Design And Fabrication Of Heat Resistant Multilayers

Abstract

Many promising applications of multilayer x-ray optical elements subject them to intense radiation. This paper discusses the selection of optima! pairs of materials to resist heat damage and presents simulations of multilayer performance under extreme . heat loadings. IntroductionMultilayers have been used in environments where x-rays or other radiation is sufficiently intense to degrade performance. Examples include such applications as synchrotron primary monochromators and end-mirrors for x-ray lasers. These two examples illustrate the range of environments and performance expectations. A multilayer in the synchrotron application will attain a moderately high steady-state temperature in a hard vacuum, and it will be expected to maintain a reliably constant thickness of its layers without warping or wrinkling. In the laser application, the multilayer is expected to-normally reflect a predetermined wavelength, but only for a short period. The damage mechanisms (and therefore the designs and fabrications) are different for the pulsed and cw applications. Damage mechanisms . r; Diffraction and reflection arises from regularly spaced, smooth layers with different refractive indexes. Damage mechanisms reduce smoothness, decrease the refractive index gradient at the interface, or cause irregularities in the layer thicknesses. Multilayers are typically made by condensation from low pressure gases (e.g., by evaporation, sputtering or chemical vapor deposition). Such methods seldom produce layers which are in chemical equilibrium with respect to crystallization, although microcrystals may be present. They may also be out of equilibrium with respect to chemical bond formation. For example, a multilayer composed of equal numbers of W and C atoms could presumably form a single, homogeneous layer of the compound WC. In addition, entropy of mixing may drive diffusion of the materials into one another. In most cases, the rate of crystallization, chemical reaction, and diffusion can be ignored at room temperature, but may become objectionable at some higher temperature.Because the temperature is proportional to the rate of absorbed radiation, materials should be selected that minimize total multilayer absorption of all incident radiation (not just the wavelengths of interest). What is more, severe strain between layers can develop if their thermal expansion coefficients are not well matched. The residual stress resulting from fabrication or annealing could cause enhancement or relief, depending upon its direction. Low absorbance could help keep the temperature below that where mechanical failure of the layers occurs.In cw applications, melting will distort the layer spacing, if not by flow under the influence of gravity, then by surface tension. In the anticipated pulsed applications, there is not enough time for liquid flow and so it appears that melting can be ignored, at least as a first approximation. Direct vaporization of solid materials (sublimation) can completely remove an unprotected material exposed to a vacuum ove...

Design of X-UV multilayers for high X-ray flux

“…Design of X-UV multilayers for high X-ray flux L. V. Knight (1), J. M. Thorne (1), A. Toor (2) and T.W. Barbee Jr. (2) (1) Departments of Physics and Chemistry, Brigham Young University, Provo, Utah 84602, U.S.A.…”

“…Toor (2) and T.W. Barbee Jr. (2) (1) Departments of Physics and Chemistry, Brigham Young University, Provo, Utah 84602, U.S.A.…”

“…Several groups have contributed to the theoretical and experimental understanding required to develop such a design code. They have considered damage mechanisms capable of degrading the performance of multilayers [1][2][3][4][5]. Thorne et al [2] have studied the optimum materials for damage resistant performance for both continuous-use and pulsed applications.…”

“…They have considered damage mechanisms capable of degrading the performance of multilayers [1][2][3][4][5]. Thorne et al [2] have studied the optimum materials for damage resistant performance for both continuous-use and pulsed applications. Methods of estimating multilayer damage as a function of time during periods of irradiation have been developed [1][2][3][4][5][6][7].…”

“…Thorne et al [2] have studied the optimum materials for damage resistant performance for both continuous-use and pulsed applications. Methods of estimating multilayer damage as a function of time during periods of irradiation have been developed [1][2][3][4][5][6][7]. Some experimental damage data has been obtained in X-ray laser experiments [8][9][10], and in experiments specifically designed to make a quantitative measurement of the reflectivity degradation of a multilayer mirror while it was subjected to a short pulse of intense X radiation [3,4,6,7,11].…”