Photocathodes for RF photoinjectors

Over the past ten years photocathodes have been extensively used as high-brightness electron sources in RF guns. In this paper, I present a general review of the alkali-based high quantum efficiency (QE) photoemitters (e.g. Cs3Sb, K2CsSb and Cs2Te), together with a comparative analysis of the different preparation procedures and the results obtained, both in the preparation chambers and in RF guns. The need to increase the photocathode reliability has provided the impetus to get an R&D activity to go over the alchemy of photocathode preparation procedure. In this paper, I will discuss the results so far obtained in different laboratories, both by using traditional investigation strategy (e.g. QE and RF behavior) and by means of surface science techniques as Auger Electron Spectroscopy (AES) and X-ray Photoelectron Spectroscopy (XPS). Alkali antimonides have been used at first in the RF gun due to the high QE response to the green light of the Nd:YLF second-harmonic radiation. Measurements have confirmed the high reactivity of the alkali antimonide photocathodes to the residual gases: this fact makes their use in RF guns not practical, mainly for short lifetimes. Further investigations have shown that the choice of the substratum preparation procedure and chemical composition plays a fundamental role in the photocathode performance, both from the point of view of the QE and the operative lifetime and ruggedness to gas exposition. Cesium telluride (Cs2Te) prepared on a molybdenum substratum seems to be, nowadays, the best compromise, in terms of preparation procedure reliability and ruggedness, that now the characteristics and drawbacks of this material are well understood (e.g. the need of an UV laser source). Future possible developments will be discussed. In particular, the measurement and the control of the thermal emittance and the time response could be an important task

Photocathodes: the state of the art and some news

Abstract The present cesium telluride cathodes have the capability to provide, for months, trains of picosecond pulses carrying a charge equal/higher than 10 nanocoulomb, for a total charge higher than the microcoulomb. However, they are very delicate. A possible way to increase their robustness is by covering them with a protective film of nanostructured carbon. Ferroelectric ceramics, as possible new robust photocathodes, showed an interesting level of emission with 532 nm–25 ps laser pulses

THEORETICAL AND EXPERIMENTAL EVALUATION OF THE WINDOWLESS INTERFACE FOR THE TRASCO-ADS PROJECT

TRASCO-ADS is a national funded program in which INFN, ENEA, and Italian industries work on the design of an accelerator driven subcritical system for nuclear waste transmutation. TRASCO is the Italian acronym for Transmutation (TRAsmutazione) of Waste (SCOrie). One of the most critical aspects in the design of an Accelerator Driven System is related to the interface region, which is the part of the beamline located between the accelerator, operating under UHV conditions, and the pressurized reactor vessel, consisting of a contained plenum of Pb-Bi eutectic (LBE). A so-called window could separate these two environments, but thermomechanical considerations and radioprotection issues point out that this component could be critical. In the windowless interface, no window is located between the linac and the spallation target. Only a suitable pumping and trapping system, for the gases and the vapors outcoming from LBE, divides the UHV accelerator and the spallation target vacuum. Vacuum gas dynamics theoretical considerations and calculations are presented in this article. The need for a validation of the theoretical models gave the motivation for an experimental work, whose results are also discussed. Scale-up of the experimental setup to the full system needs accurate analyses for a proper dimensioning of the system in the interface region

Performance analysis of the European X-ray Free Electron Laser 3.9 GHz superconducting cavities

The limits of performance of the European XFEL 3.9 GHz superconducting cavities were investigated. Most cavities exhibited high field Q slope, reaching the breakdown field at approximately $22\text{ }\text{ }\mathrm{MV}/\mathrm{m}$. We hypothesize that this limit is a feature of high frequency cavities and can be explained by a thermal model incorporating field dependent surface resistance. The results obtained from simulations were in good agreement with experimental data obtained at 2 K