Photocathode quantum efficiency of ultrathin Cs2Te layers on Nb substrates

Zikri Yusof,Noah Samuelson,Linda Spentzouris,John Power,Javier Cardenas,Mark Warren,John Zasadzinski,Adam Denchfield

doi:10.1103/physrevaccelbeams.20.123401

Zikri Yusof, Noah Samuelson + Show 6 more

Open Access

https://doi.org/10.1103/physrevaccelbeams.20.123401

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Abstract

The quantum efficiencies (QE) of photocathodes consisting of bulk Nb substrates coated with thin films of Cs2Te are reported. Using the standard recipe for Cs2Te deposition developed for Mo substrates (220 {\AA} Te thickness), a QE ~11% - 13% at light wavelength of 248 nm is achieved for the Nb substrates, consistent with that found on Mo. Systematic reduction of the Te thickness for both Mo and Nb substrates reveals a surprisingly high residual QE ~ 6% for a Te layer as thin as 15 {\AA}. A phenomenological model based on the Spicer 3-Step model along with a solution of the Fresnel equations for reflectance, R, leads to a reasonable fit of the thickness dependence of QE and suggests that layers thinner than 15 {\AA} may still have a relatively high QE. Preliminary investigation suggests an increased operational lifetime as well. Such an ultra-thin, semiconducting Cs2Te layer may be expected to produce minimal ohmic losses for RF frequencies ~ 1 GHz. The result thus opens the door to the potential development of a Nb (or Nb3Sn) superconducting photocathode with relatively high QE and minimal RF impedance to be used in a superconducting radiofrequency (SRF) photoinjector.

Highlights

Future free-electron-laser (FEL)-based light sources will require low emittance, high brightness and high average-current electron beams, necessitating high duty cycle (>1 MHz) or effectively cw operation [1]
Using the standard recipe for Cs2Te deposition developed for Mo substrates (220 Å Te thickness), a quantum efficiencies (QE) ∼ 11%–13% at light wavelength of 248 nm is achieved for the Nb substrates, consistent with that found on Mo
The result opens the door to the potential development of a Nb superconducting photocathode with relatively high QE and minimal rf impedance to be used in a superconducting radiofrequency (SRF) photoinjector

Summary

INTRODUCTION

Future free-electron-laser (FEL)-based light sources will require low emittance, high brightness and high average-current electron beams, necessitating high duty cycle (>1 MHz) or effectively cw operation [1]. High peak current is more obtained with semiconductor cathodes such as cesium telluride (Cs2Te) [12,13] It has a QE as high as 20% and has consistently produced a QE > 1% during normal accelerator operations over a period of at least a year, providing a relatively large bunch charge per laser pulse, and has been shown to be. It has been used as an electron source in SRF photoinjectors, but only as a normal-state photocathode [14] This requires a more complex engineering design to isolate the cathode from the rest of the superconducting cavity. Preliminary tests indicate a possible increased operational lifetime These results are quite promising for the development of a hybrid superconducting photocathode with high QE and low rf losses. The elevated QEs that we measured for Te25 and Te15 were predominantly from the Cs2Te layer on the Nb

EXPERIMENT

DATA ANALYSIS

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DISCUSSION

SUMMARY