Beam photography: a technique for imaging dark currents

Dark currents are unwanted electron currents that stem from surface electrons 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 was exposed to dark current electrons coming out of an 805 MHz cavity in the Lab G facility at Fermilab. This is a copper pillbox cavity in a 2.5 T solenoidal field parallel to the electric 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 dark currents sources is obtained. Single emitters, surrounded by regions of virtually no dark current, can be clearly identified.