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.

Electron dynamics in molecules occur on attosecond timescales and drive fundamental processes such as photosynthesis, catalysis, and chemical bond transformations. Understanding these phenomena requires tools with both high temporal resolution and the capability to probe molecular dynamics at high repetition rates. Here, we present the first single-shot measurements of attosecond soft x-ray pulses at the superconducting LCLS-II accelerator. Using an angle-resolving electron time-of-flight spectrometer, we perform angular streaking measurements with high energy and angular resolution, enabling a complete reconstruction of the spatial and temporal profiles of the pulses. These measurements showcase the attosecond science capabilities of LCLS-II at unprecedented repetition rates and provide the foundation for controlling and shaping x-ray pulses to study ultrafast dynamics in complex systems with precision.

We report the generation of single spike hard x-ray pulses at the Linac Coherent Light Source enabled by temporal shaping of the photocathode laser. The pulses were produced with typical pulse energies of 10 uJ and full-width at half-maximum spectral bandwidths averaging 30 eV, corresponding to a 60 attosecond Fourier-limited pulse duration. These pulses open new doors in electronic-damage-free probing of ultrafast phenomena and, eventually, attosecond hard x-ray scattering experiments. We discuss progress towards characterization of the pulses in the time domain using hard x-ray angular streaking and a hard x-ray split and delay device.

Achieving high-precision, in situ measurements of electric fields is a critical challenge in ultrafast science and accelerator diagnostics. We are developing an approach using photoconductive sampling with micro-fabricated devices to map electron beam fields with unprecedented spatiotemporal resolution. This technique enables the first direct 3D vector field measurements of electron beams, offering valuable insights into collective effects such as coherent synchrotron radiation and other phenomena impacting beam quality. These low-cost, highly flexible devices present a pathway to enhancing our understanding of beam dynamics and reducing transient effects that degrade beam quality. The devices will be initially tested on the ultrafast x-ray beamline at LCLS, and could be adapted as a diagnostic tool across other SLAC user facilities. Beyond diagnostics, this approach will also help in advancing studies of ultrafast charge transport and unlocking new science in attosecond solid-state physics.

Formation of current spike in electron bunch has direct implication for attosecond pulse generation in XFEL. In this paper, we present start-to-end simulation for tunable, short current spike generation in the LCLS copper linac using photocathode laser shaping. Our approach uses two stacked laser pulses—a long and a short pulse—to imprint a small modulation in the electron bunch as it is created in the injector. This initial modulation is then amplified as the bunch travels through the downstream bunch compressors, ultimately forming a sharp current spike. We also discuss how different shapes of the initial laser pulses influence the final current profile and the efficiency of spike generation.

In this contribution we report on the experimental generation of high energy (10 GeV), ultra-short (fs-duration), ultra-high current (∼ 0.1 MA), petawatt peak power electron beams at the FACET-II National User Facility at SLAC National Accelerator Laboratory. These extreme beams enable the exploration of a new frontier of high intensity beam-light and beam-matter interactions broadly relevant across fields ranging from high-field plasma wakefield acceleration to laboratory astrophysics and strong field quantum electrodynamics. We demonstrate our ability to generate and control the properties of these electron beams by means of a laser-electron beam shaping technique. This experimental demonstration opens the door to on-the-fly customization of extreme beam current profiles for desired experiments and is poised to benefit a broad swathe of cross-cutting applications of relativistic electron beams including optimization of advanced accelerator applications.

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.

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.

Electron dynamics in molecules occur on attosecond timescales and drive fundamental processes such as photosynthesis, catalysis, and chemical bond transformations. Understanding these phenomena requires tools with both high temporal resolution and the capability to probe molecular dynamics at high repetition rates. Here, we present the first single-shot measurements of attosecond soft x-ray pulses at the superconducting LCLS-II accelerator. Using an angle-resolving electron time-of-flight spectrometer, we perform angular streaking measurements with high energy and angular resolution, enabling a complete reconstruction of the spatial and temporal profiles of the pulses. These measurements showcase the attosecond science capabilities of LCLS-II at unprecedented repetition rates and provide the foundation for controlling and shaping x-ray pulses to study ultrafast dynamics in complex systems with precision.

We report the generation of single spike hard x-ray pulses at the Linac Coherent Light Source enabled by temporal shaping of the photocathode laser. The pulses were produced with typical pulse energies of 10 uJ and full-width at half-maximum spectral bandwidths averaging 30 eV, corresponding to a 60 attosecond Fourier-limited pulse duration. These pulses open new doors in electronic-damage-free probing of ultrafast phenomena and, eventually, attosecond hard x-ray scattering experiments. We discuss progress towards characterization of the pulses in the time domain using hard x-ray angular streaking and a hard x-ray split and delay device.

X-ray free-electron lasers have opened new frontiers in attosecond science thanks to their high pulse energy compared to traditional table top sources. To date, most attosecond experiments performed at XFELs have been impulsive, with the impinging x-ray pulses being much shorter than the timescales being studied. We present a method for attosecond pulse shaping which enables us to move beyond simple observation of ultrafast dynamics towards coherent control of quantum systems on sub-femtosecond timescales. We present experimental evidence of phase locked attosecond pulse trains from the LCLS-II. We conclude by presenting recent experiments utilizing mutually coherent pulse pairs with controllable temporal and spectral delay to launch controllable coherent electronic wavepackets in molecular systems.