Te PHOTOCATHODES ROBUSTNESS STUDIES *

Cs2Te photocathodes are used as laser driven electron sources at FLASH and PITZ. Besides many aspects of their performances, their robustness to gas exposition and the effect of pollutants on photocathode properties, and indirectly on the photoemitted electrons, are a field still unexplored. In this article we present the results of controlled exposition of Cs2Te photocathodes to gases typical present in the UHV environment of an RF gun with respect to spectral responses (QE vs. wavelength), and QE uniformity. Moreover, a comparison between polluted cathodes and fresh ones during operation in an RF Gun is presented. INTRODUCTION Since the ‘90s INFN Milano-LASA photocathode laboratory is involved in the study of the photoemissive cathodes based on alkali antimonide and telluride. Since 1998, LASA produces photocathodes for the photoinjectors at FLASH and PITZ at DESY and for the photoinjector test facility A0 at FNAL. In these years, we have studied the photoemissive properties of photocathodes with the main interest for Cs2Te films. A variety of techniques have been used such as spectroscopy techniques (Time-Of-Flight angle resolved spectrometer) and optical measurements. Moreover, we have collected a large amount of information related to cathode behaviors under operational conditions in an RF gun environment, with diagnostic on cathodes during and after their usage. Up to now, we have produced 107 cathodes, 75 with a Cs2Te film [1]. The lifetime of cathodes used at FLASH, under standard operation, varies between one and six months, depending on the vacuum conditions. However, we have observed at PITZ that for cathodes operated at higher gradients approaching 60 MV/m, their lifetime reduces to a few days only [2]. In this paper we present measurements performed at LASA on pollution of cathodes. We have investigated the robustness of Cs2Te cathodes in vacuum condition similar to the one present in the RF gun and in the case of an accidental vacuum leak. Moreover we have polluted a cathode with Oxygen to change its photoemissive properties, in order to lower its thermal emittance. CATHODE ROBUSTNESS One significant characteristic of a photocathode is its robustness that contributes to its operative lifetime. The main causes of degradation of the photoemissive film are: • Vacuum condition (in the gun, in the transport box and in the transfer devices) • Degradation of the thin film (long operative time, sparks, high gradient, dark current, etc.). The degradation of the surface during operation is typically related to sparks caused by high applied field in the RF guns (PITZ) and also to long time operation (damages on the coating, holes, etc.). Vacuum is another key parameter that influences the cathode operating lifetime. We typically observe that the QE of cathodes stored in the transport boxes is stable for very long time, since the base pressure is kept at 10 mbar. However, bad vacuum conditions age cathodes. As an example, cathodes in operation at FLASH from April ’06 to March ’07, where the vacuum in the RF gun has been poor, showed an operative lifetime of 30 days only [3]. Improvements of the vacuum condition done during the shutdown resulted in an increase of the operative lifetimes to standard values [4]. Cathode Pollution In an attempt to simulate similar conditions as in an RF gun, we have experimentally measured the change of QE of a cathode polluted by switching off the ion pump. We grew a standard cathode, following the usual recipe [5]. After the deposition, we measured the spectral response of the film, repeating the measurement also in the following days. The analysis of the spectral responses gives the low and high energy threshold (Eg+Ea) of the film [6]. Furthermore, we measured the QE map at 254 nm. Table 1 reports the main parameters of this cathode. During the pollution process, we have monitored the photocurrent emitted from the cathode, illuminated by UV light (λ = 254 nm, P = 180 nW, Φ = 2 mm). To minimize rejuvenation effects, the light has been modulated by a shutter with an open/close ratio of 5:60 s. The total pressure in the chamber has been recorded with a BayardAlpert (BA) gauge, while a Residual Gas Analyzer (RGA) recorded the evolution of the gases in the system. The measurement lasts for about two days with the ion pump switched off for about 24 h. We observed an unexpected increase of the photocurrent. The starting pressure was 2.210 mbar. After about 3 hours, the total pressure reached its maximum at 2.9 10 mbar. The main residual gas was found to be CH4 (in the 10 mbar range). Other relevant contributions are from H2 and CO (a few 10 mbar). The water vapour pressure was below 10 mbar. This is expected since we regularly bake out the vacuum chambers. These gases have a low influence on the cathode properties: more reactive gases for Cs2Te are O2 and CO2 [7] that in our case have partial pressures ___________________________________________ * Work partially supported by the European Community, contract numbers RII3-CT-2004-506008. #laura.monaco@mi.infn.it Proceedings of EPAC08, Genoa, Italy MOPC075 02 Synchrotron Light Sources and FELs T02 Lepton Sources