Photoemission studies on niobium for superconducting photoinjectors

Abstract

This work presents an investigation of the photoemission properties of niobium. The quantum efficiency (QE) of niobium cathodes was measured for a variety of surface preparations relevant to the operation of a superconducting injector. The dependence of the QE on wavelength, applied field, and laser cleaning energy was determined. The three-step model of photoemission was adapted to fit the observed emission behavior. QE values of 6.5×10−5 for 266 nm, 1.0×10−4 for 248 nm, and 2.8×10−4 for 193 nm were observed with a bias field of 1MV∕m.

Emittance, surface structure, and electron emission

“…The importance of modeling the cathode emission physics and electron dynamics to understanding the interplay between emittance and space charge has therefore been advocated for cathode research and development [22], particularly in simulation codes [26]. The intrinsic emittance of the photocathodes are emerging as the primary limitation to realizing the short wavelengths and energy recovering linac sources of x rays and makes describing the causes of cathode emittance important [22,27], with surface roughness emerging as a probable contributor [28][29][30][31][32].…”

Lead photocathodes

“…The niobium data is included for Ref. [9]. Figure 10 shows a comparison between the predicted and measured QE for the arc-deposited lead cathode.…”

“…The niobium that comprises the cavity walls is the obvious first choice, and a cavity utilizing a niobium cathode has been built and tested at BNL [8]. The low quantum efficiency (QE) measured for Nb [9] does not lend itself to producing >1 mA current with existing laser systems. Lead has been suggested as a possible alternative photocathode [1,10,11].…”

Structural and morphological properties of metallic thin films grown by pulsed laser deposition for photocathode application

“…However, their relatively high work function and low quantum efficiency (QE) urge the search of alternative substances that may substitute them as the photoemitting material [5,6]. Among the alternative candidates, yttrium (Y) and lead (Pb) seem to be the more adequate choices due to their lower work functions and superior quantum efficiencies, with respect to Cu and Nb, respectively [6].…”

“…The radiofrequency (RF) and superconducting radiofrequency (SRF) cavities are usually made of copper (Cu) and niobium (Nb), respectively [4,5]. However, their relatively high work function and low quantum efficiency (QE) urge the search of alternative substances that may substitute them as the photoemitting material [5,6].…”