For certain photonuclear experiments utilizing Compton gamma-ray beams, beam-uncorrelated background poses a significant challenge. At the High Intensity Gamma-ray Source (HIGS), we have developed methods to generate pulsed free-electron laser (FEL) beams by transversely or longitudinally modulating the storage ring FEL. Both methods enable periods of FEL interaction: one by transversely manipulating the electron beam orbit, the other by de-synchronizing the electron and FEL beams. The recently-developed longitudinal method has proven superior: it avoids beam loss and is applicable across a wide range of electron beam energies. In this work, we describe the operational principle of pulsed FEL beam generation using longitudinal modulation, and we present measurements of the macro- and micro-temporal structure of the FEL beam. Furthermore, we present experimental results demonstrating the effectiveness of using a pulsed gamma-ray beam to reduce beam background.

Powered by a storage ring with energies ranging from 240 MeV to 1.2 GeV, the Duke Free-Electron Laser (FEL) has demonstrated operation across a broad wavelength spectrum from infrared (IR) to vacuum ultraviolet (VUV): 1100 nm to 170 nm. This FEL serves as a photon source for the High Intensity Gamma-ray Source (HIGS), producing polarized, near-monochromatic, and high-flux Compton gamma-ray beams in an extensive energy range from 1 MeV to 120 MeV, with the highest flux recorded at 3.5e+10 ph/s (total) around 10 MeV. To generate high-energy gamma-ray beams above 80 MeV, the FEL must operate in the VUV region from 195 nm to 155 nm. This work describes the development and operation of VUV beam diagnostics within a nitrogen-purged enclosure, with increased difficulty as the wavelength shortens towards 155 nm. We will discuss the challenges encountered and the solutions found for VUV beam diagnostics, leading to the successful FEL lasing in the VUV region.

The linac-based single-pass FEL has been successfully operated in the EUV and x-ray regions for about two decades. However, the oscillator FEL has been limited to operating in the longer wavelength region. This limitation arises from the challenge of obtaining short-wavelength FEL mirrors with high reflectivity, thermal stability, and radiation resistance. With the Duke storage ring FEL, we have demonstrated VUV FEL lasing from 168.6 to 179.7 nm with excellent beam stability. This progress has been made possible by developing a new FEL configuration with substantially reduced undulator harmonic radiation on the FEL mirror, a thermally stable FEL optical cavity, and a new type of high-reflectivity fluoride-based multilayer coating with a protective capping layer. Employing this VUV FEL in Compton scattering, we have also produced a high-flux, circularly polarized gamma-ray beam up to 120 MeV at the High-Intensity Gamma-ray Source (HIGS). The high-energy gamma rays will open up new opportunities for experimental study of the nucleon’s structure through the lens of Chiral Perturbation Theory.

Orbital angular momentum (OAM) photon beams are excellent tools for non-contact optical manipulation of matter in a broad photon energy range. A free-electron laser (FEL) oscillator is well-suited for studying OAM beams with various features including a wide spectral coverage, wavelength tunability, two-color lasing, etc. Here, we report the first experimental demonstration of superposed OAM beams from an oscillator FEL. Lasing at around 458 nm, we have generated superposed OAM beams up to the fourth order as a superposition of two pure OAM modes with opposite helicities. These generated beams have a high beam quality, a high degree of circular polarization, and high power. Using external rf modulation with frequencies from 1 to 30 Hz, we also developed a pulsed mode operation of the OAM beams with a highly reproducible temporal structure. FEL operation showcased in this work can be extended to higher photon energies, e.g. using a future x-ray FEL oscillator. The operation of such an OAM FEL also paves the way for the generation of OAM gamma-ray beams via Compton scattering.