MEASUREMENTS OF THERMAL EMITTANCE FOR CESIUM TELLURIDE PHOTOCATHODES AT PITZ

The thermal emittance determines the lower emittance limit and its measurement is of high importance to under- stand the ultimate injector performance. In this contribu- tion we present results of thermal emittance measurements under rf operation conditions for various Cs 2Te Te cath- odes and different accelerating gradients. Measurements of thermal emittance scaling with the cathode laser spot size are presented and analyzed. The significance of the applied electric rf field in the emittance formation process is discussed. measurements of the thermal emittance as well as to solu- tions for reducing it. Everywhere the authors point out the high importance of thermal emittance measurements to un- derstand and to improve the ultimate performance of injec- tors feedinglinac drivenFELs. Amongthe variouspossible photocathodematerials,the semiconductivealkalitelluride Cs2Te shows high quantum efficiency, high robustness and long lifetime and therefore has been chosen for PITZ as well as for many other photoinjectors. For first time ther- mal emittance measurements for Cs2Te photocathodes un- der real rf operating conditions were performed at PITZ and will be presented in the following sections. The ther- mal emittance depends (3) on the laser spot size, the mo- mentum and the angular distribution of the emitted pho- toelectrons. Its typical value is small, usually in the range 0.3-0.7mm·mrad and thereforeit starts to play a significant role in the emittance formation, when the injector operat- ing parameters have been fully optimized so that very low emittances of the order of 1mm·mrad are about to be pro- duced. It is important to note that the thermal emittance is a complex quantity influenced not only by the photocath- ode material properties but also by the parameters of the cathode UV laser and the accelerating field amplitude. In planning the measurements one has to consider the depen- dence on the laser spot rms size, on lowering the surface potential barrier due to the high rf field (Schottky effect) and on poisoning of the cathode due to increased vacuum pressure.