Detail showing the transmission near (2,0)4 for a column density of ~68 × 1017 cm−2
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Figure 4. Detail showing the transmission near (2,0)4 for a column density of ~68 × 1017 cm−2. The series limit converging on He+(n = 2) is at 18.96 nm. The inset shows the single-shot spectra with the highest peak intensities at the positions of the He** absorption profiles along with the calculated average transmission (black trace).
Using the third harmonic of the FEL radiation from the SPring-8 compact SASE (self-amplified stimulated emission) source SCSS we have studied the effects on SASE pulses with central wavelengths near 20 nm due to passage through a helium gas cell. The positions of zero ionization cross-section close to wavelengths corresponding to double-excitations allow operation as an efficient wavelength filter, with effectively 100% transmitted peak intensity until the Doppler-broadening limit is reached. We discuss how the time profile of the SASE pulses is affected, and discuss potential applications.