Detail showing the transmission near (2,0)<sub>4</sub> for a column density of ~68 <b>×</b> 10<sup>17</sup> cm<sup>−2</sup>

<p><strong>Figure 4.</strong> Detail showing the transmission near (2,0)<sub>4</sub> for a column density of ~68 <b>×</b> 10<sup>17</sup> cm<sup>−2</sup>. The series limit converging on He<sup>+</sup>(<em>n</em> = 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).</p> <p><strong>Abstract</strong></p> <p>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.</p>