The ion channel laser (ICL) is similar to the free electron laser (FEL) but utilizes the electric field from a blowout regime plasma wake rather than the magnetic field from an undulator to oscillate particles. Compared to the FEL, the ICL can lase with much larger energy spread beams and in much shorter distances, making it an attractive candidate for a future compact plasma accelerator driven coherent light source. We present a novel full 3D theory of the ICL accounting for numerous effects including transverse guided mode shape, diffraction, frequency and Betatron phase detuning, and nonzero spread in energy and undulator parameter. This theory is used to predict the gain, radiation mode profile, gain bandwidth, and emittance and energy spread constraints of the ion channel laser.

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.

We present a study of microbunching amplification in linear accelerators, focusing on the combined effects of coherent synchrotron radiation (CSR) and longitudinal space charge (LSC). We also investigate the role of a laser heater, which is designed to suppress microbunching by decreasing the relative correlated energy spread early in the beamline. Simulations are performed for the FACET linac (SLAC), enabling direct comparison with existing theoretical predictions for CSR-induced microbunching in the presence of a laser heater. In addition to this comparison, we analyze microbunching amplification due to CSR and LSC both individually and jointly, highlighting their interplay. This work lays the foundation for upcoming experimental studies at FACET aimed at validating both theoretical models and numerical simulations.

We present a study of intra-beam scattering (IBS) that is important for high-brightness electron beams, including a recent theory incorporating enhanced temporal correlations of electric field fluctuations. These correlations primarily arise from the periodic betatron motion of particles within the beam that is not accounted for in conventional theories. To enable direct verification of the theoretical calculations, we perform simulations with particle distributions preserved over time, ensuring conditions compatible with theoretical assumptions. We focus our study on the energy spread increase in high-brightness electron injectors. Energy spread growth is extracted from simulations in two ways: through the theoretical connection with field correlations, and directly from accumulated energy changes of individual particles. Comparisons are performed across multiple beam distributions and dynamics, from linear motion in an infinite uniform plasma to betatron oscillations in a Gaussian bunch.

Cavity-based X-ray free-electron lasers present a promising path toward fully coherent, high-brightness X-ray sources with enhanced stability and spectral purity. By using Bragg-reflecting crystal cavities to recirculate and amplify an X-ray seed pulse over multiple passes, CBXFELs offer the potential for orders-of-magnitude improvements in coherence and brightness compared to single-pass FELs. This talk will present an overview of the CBXFEL concept and the proof-of-principle experiment currently under development at SLAC. Recent progress will be presented, along with ongoing efforts in beam–X-ray overlap diagnostics and cavity alignment. The talk will also address the key technical challenges ahead for CBXFELs and briefly explore alternative cavity-based XFEL designs as promising paths forward.

We present a study of microbunching instability in the FACET-II linac, in which the amplification and damping mechanisms are analyzed separately. Our simulations investigate the gain induced by Longitudinal Space Charge (LSC) and, critically, the damping caused by nonlinear terms in the beam transport transfer map. We show theoretically and through simulation that these nonlinear effects can produce damping several orders of magnitude stronger than predicted by linear theory. Experimental evidence validates these findings. A quadrupole scan performed in the FACET-II dogleg reveals that an interaction between coherent betatron oscillations and transfer map nonlinearities shifts the point of minimal damping away from the expected linear condition of $R_{51}=0$. The strong agreement between our simulations and the experimental data demonstrate that a thorough understanding of nonlinear dynamics is essential for high-brightness beam transport.

We present a study of intra-beam scattering (IBS) in high-brightness electron beams, incorporating a recent theory that accounts for enhanced temporal correlations of electric field fluctuations. These correlations, absent in conventional binary-collision models, arise from the periodic betatron motion of particles within the beam. To enable direct verification of the theoretical calculations, we perform simulations in a computer code specifically written for that purpose. In the code, the particle distribution is preserved over time, ensuring conditions compatible with theoretical assumptions, and the IBS is neatly separated from the conventional Space Charge (SC) effect. The simulations, benchmarked against an exactly solvable case of an infinite isotropic uniform plasma, show good agreement with both uncorrelated models, such as Piwinski’s, and the new correlation-based theory, across various bunch distributions and dynamical regimes. This validates the simulation approach and highlights the role of time-correlated fields in accurate IBS modeling.

