Nonlinear Photoemission for Bright Beams in X-Band Photoinjectors

Abstract: The ongoing development of low-cost and compact x-ray light sources is vital for increasing the number of experiments that can be performed at the atomic scale. Smaller sources can be operated in blow-out mode, in which a high-charge, short-bunch length electron beam self compresses into a uniformly ellipsoidal shape. This mode necessitates a photoinjector whose laser has a pulse duration on the order of $10^2$ fs. In this case, linear photoemission with an ultraviolet laser presents significant technical challenges, as many optical substrates degrade at these wavelengths. In this paper, we present an alternative design that uses nonlinear photoemission to produce electron beams with high charge and short bunch length. This photoinjector design produces an electron beam with a bunch charge of up to $200$ pC, an RMS bunch length of $140$ fs, a normalized transverse emittance of $1$ to $1.4$ mm$\cdot$mrad, and an energy spread of $10<sup>{-4}$. This results in a beam brightness of $9.74\times10<sup>{18}$ A/m$<sup>2$, which is bright enough for our beamline's purposes. Ongoing efforts are aimed at further increasing the electron beam brightness by using a spatial light modulator to shape the laser