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The GaAs photo-cathode is a unique device which is able to generate highly polarized electron beam up to 90% and an extremely small emittance beam. The photo-electron emission is possible with IR (Infrared Red) and visible light and the quantum efficiency can be more than 20%. These unique features depend on the negative electron affinity (NEA) surface made by adsorption of Cs and O 2 /NF 3 , but this surface is easily damaged by residual gas adsorption, ion back-bombardment, etc. The exact structure of the NEA surface is not known, even there are several hypotheses. In this study, we performed the NEA activation on a cleaned GaAs surface with CO 2 , CO, N 2 , and O 2 gases and compared the results to improve our understanding on the NEA surface. We found that the NEA activation with CO 2 is similar to that with O 2 , but the maximum QE value is significantly lower than that with O 2 . We also found that N 2 and CO gases did not contribute to NEA activation at all. By analyzing CO 2 activation, we found that atomic oxygen activates the NEA surface and CO degrades the NEA surface simultaneously. We found that the NEA activation ability of atomic oxygen is almost a half of that of O 2 molecule. [ABSTRACT FROM AUTHOR]

Abstract: ERL is considered to be one of the future light source, which is operated in a high average current up to 100mA with an extremely low emittance down to 0.1mm-mrad. It is also applicable to generate short electron bunch down to 100fs with a bunch compression. Ultra-short photon pulse can be generated by the inverse laser Compton scattering or FEL with the short pulse from ERL. In ERL, one of the most critical device is the electron gun. The electron beam is generated by GaAs photo-cathode driven by a high-average power laser. Enough quantum efficiency and a reasonably long operational lifetime are required. The Negative Electron Affinity (NEA) surface, which is generated by artificial treatments on GaAs crystal, has an important roll to generate the high brightness electron beam. In a real operation, 15W laser is illuminated on the GaAs cathode in a small spot size (typically a few mm) and the cathode temperature is increased by this drive laser. The operational lifetime of GaAs cathode is an important issue in such tough condition. In this article, we examine GaAs lifetime as a function of temperature to evaluate effects of heating. In our estimation, temperature rise is 60K for 5mm laser radius. We observed a significant degradation of the lifetime by heating. By analyzing the lifetime as desorption process, the binding energy of “NEA molecule” on the GaAs surface is found to be 1.20eV. According to the result, the lifetime degradation by heating is one of the most critical problem on the ERL gun and should be solved to develop a robust cathode, otherwise we should consider an active cooling of the cathode. [Copyright &y& Elsevier]

Abstract: A laser-driven RF gun with a Cs2Te photocathode, a device modeled after the BNL-IV RF gun, has been in development at KEK since 2002. This gun has been operated as an electron source for the Accelerator Test Facility (ATF) and generates a beam with an operational intensity of up to 3.2nC per bunch. Based on successful results at the ATF, this gun has been applied several times for new research at universities. From 2002 onwards, successive improvements have been incorporated into newer models of the RF gun. Most recently, in 2008, a new gun incorporating all of the earlier modifications was produced for the ATF. Tests have confirmed a significant improvement of the Q-value of the latest model. A typical transverse emittance of 1.3 π mmmrad has been obtained under the following conditions: solenoid field of 0.18T, beam intensity of 1.6nC per bunch, and RF power of 9MW. This RF gun equipped with the Cs2Te photocathode is being used for the advanced research programs at the ATF. A survey of its performance has recently been completed. [Copyright &y& Elsevier]