the divertor plates, adopting new operational techniques to disperse the heat-load distribution, improving the ICRF antenna configurations, and modifying the divertor configuration. Interruptions in long-pulse discharges were statistically analyzed using experimental data in three previous experimental campaigns, demonstrating a history of continuous efforts to extend the plasma discharge duration in LHD. The presentation highlights the contributions of image observations over the past two decades, which have revealed inherent limitations in conventional magnetic plasma confinement devices that utilize carbon and iron plasma-facing components in steady-state plasma discharge operations, providing valuable insights for optimizing next-generation plasma confinement devices.