Speaker
Description
We report on the design, performance and applications of FHI FEL, a dual-oscillator two-color infrared free-electron laser which we operate at the Fritz Haber Institute (FHI) in Berlin, Germany [1]. It includes a 50 MeV normal-conducting linac supplying two oscillator FEL branches; a mid-infrared (MIR) FEL (commissioned in 2013) and a new far-infrared (FIR) FEL. What makes the FHI FEL unique among present and previous FELs worldwide is the two-color mode of simultaneous operation of two oscillator FELs powered by a single accelerator. In this mode the high-repetition rate (1 GHz) electron bunch train provided by the accelerator is split by a transverse deflection kicker cavity into two bunch trains, each one having half the repetition rate (500 MHz). The half-repetition-rate bunch trains are separated by an angle of 4 degree and steered into the MIR and FIR FELs. The lasing wavelengths of both FELs can be tuned independently from one another by undulator gap variation. This enables continuous variation of the MIR-to-FIR wavelengths ratio by as much as a factor of 10. We provide experimental evidence indicating that the MIR and FIR radiation pulses are highly synchronized and, hence, well suited for a wealth of novel applications that have not been possible before such as, pump-probe experiments.
The FIR FEL was designed as a short-Rayleigh-range oscillator FEL, thereby optimizing the achievable micro-pulse energy [1]. In a recent line of experiments we were able to boost the micro-pulse energy to more than 150 µJ at a wavelength of 8 µm when running the FIR FEL at a small cavity-length detuning of less than a wavelength. At these conditions we observed a very wide spectrum indicating a short (sub-ps) pulse duration, as expected for small FEL cavity desynchronism. Upon tightly focusing these pulses into a Xe gas we observed ionization of Xe atoms, which is remarkable considering that the Xe atom’s ionization energy (12.1 eV) equals about 78 times the photon energy at 8 µm. We interpret this observation as evidence that electric fields strengths strong enough to cause tunneling ionization of atoms can be achieved. This, in principle, opens up the possibility to observe HHG (high-order harmonic generation). Experiments to explore the feasibility of generating ultra-short XUV pulses by HHG using the FHI FIR FEL are being prepared.
[1] W. Schöllkopf et al., arXiv:2604.16189