Abstract
Measuring the quantum efficiency (QE) map of a photocathode injector typically requires laser scanning, an invasive operation that involves modifying the injector laser focus and rastering the focused laser spot across the photocathode surface. Raster scanning interrupts normal operation and takes considerable time to setup. In this paper, we demonstrate a novel method of measuring the QE map using a ghost imaging framework that correlates the injector laser spatial variation over time with the total charge yield. Ghost imaging enables passive, real-time monitoring of the QE map without manually modifying the injector laser or interrupting injector operation. We first demonstrate the method at the UCLA Pegasus photoinjector with the help of a digital micromirror device (DMD) and a piezoelectric mirror to increase our control of the overall transverse variance of the illumination profile. The reconstruction algorithm parameters are fine-tuned using simulations and the results are validated against the ground truth map acquired using the traditional rastering method. Finally, we apply the technique to data acquired parasitically from the LCLS photoinjector, showing the feasibility of this method to retrieve a QE map without interrupting normal operation.
Highlights
Modern electron accelerators use photoemission to generate high brightness electron beams [1,2,3]
We show how the coefficient of variation (CV) and principal component analysis (PCA) can help assess the amount of laser spatial variation needed for a reconstruction
We present a novel method for measuring the spatial features in quantum efficiency (QE) of injector cathodes based on a ghost imaging reconstruction framework
Summary
Modern electron accelerators use photoemission to generate high brightness electron beams [1,2,3]. In this process, an optical drive laser strikes the surface of a photocathode, emitting electrons due to the photoelectric effect. The QE map is measured by focusing the drive laser to a small spot size and scanning it across the cathode surface. In this configuration, the emitted charge at each location of the focused laser spot maps out the QE, assuming the emitted
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