Increase of intrinsic emittance induced by multiphoton photoemission from copper cathodes illuminated by femtosecond laser pulses

Chenjie An,Dapeng Yu,Jun Xu,Jiasen Zhang,Rui Zhu,Xiaopeng Hu,Yaqi Liu

doi:10.1063/1.5029387

Abstract

Electron sources driven by femtosecond laser have important applications in many aspects, and the research about the intrinsic emittance is becoming more and more crucial. The intrinsic emittance of polycrystalline copper cathode, which was illuminated by femtosecond pulses (FWHM of the pulse duration was about 100 fs) with photon energies above and below the work function, was measured with an extremely low bunch charge (single-electron pulses) based on free expansion method. A minimum emittance was obtained at the photon energy very close to the effective work function of the cathode. When the photon energy decreased below the effective work function, emittance increased rather than decreased or flattened out to a constant. By investigating the dependence of photocurrent density on the incident laser intensity, we found the emission excited by pulsed photons with sub-work-function energies contained two-photon photoemission. In addition, the portion of two-photon photoemission current increased with the reduction of photon energy. We attributed the increase of emittance to the effect of two-photon photoemission. This work shows that conventional method of reducing the photon energy of excited light source to approach the room temperature limit of the intrinsic emittance may be infeasible for femtosecond laser. There would be an optimized photon energy value near the work function to obtain the lowest emittance for pulsed laser pumped photocathode.

Highlights

Increase of intrinsic emittance induced by multiphoton photoemission from copper cathodes illuminated by femtosecond laser pulses
Metallic photocathodes are used for high brightness electron beams with applications such as the free electron laser (FEL)[1] and ultrafast electron diffraction (UED)[2,3] due to their prompt temporal response and robustness to poor vacuum conditions
Where σx is the RMS beam size, which can be replaced by the laser spot size on the cathode for the intrinsic emittance, px is the transverse momentum of the electrons in x direction, me and c are the electron rest mass and the speed of light, respectively

Summary

Introduction

Increase of intrinsic emittance induced by multiphoton photoemission from copper cathodes illuminated by femtosecond laser pulses. With an isotropic assumption at 0 K, the RMS transverse momentum is determined by the incident photon energy ω and the effective work function of the photocathode φeff,[7] This letter reports the measurement of intrinsic emittance of a copper cathode illuminated by femtosecond laser pulses at photon energies above and below the work function of the cathode.

Results

Conclusion