High-power 2- to 6-μm window material figures of merit with edge cooling and surface absorption included

Abstract

Values of the total power $\mathcal{P}$ that a window can transmit under specified conditions are calculated for use as figures of merit for window materials. New features of the figures of merit are consideration of edge as well as face cooling, treatment of surface as well as bulk absorption, use of $\mathcal{P}$ rather than intensity as the figure of merit, effects of improving materials, and calculation for 3.8 and 5.25 μm. New results include the following: For continuous operation, large‐diameter windows should be face cooled, but small‐diameter window may be edge cooled, and $\mathcal{P}$ often decreases as the diameter D increases. For pulsed operation, $\mathcal{P}$ increases as D increases, as was intuitively expected. Values of the diameter D_EF, above which face cooling should be used, are surprisingly large, ranging between 7 and 100 cm, typically. For pulsed operation and for D > D_EF in cw operation, $\mathcal{P}$ is independent of thermal conductivity K while for D < D_EF in cw operation, $\mathcal{P}\mathrm{\propto K}$ . The alkaline‐earth fluorides have the greatest figures of merit for large‐diameter windows with $\mathcal{P}\text{=76\hspace{0.17em}}\mathrm{MW}$ , ΔT = 50 K, and l=0.5 cm for a 1‐sec pulse on a 10‐cm‐diam window of BaF₂ at 3.8 μm with a bulk absorption coefficient of 10⁻⁴ cm⁻¹. For small‐diameter windows (1‐cm diameter), Si, BaF₂, and GaAs have the greatest values of $\mathcal{P}\text{=1.8}$ , 0.94, and 0.76 MW, respectively, for cw or 1‐sec pulse duration.