SLAC’s LCLS-II is pioneering high-repetition-rate attosecond X-ray science, enabling new opportunities to optimize X-ray generation by controlling the electron beam at its source—the photoinjector. LCLS-II employs a 20 ps Gaussian UV laser pulse to drive the photocathode, with an added narrow modulation to induce microbunching for extended modes. Recent advances in laser pulse shaping and frequency upconversion now allow for more sophisticated tailoring of the electron beam at the injector. We present a novel approach using spectral amplitude and phase shaping of the IR laser, followed by dispersion-controlled nonlinear synthesis—relying on phase-modulated noncollinear sum-frequency generation—for UV upconversion. This enables diverse UV temporal profiles, including flattop and double/triple spikes, offering new degrees of freedom for shaping. Preliminary results from LCLS-II beam time show these modulations produce effective downstream perturbations to the electron bunch at the undulators, demonstrating feasibility for programmable bunch formation. We are integrating this shaping into a start-to-end simulation framework, enabling digital twin modeling of the XFEL chain—from photoinjector laser to X-ray output—laying the groundwork for fully tunable, end-to-end optimized, application-specific X-ray pulses.

Free-electron lasers (FEL) seeded by short radiation pulses can exhibit superradiant behavior. In the superradiant regime, the pulse simultaneously compresses and amplifies as it propagates through the FEL, making superradiance very promising for pushing the performance limits of attosecond x-ray FELs. To date, this regime has been studied in asymptotic limits, but there is no model for how the initially linear dynamics of the seeded FEL transition into the nonlinear superradiant behavior. We derive an analytical model for the 1D FEL seeded by a short pulse which accurately captures the linear dynamics, the nonlinear superradiant evolution, and the smooth transition between them. Our model fills a critical gap in our understanding of FEL superradiance and nonlinear time-dependent FEL physics more broadly, and may provide a bridge to the corresponding problem in three-dimensions, and analogous problems in other fields exhibiting soliton behavior.

SLAC’s LCLS-II is pioneering high-repetition-rate attosecond X-ray science, enabling new opportunities to optimize X-ray generation by controlling the electron beam at its source—the photoinjector. LCLS-II employs a 20 ps Gaussian UV laser pulse to drive the photocathode, with an added narrow modulation to induce microbunching for extended modes. Recent advances in laser pulse shaping and frequency upconversion now allow for more sophisticated tailoring of the electron beam at the injector. We present a novel approach using spectral amplitude and phase shaping of the IR laser, followed by dispersion-controlled nonlinear synthesis—relying on phase-modulated noncollinear sum-frequency generation—for UV upconversion. This enables diverse UV temporal profiles, including flattop and double/triple spikes, offering new degrees of freedom for shaping. Preliminary results from LCLS-II beam time show these modulations produce effective downstream perturbations to the electron bunch at the undulators, demonstrating feasibility for programmable bunch formation. We are integrating this shaping into a start-to-end simulation framework, enabling digital twin modeling of the XFEL chain—from photoinjector laser to X-ray output—laying the groundwork for fully tunable, end-to-end optimized, application-specific X-ray pulses.