Soft x-ray enhancement from a porous nano-layer on metal targets irradiated by long laser pulses

Soft x-rays are produced when high intensity pulsed lasers irradiate solid targets. The emitted x-rays can be suitable for different applications such as lithography and biological imaging. The free-bound and free-free (bremsstrahlung) x-ray emission at wavelengths 2.3-4.4 nm and 13.6 nm from targets with porous surfaces irradiated by longer (>50 ps) laser pulses has been investigated numerically. The computer code EHYBRID was used to simulate the hydrodynamics of a copper plasma produced by a 532 nm wavelength laser pulse irradiating a copper nano-layer on a copper target at intensities up to 1015 W cm-2. The target is modelled by assuming a uniform solid metal substrate coated by a nano-layer with density less than the solid density. The simulation results show that the x-ray yield can be enhanced significantly by using such low-density targets and is sensitive to the nano-layer density. For any specific condition there is a density ratio at which an optimum x-ray yield occurs. Using a pre-pulse to increase x-ray yield was also investigated and the results are compared to the x-ray yield for low-density targets.