Kelvin probe studies of cesium telluride photocathode for AWA photoinjector

Cesium telluride is an important photocathode as an electron source for particle accelerators. It has a relatively high quantum efficiency (QE) (> 1%) ,i s sufficiently robust in a photoinjector, and a long lifetime. This photocathode is grown in-house for the new Argonne Wakefield Accelerator (AWA) to produce high charge per bunch (50 nC). We present a study of the work function of a cesium telluride photocathode using the Kelvin Probe technique. The study includes an investigation of the correlation between QE and work function, the effect of photocathode aging, the surprising effect of UV exposure on the work function, and the puzzling behavior of the work function during and after photocathode rejuvenation via heating. A proven high-charge electron source for particle accelerators, Cesium telluride (Cs2Te) photocathodes have many distinguishing characteristics including high quantum efficiency (QE) (10% at 4.9 eV photon energy), long lifetime (months) and survival in a high gradient environment [1]. The RF photocathode drive gun at the Argonne Wakefield Accelerator (AWA) is a new high peak-current electron beam source for the new 75 MeV linear electron accelerator. Primary application will be to create wakefields in dielectric-loaded accelerating (DLA) structures and other novel structures [2]. AWA’s experimental program requires the ability to produce high-charge bunches, often in long bunch trains. This calls for a high QE photocathode such as Cs2Te. The AWA fabricates Cs2Te photocathodes for use in the new high-charge, 1.3 GHz photoinjector [3]. Electron bunch train of 30 bunches with up to 50 nC per bunch is planned. A thorough understanding of the photocathode is required because of the high performance demand. The QE at a particular photon energy and the work function (φ) are two key parameters of electron emission. We present some results of Kelvin probe measurements of φ on Cs2Te photocathodes [4]. We examined the correlation between the changes in QE and the work function; how QE and the work function evolved with photocathode aging; effects of rejuvenation of the photocathode via heating, and effects on the work function upon exposure to UV light. The Kelvin probe method is a non-contact, nondestructive technique used to measure work function. The theory and details of the method have been described in de