Characterization of the collisionally pumped optical-field-ionized soft-x-ray laser at 41.8 nm driven in capillary tubes

We report on experimental and theoretical studies of a collisionally pumped, optical-field-ionized soft-x-ray laser (SXRL) at 41.8 nm driven in capillary tubes with smooth inner surface. A detailed experimental study has been conducted in order to understand the key effects related to guiding in this configuration. The amplifying plasma was created inside few-cm-long capillary tubes, and maximum extreme ultraviolet emission was obtained when operating in a multimode guiding regime with an optimized lasing signal from a 25-mm-long capillary a factor of 3 higher than that of a gas cell and with a beam divergence reduced by a factor of 3. A numerical code, named COFIXE, has been developed to calculate the SXRL signal emitted by the plasma source. It includes the calculation of the pump beam propagation, the determination of the plasma state just after the interaction with the pump beam, the calculation of the evolution of the local properties of the plasma during the subsequent few ps, and the calculation of the amplification and transport of the SXRL emission. Excellent agreement has been obtained between experiment and theory for several features such as the divergence of the beam, the correlation between guided pump beam transmission and SXRL energy, and the enhancement factor induced by guiding.