Abstract
Modern electron beams have demonstrated the brilliance needed to drive free electron lasers at x-ray wavelengths with major advances occurring since the invention of the photocathode gun and the realization of emittance compensation. These state-of-the-art electron beams are now becoming limited by the intrinsic thermal emittance of the cathode. In both dc and rf photocathode guns details of the cathode emission physics strongly influence the quantum efficiency and the thermal emittance. Therefore improving cathode performance is essential to increasing the brightness of beams. It is especially important to understand the fundamentals of cathode quantum efficiency and thermal emittance. This paper investigates the relationship between the quantum efficiency and the thermal emittance for metal cathodes using the Fermi-Dirac model for the electron distribution. We use a consistent theory to derive the quantum efficiency and thermal emittance, and compare our results to those of others.
Highlights
In this paper we use the three-step photoemission model of Puff [1] and Spicer [2,3] to obtain expressions for the quantum efficiency and the uncorrelated or thermal emittance of metal cathodes
We developed analytic expressions for the quantum efficiency and thermal emittance based on the three-step model ignoring electron-electron scattering [4]
We derive the thermal emittance for photoelectric emission by applying the same Fermi-Dirac model as used to obtain the quantum efficiency and closely follows that performed for Cs2Te cathodes by Floettmann [11]
Summary
In this paper we use the three-step photoemission model of Puff [1] and Spicer [2,3] to obtain expressions for the quantum efficiency and the uncorrelated or thermal emittance of metal cathodes. In a previous publication [5], we calculated the QE using this approach using the work function as the only free parameter In this model, the electron is emitted by means of three sequentially independent processes: (1) absorption of the photon with energy @!, (2) migration, including e-e scattering, to the surface, and (3) escape for electrons with kinematics above the barrier. The quantum efficiency is obtained from Fermi-Dirac statistics for fermions This is followed by a derivation of the thermal or photoelectric emittance. In the last two sections, the relationship between the cathode emittance and the quantum efficiency is discussed for metallic emitters, and the results are compared with previously published work
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callMore From: Physical Review Special Topics - Accelerators and Beams
Disclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
