Filter bandwidths using the (2,0)<sub><em>n</em></sub> absorption profiles as functions of helium column density

<p><strong>Figure 6.</strong> Filter bandwidths using the (2,0)<sub><em>n</em></sub> absorption profiles as functions of helium column density. For <em>n</em> = 2 (red) the two half-widths (dashed lines) are shown along with the full-width at half-maximum. Only the FWHMs are shown for <em>n</em> = 3 (blue), <em>n</em> = 4 (green), and <em>n</em> = 5 (purple). The dashed vertical lines show the maximum column density used in this work (68 <b>×</b> 10<sup>17</sup> cm<sup>−2</sup>) and the column density which would correspond to 2 m of gas at atmospheric pressure (5 <b>×</b> 10<sup>21</sup> cm<sup>−2</sup>). The solid black circles show the bandwidths obtained in the spectra plotted in figures <a href="http://iopscience.iop.org/0953-4075/46/16/164021/article#jpb465316f3" target="_blank">3</a> and <a href="http://iopscience.iop.org/0953-4075/46/16/164021/article#jpb465316f4" target="_blank">4</a>.</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>