Charles K. Sinclair

Jefferson Laboratory

Newport News, VA

Photoemission offers a number of very significant advantages to the designer of electron sources for accelerator, lithography, or surface science applications. The beams are produced from a cold cathode, avoiding any problems from possible thermal or chemical effects of hot cathodes, and eliminating the need for any electronics at cathode potential. Spatial or temporal modulation is practical to very high frequencies. The cold cathode produces an inherently bright beam. And, if an appropriate semiconductor is chosen as the photocathode material, it is possible to produce highly spin-polarized electron beams.

These benefits do not come without associated costs. The beam current produced is directly proportional to the product of the quantum efficiency of the cathode and the optical power illuminating it. In many applications, this precludes the use of metallic cathodes, which have very low quantum efficiency and provide useful yield only with extreme ultraviolet illumination. Modulation of the electron beam typically requires a modulated laser to illuminate the photocathode, and such lasers can easily be quite sophisticated and expensive. Polarized electrons are produced only by photoemission near the semiconductor bandgap, which is typically in the near infrared, where high quantum efficiency can be difficult to achieve. The highest electron polarization is produced only in exceptionally thin cathodes, which have low quantum efficiency. Finally, to obtain reasonable operational lifetimes for high quantum efficiency photoemission cathodes operated in actively pumped vacuum systems, excellent vacuum conditions are required.

The quantum efficiency of photoemission cathodes operating in the visible or near infrared may be reduced either by chemical poisoning, or by ion back bombardment when an electron beam is present. The residual gas responsible for these effects may be present in the electron source under static conditions, or produced by electrons striking interior surfaces of the vacuum system. These electrons in turn arise from either field emission from the electrode structures, or from losses of beam electrons. In many cases it is possible, by mapping the cathode quantum efficiency, to determine whether chemical poisoning or ion back bombardment is the source of cathode degradation.

At Jefferson Laboratory, we operate two electron accelerators. One delivers highly polarized high energy electron beams of up to 200 mA average current for basic research in nuclear physics. The other provides a high brightness beam of 5 mA average current to drive a very high average power free electron laser. The beams for both of these accelerators are provided from negative electron affinity GaAs photocathodes. Both of these electron sources have been developed to the point where quantum efficiency degradation by ion back bombardment is the principal operational lifetime limitation.

In this paper, we will give a brief description of the processes we use to produce our negative electron affinity photocathodes. The vacuum systems in which these cathodes are operated will be described. Evidence of ion back bombardment damage will be presented. Improvements to the electron gun and beam line vacuum systems have markedly improved the operational lifetime of these cathodes. Presently, our best guns show photocathode 1/e lifetimes in excess of 50,000 coulombs/cm². Further pressure reductions, into the XHV regime, are necessary to meet the requirements of proposed electron sources for future applications.