Absorption cross-section σ (red, left-hand <em>y</em>-axis), and transmission (right-hand <em>y</em>-axis) for different column densities (/10<sup>17</sup> cm<sup>−2</sup>)

<p><strong>Figure 1.</strong> Absorption cross-section σ (red, left-hand <em>y</em>-axis), and transmission (right-hand <em>y</em>-axis) for different column densities (/10<sup>17</sup> cm<sup>−2</sup>).</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>