Large field double Kirkpatrick–Baez microscope with nonperiodic multilayers for laser plasma imaging

Abstract

The double Kirkpatrick–Baez (DBA) microscope is derived from the grazing x-ray Kirkpatrick–Baez (KB) microscope. The KB is compound of two concave spherical mirrors working at grazing incidence and in an energy range about 100 eV–10 keV. The combination of two similar mirrors in the DKB increases the useful field. This device only requires spherical mirrors, more easy to manufacture (precision and roughness) than aspherical ones. In order to image a laser plasma source in a large field of view and within a bandpass of 0.6 keV around 3.4 keV, a KB optic covered with multilayers is developed. In fact, a compromise has to be found between the resolution of the optic (better with a less grazing angle), and the reflectivity (better with more grazing angle). We have chosen to keep the average grazing incidence on the four mirrors around 2°–3° just as for a first uncoated DKB made in our laboratory. This allows us to keep the same radius of curvature for the mirrors. At this energy multilayers are needed, due to the required reflectivity of better than 7% for each mirror. A difficulty appears concerning the energy and angular acceptance of multilayers. It is shown that a nonperiodic multilayer structure can be calculated to solve this problem, in spite of the absorption of the layers at the low energy of our application. The variation of periods is not continuous as in known classical supermirrors, and all the experimental parameters such as complex index of refraction and roughness have to be known. Thicknesses can then be optimized individually. The multilayers were deposited, tested, and the defects identified and corrected. Final experimental results of such stacks are given.