Next-generation electron photoinjector accelerators, such as the LCLS-II photoinjector, have increasingly tight requirements on the excitation lasers, often calling for tens of picosecond, temporally flat-top, ultraviolet (UV) pulse trains to be delivered at up to 1 MHz*. We present an experimental demonstration of temporal pulse shaping for the LCLS-II photoinjector laser resulting in temporally flat-top pulses with 24 ps durations. Our technique is a non-colinear sum frequency generation scheme wherein two identical infrared optical pulses are imparted with equal and opposite amounts of spectral dispersion. The mixing of these dispersed pulses within a thick nonlinear crystal generates a second harmonic optical pulse that is spectrally narrowband with a designed temporal profile**. In experiment we achieve upwards of 40% conversion efficiency with this process allowing this to be used for high average and peak power applications. These narrowband pulses can then be directly upconverted to the UV towards use in driving free electron laser photocathodes. Additionally, we present a theoretical framework for adapting this method to shape optical pulses driving other photoinjector based applications.

The next generation of augmented brightness XFELs, such as LCLS-II, promises to address current challenges associated with systems with low X-ray cross-sections. Typical photoinjector lasers produce coherent ultraviolet (UV) pulses via nonlinear conversion of an infrared (IR) laser. Fast and active beam manipulation is required to capitalize on this new generation of XFELs, and controlling the phase space of the electron beam is achieved by shaping the UV source. However current techniques for such shaping in the UV rely on stacking pulses in time, which leads to unavoidable intensity modulations and hence space-charge driven microbunching instabilities [1]. Traditional methods for upconversion do not preserve phase shape and thus require more complicated means of arriving at the desired pulse shapes after nonlinear upconversion [2]. Upconversion through four-waving mixing (FWM) allows direct phase transfer, convenient wavelength tunability by easily changeable phase matching parameters, and also has the added advantage of greater average power handling than traditional χ(2) nonlinear processes [3, 4,]. Therefore, we examine a possible solution for e-beam shaping using a machine learning (ML) implementation of real-time photoinjector laser manipulation which shapes the IR laser source and then uses FWM for the nonlinear upconversion and shaping simultaneously. Our presentation will focus on the software model of the photoinjector laser, the associated ML models, and the optical setup. We anticipate this approach to not only enable active experimental control of X-ray pulse characteristics but could also increase the operational capacity of future e-beam sources, accelerator facilities, and XFELs. References: [1] S. Bettoni, et al. “Impact of laser stacking and photocathode materials on microbunching instability in photoinjectors”, Phys. Rev. Accel. Beams 23, 024401 (2020) [2] Lemons, Randy, et al. “Dispersion-controlled Temporal Shaping of Picosecond Pulses via Non-colinear Sum Frequency Generation.” Phys. Rev. Accel. Beams 25, 013401 (2022) [3] P. Zuo, T. Fuji, and T. Suzuki, "Spectral phase transfer to ultrashort UV pulses through four-wave mixing," Opt. Express 18, 16183-16192 (2010) [4] John E. Beetar, M. Nrisimhamurty, Tran-Chau Truong, Yangyang Liu, and Michael Chini, "Thermal effects in molecular gas-filled hollow-core fibers," Opt. Lett. 46, 2437-2440 (2021)

Shaping techniques traditionally used to produce few femtosecond and even sub femtosecond soft X-ray FEL pulses at LCLS do not scale well to high repetition rates. Here we present the progress of the LCLS-II X-ray temporal shaping project which uses infrared and ultraviolet picosecond lasers to shape the electron beam of the LCLS-II superconducting linac. Quickly switching these shaping lasers on and off will enable multiplexing different beams to different beamlines.