Shaping ultraviolet (UV) laser beams is critical for optimizing photoinjector performance for applications in free-electron lasers (FELs). It has been shown that a 50% truncated Gaussian beam can achieve the lowest emittance via space charge compensation at LCLS-I. However, conventional shaping techniques to prepare this beam are limited by significant power losses or are not adapted for UV light. Here we report a high-precision transverse-shaping technique based on custom fused-silica phase plates with >99 % transmission at 253 nm. This approach enables spatial beam profile tailoring and significantly enhances beam stability at the photocathode. Using IMPACT-T simulations, we predict a 33% (from 0.67um to 0.45um) reduction in normalized emittance for a 250 pC bunch at LCLS-I. Experimental implementation at FACET-II demonstrated a 37% emittance reduction (from 5.4um to 3.4um) at 1.6 nC. These results establish phase-plate beam shaping as a high-fidelity, low-loss approach for high-brightness photoinjectors. Implementation at LCLS-II which will enable stable operation at megahertz repetition rates is underway.

The ion channel laser (ICL) is similar to the free electron laser (FEL) but utilizes the electric field from a blowout regime plasma wake rather than the magnetic field from an undulator to oscillate particles. Compared to the FEL, the ICL can lase with much larger energy spread beams and in much shorter distances, making it an attractive candidate for a future compact plasma accelerator driven coherent light source. We present a novel full 3D theory of the ICL accounting for numerous effects including transverse guided mode shape, diffraction, frequency and Betatron phase detuning, and nonzero spread in energy and undulator parameter. This theory is used to predict the gain, radiation mode profile, gain bandwidth, and emittance and energy spread constraints of the ion channel laser.

This work presents an experimental study emulating a two-pass gain scenario in a cavity-based X-ray free-electron laser (CBXFEL) using a self-seeding configuration at LCLS. In this “7+7” arrangement, radiation generated by the first seven hard X-ray undulators (HXUs) is spectrally filtered by a high-resolution self-seeding crystal monochromator and used to seed a second set of seven undulators downstream. This setup mimics the regenerative amplification process expected in the CBXFEL cavity, where the seed pulse is recirculated and overlapped with a second trailing electron bunch. By systematically reducing the number of post-crystal undulators (from 13 to 5), we quantified the spectral amplification ratio by comparing the self-seeded peak signal to the SASE background. These results confirm the feasibility of seeding with the initial seven HXUs and provide a valuable benchmark for extrapolating gain in future two-bunch CBXFEL demonstration experiments.

We report on the commissioning results of the newly implemented Beam Overlap Diagnostic (BOD) instrument, known as Station F, at the hard X-ray line of the Linac Coherent Light Source (LCLS). As part of the CBXFEL project at SLAC, Station F is designed to facilitate alignment between the relativistic electron beam entering the undulator hall and the X-rays returning from the CBXFEL cavity via the return line. The station features two interchangeable targets: (1) a diamond screen for direct imaging of the LCLS electron beam, enabling measurement of its transverse size and position; and (2) a YAP:Ce scintillator for detecting faint returning X-rays when the diamond’s sensitivity is insufficient. Emission from either target, whether generated by electron-induced cathodoluminescence or X-ray-induced scintillation, is captured using a fast-gated optical/UV camera. We present results from recent commissioning runs, including direct electron beam imaging, beam size and position characterization in both single- and two-bunch modes, and observations of coherent radiation linked to early microbunching, with implications for the laser heater configuration.

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.

Cavity-based X-ray Free electron lasers (CBXFELs) such as the X-ray regenerative amplifier FEL (XRAFEL) and the XFEL oscillator (XFELO) have been proposed to produce highly coherent and stable hard X-rays. While the XRAFEL produces high-peak power X-rays with the bandwidth limited by the Bragg crystals, XFELO produces much lower peak power with extremely narrow bandwidth. In this report, we discuss methods to increase the CBXFEL peak power and reduce the bandwidth altogether. We show in simulations how these methods can be applied to high-repetition rate FEL facilities to generate ultra-bright X-ray pulses.