ヘリウム2電子励起状態を使ったSASE光の単色化

A Gas monochromator for SASE pulses using absorption in the vicinity of the doubly-excited states in helium

Harries, J.; 岩山 洋士*; 永園 充*; 繁政 英治*

Harries, J.; Iwayama, Hiroshi*; Nagasono, Mitsuru*; Shigemasa, Eiji*

Using an idea suggested for use in a self-seeding scheme, we have demonstrated an efficient filter for SASE pulses at wavelengths near 20 nm using a helium gas cell. In this wavelength region absorption occurs due to double-excitation. The cross-sections for absorption show Fano profiles, with a region of transparency. For the 2lnl' doubly-excited states the cross-section drops to exactly zero, and absorption only occurs due to Doppler broadening, which is around 0.4 meV at 298 K. Using a gas cell inserted in the beamline of the SPring-8 compact SASE source SCSS, we have demonstrated efficient monochromatisation of the SASE pulses at wavelengths corresponding to the 2lnl' states, using the 3rd FEL harmonic. Due to the absorption there is also a profound effect on the time profile of the SASE pulses, and this has also been investigated. Multi-colour filtering has also been demonstrated.