Absorption cross-section σ (red, left-hand y-axis), and transmission (right-hand y-axis) for different column densities (/10¹⁷ cm⁻²)

Figure 1. Absorption cross-section σ (red, left-hand y-axis), and transmission (right-hand y-axis) for different column densities (/10¹⁷ cm⁻²).

Abstract

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.