Beam Photography: A technique for Imaging Dark Currents

Dark currents are unwanted electron currents that stem from surface electrons that are emitted in rf cavities and accelerated in their electric field. This paper describes a novel technique to produce a 2D-image of dark currents and presents some results. Ordinary black and white photographic paper has been exposed to dark current electrons stemming from an 805Mhz cavity in the Lab G facility at Fermilab. This cavity is a closed-cell cavity in a 2.5 T solenoidal field parallel to the electrical field. Thin rf and vacuum windows allow the electrons to exit the cavity. Due to the focusing effect of the magnetic field, a sharp picture of the sources of the dark currents is obtained. Single emitters, surrounded by regions of virtually no dark current, can be clearly identified.