Kang, Heung -Sik | Engineering Papers

DOE OSTI2019

X-ray free electron laser tuning for variable-gap undulators

A tuning method for hard x-ray free electron lasers is presented which combines electron-beam-based alignment (EBBA) with undulator radiation spectrum analysis. This method is conducive to achieving a dispersion-free orbit, finding the undulator field center, and determining the accurate value of the gap distance for the operating undulator K for each undulator segment, and it proves to be highly reliable and robust for free electron laser lasing with variable-gap undulators. The effect of the natural focusing of the undulator on the EBBA is confirmed with measurements and a theoretical analysis. The tilt of an undulator segment subject to ground movement introduces an undulator field change which the electron beam interacts with and can be corrected by K tuning to find a precise compensating gap distance. Lattice matching with a wire-scanner-based Twiss parameter measurement along the undulator line enables the EBBA reference orbit to coincide with the e-beam distribution.

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DOE OSTI2019

Hard X-ray self-seeding commissioning at PAL-XFEL

A wake monochromator based on a large-area diamond single crystal for hard X-ray self-seeding has been successfully installed and commissioned in the hard X-ray free-electron laser (FEL) at the Pohang Accelerator Laboratory with international collaboration. For this commissioning, the self-seeding was demonstrated with a low bunch charge (40 pC) and the nominal bunch charge (180 pC) of self-amplified spontaneous emission (SASE) operation. The FEL pulse lengths were estimated as 7 fs and 29.5 fs, respectively. In both cases, the average spectral brightness increased by more than three times compared with the SASE mode. The self-seeding experiment was demonstrated for the first time using a crystal with a thickness of 30 µm, and a narrow bandwidth of 0.22 eV (full width at half-maximum) was obtained at 8.3 keV, which confirmed the functionality of a crystal with such a small thickness. In the nominal bunch-charge self-seeding experiment, the histogram of the intensity integrated over a 1 eV bandwidth showed a well defined Gaussian profile, which is evidence of the saturated FEL and a minimal electron-energy jitter (~1.2 × 10 –4 ) effect. Here, the corresponding low photon-energy jitter (~2.4 × 10 –4 ) of the SASE FEL pulse, which is two times lower than the Pierce parameter, enabled the seeding power to be maximized by maintaining the spectral overlap between SASE FEL gain and the monochromator.

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