Intrinsic Emittance Reduction of an Electron Beam from Metal Photocathodes

Abstract

Electron beams in modern linear accelerators are now becoming limited in brightness by the intrinsic emittance of the photocathode electron source. Therefore it becomes important for large scale facilities such as free electron lasers to reduce this fundamental limit. In this Letter we present measurements of the intrinsic emittance for different laser wavelength (from 261 to 282 nm) and for different photocathode materials such as Mo, Nb, Al, Cu. Values as low as 0.41±0.03 mm·mrad/mm laser spot size (rms) were measured for a copper photocathode illuminated with a 282 nm laser wavelength. The key element for emittance reduction is a uv laser system which allows adjustment of the laser photon energy to match the effective work function of the cathode material and to emit photoelectrons with a lower initial kinetic energy. The quantum efficiency over the explored wavelength range varies by less than a factor of 3.

Dielectric laser accelerators

“…From Eqn. 68 it is clear that the intrinsic normalized emittance can, in principle, approach zero if either the laser spot size approaches zero, or the laser photon energy equals the effective barrier height (Hauri et al, 2010). In the latter case, however, the quantum efficiency of the emission process can be shown to approach zero faster than ǫ n , hence this is not a useful approach.…”

Reduction of Intrinsic Electron Emittance from Photocathodes Using Ordered Crystalline Surfaces

“…From the same 3-step based photoemission model, it can be shown that the quantum efficiency (QE) is proportional to the square of the excess energy [12]. In the past decade, experimental studies of metal cathodes with normally incident light have validated this model both in terms of QE [12] and emittance [11,13,14]. A consequence of this model is that the QE depends on the 4th power of the emittance that is required.…”

Highlights on photocathodes based on thin films prepared by pulsed laser deposition

“…Nevertheless, the challenge to obtain future x-ray FELs requires further improvements of the quality of the electron beams. Many laboratories are interested in R&D on photocathodes to find new materials capable of producing bright electron beams for the next FEL sources [5][6][7][8][9][10]. A good cathode is characterized by high robustness, promptness of emission, high quantum efficiency (QE), and low thermal emittance.…”