Abstract

Multilayer mirrors can have a dramatic reduction in reflectivity as a result of large optical and x-ray fluxes incident on the surface of the mirror. We consider the effect of a short pulse of x-rays on the reflectivity of a variety of mirrors including W/C, WC/C, Cr₃C₂/C, Mo/Si, and MoSi₂/Si multilayers. We use the measured x-ray spectrum from a gold target heated by a 0.5 ns laser for the case of no filter between the gold and the mirror and when various low energy filters are used. One tool we use to determine the reduction in reflectivity is a multilayer damage code ROMULS.¹ We have modified the code to allow non-normal incidence and for a spectrum of x rays In the figure we show the calculated normal incidence reflectivity of a W/C multilayer mirror at 44.8 Å. The x-ray flux from the gold target is incident at 50° from normal and we show the reflectivity for three values of integrated fluence. The temporal dependence of the x-ray flux used in the calculation is shown. Two of the curves use an x-ray spectrum measured with no filter and one uses the spectrum after passing through a low energy filter. In all cases, the calculated reflectivity is reduced dramatically during the x-ray pulse. The calculated reflectivities with no damage are approximately a factor of four larger than measured. The calculation assumes smooth layers and no diffusion between the layers. We will discuss the reduction of reflectivity as a result of rough surfaces and diffusion between layers. Our modeling results will be compared with recent experiments that measured the reflectivity both temporally and spectrally.

© 1992 Optical Society of America