Amorphous NEA Silicon Photocathodes-A Robust RF Gun Electron Source

Energy recovery linacs and other CW accelerators mandate the use of radio frequency (RF) photoinjectors to generate the low emittance, high current, high-peak-charge electrons used in collisions with hadron beams, and for electron cooling of hadron beams. However, RF guns using thermionic cathodes are not able to supply the high-phase-space densities required. Amorphous silicon shows great promise as a negative-electron-affinity (NEA), visible-wavelength photocathode suitable for RF gun systems. The advantages of amorphous silicon include a high degree of immunity to charged particle flux, low thermal emittance, bandgap tunability when grown as a germanium alloy, and low production cost. Phase I involved measurements on amorphous silicon to determine activation procedure, photoresponse, and neutral and charged particle interactions. Amorphous silicon surface preparation as well as in vacuo treatment was established to achieve negative electron affinity performance. Photoresponse was measured as a function of wavelength and lifetime. In Phase II, amorphous silicon photoemitter quality will be improved by using RF plasma-enhanced chemical vapor deposition (PECVD). Characteristics relevant to RF gun operation, including emission angle and current density, will be measured. Other features important for RF gun use, including substrate diffusion blocking, will be optimized. The increased quality will lead to improvements in electron diffusion length and thereby the yield. Commercial Applications and Other Benefits as described by the awardee: Amorphous, silicon-based RF photoinjectors should find use in energy recovery linacs, free electron lasers, and other applications that require the lowest possible source emittance. Reliable, low-cost, photocathode driven RF gun systems could become ready replacements for the diode and triode guns used on medical accelerators (typically S band 5-20 MeV) for the production of clinical photon beams and therapy electron beams.