The ongoing Plasma-driven Attosecond X-ray source experiment (PAX) at FACET-II aims to produce coherent soft X-ray pulses of attosecond duration using a Plasma Wakefield Accelerator [1]. These kinds of X-ray pulses can be used to study chemical processes where attosecond-scale electron motion is important. For this first stage of the experiment, PAX plans to demonstrate that <100 nm bunch length electron beams can be generated using the 10 GeV beam accelerated in the FACET-II linac and using the plasma cell to give it a percent-per-micron chirp. The strongly chirped beam is then compressed in a weak chicane to sub-100 nm length, producing CSR in the final chicane magnet at wavelengths as low as 10s of nm. In this contribution we describe the commissioning of the spectral diagnostics as well as the results expected of this experiment. Additionally, we describe a future iteration of the experiment in which short undulators are used to drive coherent harmonic generation to produce attosecond gigawatt X-ray pulses at 2 and 0.4 nm, with lengths comparable to the shortest attosecond pulses ever measured at 2 nm using HHG.

Echo-enabled harmonic generation (EEHG), a free-electron laser (FEL) scheme relying on two modulating sections, each consisting of an optical seed laser, an undulator and a magnetic chicane, has recently been implemented on the FEL-1 radiator line at the FERMI FEL user facility in Trieste, Italy. This setup imprints a density modulation onto a relativistic electron beam at a high harmonic of the seed frequency before injecting the electrons into the radiator, where they emit coherent soft x-rays. We have experimentally studied EEHG performance as a function of the properties of both seeds (modulation amplitude, frequency chirp) and the electron beam (slice energy spread, energy profile). We measured a relatively low output sensitivity to the properties of the first and a high sensitivity to the properties of the second seed, and simultaneously a high tolerance both to slice energy spread and to non-linear terms in the electron-beam energy profile. All of these observations are consistent with theoretical predictions. The emission of coherent, shot-to-shot stable radiation at harmonics of the second seed frequency as high as 50 sets the stage for a future upgrade of the FEL-2 line.

Light with polarization structure is called Fully Structured Light (FSL). We present an experimental demonstration of coherent FSL EUV light with spatially varying states of polarization generated at the FERMI free electron laser (FEL) in Trieste, Italy. Control of the polarization is obtained through the overlap of radiation emitted in orthogonally polarized helical undulators with different transverse phase distributions. The spatial polarization structure was mapped by imaging the light downstream of a polarizer, and two classes of FSL were observed and characterized: cylindrical vector and Poincare beams.

The ongoing Plasma-driven Attosecond X-ray source experiment (PAX) at FACET-II aims to produce coherent soft X-ray pulses of attosecond duration using a Plasma Wakefield Accelerator [1]. These kinds of X-ray pulses can be used to study chemical processes where attosecond-scale electron motion is important. For this first stage of the experiment, PAX plans to demonstrate that <100 nm bunch length electron beams can be generated using the 10 GeV beam accelerated in the FACET-II linac and using the plasma cell to give it a percent-per-micron chirp. The strongly chirped beam is then compressed in a weak chicane to sub-100 nm length, producing CSR in the final chicane magnet at wavelengths as low as 10s of nm. In this contribution we describe the commissioning of the spectral diagnostics as well as the results expected of this experiment. Additionally, we describe a future iteration of the experiment in which short undulators are used to drive coherent harmonic generation to produce attosecond gigawatt X-ray pulses at 2 and 0.4 nm, with lengths comparable to the shortest attosecond pulses ever measured at 2 nm using HHG.

Advances in high-performance computing have enabled detailed physics simulations, including those with nonlinear collective effects such as space charge, to be deployed online in a control room setting to aid operator intuition and be used directly in automatic tuning. Simultaneously, machine learning (ML) has enabled deployment of detailed models online with sub-second execution time, opened up new avenues for adapting simulation models to more closely match real accelerator behavior, and enabled novel ways to combine detailed physics simulations and ML-based tuning. This contribution will provide an overview of how these tools are being developed and successfully applied at SLAC, with an emphasis on experimental demonstrations. This includes improvements in adaptive calibration methods, novel approaches to simulation (e.g. differentiable physics combined with ML), and the use of system models in ML-based tuning (e.g. Bayesian optimization with system model priors, iterative simulation and ML tuning to aid LCLS-II injector commissioning). Discussion of the software infrastructure required to achieve this and deploy these solutions into regular operation will also be discussed.

The FERMI free-electron laser (FEL) facility has recently achieved a significant milestone with the successful implementation of the echo-enabled harmonic generation (EEHG) scheme in the FEL-1 amplifier line. This advancement is part of a broader upgrade strategy aiming at expanding the covered spectral range of the facility to the entire water window and beyond. Through this upgrade, the maximum photon energy of FEL-1 has been doubled and spectral quality has been enhanced. The updated FERMI FEL-1 is the first user facility operating in the spectral range 20-10 nm utilizing the EEHG scheme. It will serve also as the ideal test bench for conducting new machine studies in the perspective of future developments. In this contribution, we present the results obtained during the commissioning phase and the first user experiments.