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 $1.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ 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 $0.92\text{ }\text{ }\mathrm{mm}\text{ }\mathrm{mrad}/\mathrm{mm}$ at $50\text{ }\text{ }\mathrm{MV}/\mathrm{m}$ due to the cathode surface roughness effect, which is consistent with cathode surface morphology measurements.
Highlights
Photoinjector technology has witnessed enormous improvements during the past decade [1,2,3,4,5,6] and its sensational beam brightness has enabled the success of linac coherent light sources (LCLS) [7], the first hard x-ray free electron laser, and many other light sources [8,9]
While the average beam brightness relies on high average power laser system and high quantum efficiency (QE) photocathodes [12], the peak beam brightness is limited by the rf gun acceleration gradient and thermal emittance [13]
The acceleration gradient is boosted by moving to higher frequencies, such as X-band and C-band [14,15], and a gradient of 200 MV=m has been realized in an X-band rf gun [14], which is predicted by simulation to deliver an electron beam with beam brightness a factor of $5 higher than that from the LCLS photoinjector [16]
Summary
Photoinjector technology has witnessed enormous improvements during the past decade [1,2,3,4,5,6] and its sensational beam brightness has enabled the success of linac coherent light sources (LCLS) [7], the first hard x-ray free electron laser, and many other light sources [8,9]. Volume photoemission thermal emittance depends on photon energy, cathode work function, Schottky effect, and surface roughness [22,23,24,25,26], and it was proposed to minimize the thermal emittance by matching the photon energy with the effective work function at the sacrifice of QE [27,28]. Electric field dependence of QE and thermal emittance of copper are measured in a photocathode rf gun, and thermal emittances contributed by the gap between photon energy and work function, Schottky effect, and surface roughness are numerically fitted, which in turn extract work function and field enhancement factor.
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