Review of various approaches to address high currents in SRF electron linacs

The combination of high-brightness electron sources and high-current SRF Energy Recovery Linacs (ERL) leads to a new emerging technology: high-power, high-brightness electron beams. This technology enables extremely high average power free-electron lasers, a new generation of extreme brightness light sources, electron coolers of high-energy hadron storage rings, polarized electron–hadron colliders of very high luminosity, compact Thomson scattering X-ray sources, terahertz radiation generators and much more. What is typical for many of these applications is the need for very high current, defined here as over 100 mA average current, and high brightness, which is charge dependent, but needs to be in the emittance range of between submicron up to perhaps 50 μm, usually the lower – the better. Suffice it to say that while there are a number of projects aiming at this level of performance, none is anywhere near it. This work will review the problems associated with the achievement of such performance and the various approaches taken in a number of laboratories around the world to address the issues.