Study of luminous phenomena observed on contaminated metallic surfaces submitted to high RF fields

The primary limitation to the increase of the attainable accelerating gradient in superconducting cavities is the strong field emission (FE) of electrons from the RF surface. Many studies devoted to the DC field emission have shown that the electron emission on a broad metallic surface stems from a few very localized sources, called emitters. Superficial dust particles (metallic or dielectric) or semiconducting impurities embedded in the surface can originate the emission. Different experimental results have led to several theoretical models explaining the physical mechanisms of the field emission. We have investigated the RF field emission from a sample subjected to high RF fields (Epeak > 40 MV/m) in a copper cavity. Our study is focused on the luminous emissions occurring on the RF surface simultaneously with the electron emission. The optical apparatus attached to the cavity permits one to observe the evolution of the emitters and the direct effect of the surface conditioning. Also, the parameters of the emitted radiation (intensity, glowing duration, spectral distribution) may provide additional information on the field emission phenomena. This paper relates some results concerning samples intentionally contaminated with particles (metallic or dielectric).