Study on Improving Durability of Cs-Te Photocathode for an RF-gun

At Waseda University, we have been studying for high quality electron beam generation using 1.6 cell Cs-Te photocathode rf-gun. We use photocathode as the electron source, which can generate highquality electron beam such as low emittance, and short bunch. The performance of photocathode is evaluated mainly in terms of quantum efficiency (Q.E.) and lifetime. Cs-Te photocathode used in the rf-gun is known for high Q.E. about 10% with UV light and relatively longer lifetime among semiconductor photocathodes. Since it is a hard environment for photocathode inside the gun, it is necessary to replace the photocathode every several months. In other words, in order to achieve longterm operation of rf-gun, it is necessary to find highly durable photocathode recipe. It has been reported that the CsTe photocathode by co-evaporation can produce a photocathode having a longer lifetime as compared with the sequential evaporation. Moreover, we have done studies to improve lifetime and durability of Cs-Te photocathode by coating the cathode surface with CsBr thin film. In this conference, we report the evaluation results of Cs-Te photocathode by co-evaporation, CsBr coating and future prospects. INTRODUCTION In recent years, the demand for high brightness electron beam is rising for practical applications such as the free electron laser (FEL) and the energy recovery linac (ERL)[1-5]. Photocathodes are useful electron sources for generating high brightness and low-emittance beam. Semiconductor photocathodes, which have higher Q.E. compared with metal photocathodes, [6, 7] are especially expected to be the candidates for an electron source that requires high brightness and high current beam. However, there is an issue that semiconductor photocathodes generally have a shorter lifetime and require ultra-high vacuum or an even better vacuum environment [8]. Though, photocathodes are used in both DC guns and rfguns [9], the requirements of characteristics for the photocathodes are different in each electron gun. At present, we are using the photocathode rf-gun for the generation of high brightness electron beam. In the rf-gun cavity, it is difficult to achieve both extreme high vacuum and high efficiency (i.e. high Q.E.). Deterioration of vacuum degree inside the cavity due to Joule heat and discharge is to be caused by the rf electric field. The rf electric field itself directly causes deterioration of the cathode. Cs-Te photocathode, which has relatively long lifetime, is often used in rfguns [10,11]. In our case, it is necessary to exchange the photocathode every several months due to decrease of Q.E[12]. The purpose of this research is to make the Cs-Te photocathode withstand longer-term rf-gun operation even under the above-mentioned environmental conditions. It has been reported that Cs-Te photocathode by co-evaporation tends to have a longer lifetime as compared to that by sequential evaporation process [13]. At Waseda University, we have been studying an S-band rf electron gun [14,15] and the application of high quality electron beams to laser Compton scattering experiments [16], pulse radiolysis [17] and so on [18]. We usually use Cs-Te photocathode as an electron source and maintain high Q.E. by exchanging the cathode every six months. Thus, we have been investigating the effectiveness of CsTe photocathode by co-evaporation in order to reduce the number of the cathode exchange. EXPERIMENTAL DETAILS