High-brightness electron guns for linac-based light sources

Abstract

Most proposed linac-based light sources, such as single-pass free-electron lasers and energy-recovery-linacs, require very high-brightness electron beams in order to achieve their design performance. These beam requirements must be achieved not on an occasional basis, but rather must be met by every bunch produced by the source over extended periods of time. It is widely assumed that the beam source will be a photocathode electron gun; the selection of accelerator technique (e.g., dc or rf) for the gun is more dependent on the application.The current state of the art of electron beam production is adequate but not ideal for the first generation of linac-based light sources, such as the Linac Coherent Light Source [1] (LCLS) x-ray free-electron laser (X-FEL). For the next generation of linac-based light sources, an order of magnitude reduction in the transverse electron beam emittance is required to significantly reduce the cost of the facility. This is beyond the present state of the art, given the other beam properties that must be maintained. The requirements for current and future linac-based light source beam sources are presented here, along with a review of the present state of the art. A discussion of potential paths towards meeting future needs is presented at the conclusion.

High-Brightness Injector Modeling

“…For a linac-based light source intended mainly to increase peak brightness over existing third-generation synchrotrons, greater bunch compression (e.g., higher peak current) improves radiation beam brightness faster than reducing the transverse emittance. For an X-FEL injector, taking advantage of emittances lower than about 0.1 µm would require fundamental improvements in undulator technology [4].…”

Modeling the resupply, diffusion, and evaporation of cesium on the surface of controlled porosity dispenser photocathodes

“…The theoretical evaporation rates of sub-monolayer surface coverage of Cs compare well to the data of Taylor Laser switched photocathodes have long been advocated as a low emittance, high performance electron source for rf photoinjectors used by Free Electron Lasers (FELs) 1-3 and next generation light sources. 4,5 In particular, following the introduction of photocathodes into FELs in 1985 at Los Alamos (LANL), 6 the demonstrated performance of a photoinjector in 1993 using a CsK 2 Sb photocathode by Dowell et al 7 stimulated interest in FELs as a tunable light source for Directed Energy applications, 8 even though multi-alkali photocathodes degrade due to vacuum quality, surface conditions, contamination, and ion back bombardment, all of which limit the cathode lifetime. 5,7,[9][10][11][12] An rf photoinjector typically chooses between high quantum efficiency (QE) (ratio of emitted electrons to absorbed photons) or long lifetime (reduction by 1=e of initial value) because both are generally not available simultaneously.…”

Future Directions in Electron Sources

“…Demands on the injectors of linac-based x-ray light sources will focus on reliability as well as on improving beam brightness. Generally speaking, the single-bunch parameters for both incoherent and coherent linac-based x-ray light sources are quite similar [5]; injectors will have additional demands based on required average beam currents. For instance, incoherent linac-based light sources intended to replace storage ring sources typically require 10 -100 mA average beam current [6].…”