Об излучении гармоник в рентгеновских лазерах на свободных электронах с изменяемым параметром дипольности ондуляторов

X-ray free electron lasers (FELs) produce ultra short bursts of coherent radiation at the wavelengths ~1–100 . In the absence of ready seed in this range, the fundamental tone is usually radiated in self-amplified spontaneous emission (SASE) FELs. With the goal to achieve maximum performance in minimum size and cost of a FEL, we study the harmonic multiplication in resonating undulator cascades as viable alternative to the SASE fundamental radiation. The harmonic power evolution along FELs is analyzed for the SASE and the harmonic self-seed. The analytical results are compared with available data for soft X-rays FLASH2. The possibility of harmonic radiation is studied for FELs with variable deflection parameter k: FLASH2, SwissFEL and currently built LCLS-II. We demonstrate earlier growth of the harmonic power in a buncher as compared with the SASE regime at the same wavelength. The difference is explained theoretically by the harmonic bunching growth in the buncher, as compared with that in the SASE FEL at the same wavelength. Excellent characteristics of the SwissFEL allow amplification of its third harmonic. We propose the harmonic self-seed, which would allow 10 m shorter undulator length for saturation at the same power, and up to 30% shorter radiation wavelength with the existing electron beam. At the LCLS-II, good bunching at the third harmonic wavelength in a dedicated buncher allows early and strong third harmonic growth. However, further resonating amplifier requires small value of k in LCLS-II undulators, which makes the amplification slow. We propose using the full amplification of the third FEL harmonic in LCLS-II undulators with bunching disruption between them at the wavelength of the fundamental. We show that this scheme allows >1 GW FEL power at λ=0.25 nm wavelength achievable already at ~40 m of the undulators with the electron beam energy E=4 GeV.

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. 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.

Towards an extremely high resolution broad-band flat-field spectrometer in the `water window'

The optical design of a novel spectrometer is presented, combining a cylindrically convex pre-mirror with a cylindrically concave varied-line-spacing grating (both in the meridional) to deliver a resolving power of 100000–200000 in the `water window' (2–5 nm). Most remarkably, the extremely high spectral resolution is achieved for an effective meridional source size of 50 µm (r.m.s.); this property could potentially be applied to diagnose SASE-FEL and well resolve individual single spikes in its radiation spectrum. The overall optical aberrations of the system are well analysed and compensated, providing an excellent flat-field at the detector domain throughout the whole spectral range. Also, a machine-learning scheme – SVM – is introduced to explore and reconstruct the optimal system with high efficiency.