Light emission phenomena in superconducting niobium cavities

During the investigation of field emission limitations of superconducting niobium cavities, a CCD camera was inserted at the end of the beam pipe on a single-cell 1500 MHz cavity. When operating the cavity in field emission, glowing filaments of light were observed trapped by RF fields in closed-orbit trajectories. These filaments were traveling at frequencies much lower than the oscillating RF fields and formed various patterns of light for up to several seconds. This experiment was then repeated on a production CEBAF five-cell cavity with similar results. Events from both experiments were captured on video tape and are presented in this paper along with a discussion of the possible origin of these types of light patterns and the plans to further investigate the phenomena.