Abstract
The photocathode rf gun is one of the most critical components in x-ray free electron lasers. The drive laser strikes the photocathode surface, which emits electrons with properties that depend on the shape of the drive laser. Most free electron lasers use photocathodes with work function in the ultraviolet, a wavelength where direct laser manipulation becomes challenging. In this paper, we present a novel application of a digital micromirror device (DMD) for the 253 nm drive laser at the Linear Coherent Light Source. Laser profile shaping is accomplished through an iterative algorithm that takes into account shaping error and efficiency. Next, we use laser shaping to control the X-ray laser output via an online optimizer, which shows improvement in FEL pulse energy. Lastly, as a preparation for electron beam shaping, we use the DMD to measure the photocathode quantum efficiency across cathode surface with an averaged laser rms spot size of $59\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$. Our experiments demonstrate promising outlook of using DMD to shape ultraviolet lasers for photocathode rf guns with various applications.
Highlights
The x-ray free electron laser (FEL) is the fourth generation light source that produces high power, tunable, and coherent x-rays
We show the measurement of Linear Coherent Light Source (LCLS) photocathode quantum efficiency (QE) map using the digital micromirror device (DMD) and discuss future work in electron beam shaping
We show that we can use a digital micromirror device to produce arbitrary laser profiles at the LCLS injector
Summary
The x-ray free electron laser (FEL) is the fourth generation light source that produces high power, tunable, and coherent x-rays. The transverse profile shaping is complicated since it involves nonuniformities from the drive laser and the photocathode QE spatial variation. These structures can carry over to the electron beam and can degrade beam brightness and FEL performance. Recent studies have used liquid crystal based spatial light modulators (SLMs) to achieve drive laser shaping for Cornell’s high voltage dc gun at 532 nm [18,19] and for PITZ drive laser at 1030 nm [20]. We briefly discuss the algorithm to achieve laser shaping and present results of the LCLS drive laser beam. We show the measurement of LCLS photocathode QE map using the DMD and discuss future work in electron beam shaping. We discuss potential applications of laser shaping and further improvements to our experiments
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