Abstract
An electron beam's quality is fundamentally limited by its attributes at the cathode. The emission from photocathodes can be bright, but not necessarily uniform. Quantum efficiency (QE) maps generated by selectively illuminating the cathode surface reveal this nonuniformity. In this paper a proof-of-principle experiment is described in which a high resolution map of the QE is generated using a digital micromirror device. We show a substantial improvement over the best results reported for laser raster scanning.
Highlights
Photocathodes are promising electron sources for high performance free electron lasers and future acceleratorbased light sources [1]
We report a new method of generating quantum efficiency (QE) maps which uses a Texas Instruments digital micromirror device (DMD)
QE maps are made with different step sizes by programming groups of DMD pixels to switch together
Summary
Photocathodes are promising electron sources for high performance free electron lasers and future acceleratorbased light sources [1]. The initial transverse current density profile is driven in part by the variation in photoemission (variation in QE) at the cathode surface. Measured QE is a weighted average over the illuminated area, with the weighting determined by the transverse laser intensity profile. QE can be mapped by using a laser spot smaller than the diameter of the drive laser used for electron beam generation. If two different cathodes have the same average QE over the typical drive laser spot size, a QE map using a smaller laser spot size can determine which is more uniform. If the drive laser spot is smaller than the cathode and there is freedom in choosing the photoemission site, a QE map of the full cathode can reveal favorable positions for illumination
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