Experimental investigation of thermal emittance components of copper photocathode

Abstract

With progress of photoinjector technology, thermal emittance has become the primary limitation of electron beam brightness. Extensive efforts have been devoted to study thermal emittance, but experiment results differ between research groups and few can be well interpreted. Besides the ambiguity of photoemission mechanism, variations of cathode surface conditions during cathode preparation, such as work function, field enhancement factor, and surface roughness, will cause thermal emittance differences. In this paper, we report an experimental study of electric field dependence of copper cathode quantum efficiency (QE) and thermal emittance in a radio frequency (rf) gun, through which in situ cathode surface parameters and thermal emittance contributions from photon energy, Schottky effect, and surface roughness are extracted. It is found the QE of a copper cathode illuminated by a 266 nm UV laser increased substantially to 1:5 Â 10 À4 after cathode cleaning during rf conditioning, and a copper work function of 4.16 eV, which is much lower than nominal value (4.65 eV), was measured. Experimental results also show a thermal emittance growth as much as 0:92 mm mrad=mm at 50 MV=m due to the cathode surface roughness effect, which is consistent with cathode surface morphology measurements.

Nanometer emittance ultralow charge beams from rf photoinjectors

“…Particularly, the measured emittance numbers converge toward a same thermal emittance value at low charge. For both the long and short laser cases, the laser spot sizes on the cathode were 30 m rms and the measurement indicates a thermal emittance level of 0.8 mm mrad/mm rms which is consistent (within 20%) with the results from other groups [10,26,27]. We would like to emphasize that the emittance values reported in Fig.…”

Cold electron beams from cryocooled, alkali antimonide photocathodes

“…Including the Schottky work function lowering scaled by the field enhancement coefficient obtained from QE measurement in the photoinjector gun electric field dependence is expected on MTEs and values as large as ∼115 meV should be observed at 300 K and at 3.4 MV=m. In addition, it is known that the electron beam intrinsic emittance can be degraded by the cathode surface roughness [21][22][23]. This whole picture looks in contrast with our smaller measured MTEs indicating that a model simply scaled on electrons excess energy cannot explain our experimental result.…”

“…We believe this discrepancy is due to an emittance growth caused by photocathode surface roughness. The scientific community is attempting to address the question of how the roughness affects the intrinsic emittance of a photocathode surface and several models have been proposed in recent years to describe this phenomenon [21][22][23]. Some of these models indicate that the intrinsic emittance contributions due to the surface roughness can be due to a simply geometrical effect due to the relative orientation of local surfaces from where the electron emission takes place and to an increase of transverse momentum due to transverse electric field components induced by surface roughness.…”

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].…”

“…Qian et al [32] consider a variation of the 2D sinusoidal surface roughness model (the analysis was expanded upon by Dowell et al [41]). They argue that for photoemission, the planar emittance should be augmented by a roughness term according to ε 2 total ¼ ε 2 smooth þ ε 2 rough , where ε smooth is the planar (Dowell-Schmerge) formula of Eq.…”