Dark-lifetime degradation of GaAs photo-cathode at higher temperature

ERL is considered to be one of the future light source, which is operated in a high average current up to 100 mA with an extremely low emittance down to 0.1 mm-mrad. It is also applicable to generate short electron bunch down to 100 fs 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, 15 W 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 60 K for 5 mm 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 1.20 eV. 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.