Soft X-ray enhancement from porous target irradiated by high power sub-nanosecond laser beam and the effect of plasma opacity

Abstract

Abstract In this paper, we investigate numerically the enhancement of the soft continuum X-ray yield emitted from porous aluminum targets irradiated by sub-nanosecond high power pulsed laser beam. The calculations were conducted for two different conditions: (i) ignoring plasma opacity, and (ii) including it in the calculations; and finally the results were compared. In the calculations, the main emission mechanisms were assumed to be the free–bound and free–free (bremsstrahlung) transitions at water windows wavelengths ranges 2.3–4.4 nm and also 12.6–14.6 nm, which is useful for lithography applications. The free–free and bound–free transitions were also considered for calculating the opacity. The results show that for both conditions, the X-ray yield rises with increasing the porosity and the increase rate is almost equal in both cases; however, the calculations are not appropriate for estimating the absolute values of the X-ray yield when the plasma opacity is ignored.