Multidirectional Observations of the High-Performance Plasmas by Visible Imaging Diagnostics on the HL-2M Tokamak

Three visible imaging diagnostics, including a tangential view, an extreme wide-angle view, and a downward-looking diagnostic, are installed on the HL-2M tokamak for high-performance plasmas. The entire field of view (FOV) covers up to 80% of the vacuum vessel. They aim at meeting the requirements of safe machine operation, plasma position identification, magnetohydrodynamic (MHD) instabilities study, and so on. The tangential and extreme wide-angle view diagnostics are based on the bilateral telecentric relay optics, while the downward-looking view diagnostic utilizes the imaging guide bundle with large format size, realizing the long optical paths. Plasma movement trends observed from the tangential view are checked with the magnetic measurements, and the optical and magnetic measurements for the plasma positions are consistent. Some filamentary structures (blobs) wandering at the extreme plasma edge at the low field side are also snapped by the high-speed camera. A highlight in this article is the field of the extreme wide-angle view, which reaches 112°. Without showing the distortion of conventional wide-angle lenses, nearly one-third of the vacuum vessel is in its eye. The evolution process of multifaceted asymmetric radiation from the edge (MARFE) phenomenon is recorded by the three views. It occurs as a bright belt on the inner side of the torus under the limiter configuration. The underlying cause of the MARFE is discussed.