Grazing incidence liquid metal mirrors (GILMM) for radiation hardened final optics for laser inertial fusion energy power plants

A thin film of liquid metal is suggested as a grazing incident mirror for r obust final optics in a laser inertial fusion energy (IFE) power plant. The amount of laser light the grazing incidence liquid metal mirror (GILMM) ca n withstand, called the damage limit is limited by the surface disturbances initiated by rapid laser heating. For 0.35-mum light, the damage limit for a sodium film 85 degrees from normal is calculated to be 57 J/cm(2) normal to the beam for a 20 ns pulse and 1.3 J/cm(2) for a 10-ps pulse (2 and 90 m(2) of mirror area per 100 kJ of laser energy at 20 ns and 10 ps, respecti vely). Feasibility relies on keeping the liquid surface flat to the require d accuracy by a combination of polished substrate, adaptive (deformable) op tics, surface tension and low Reynolds number, laminar flow in the film. Th e film's substrate must be polished to +/- 0.015 mum. Then surface tension keeps the surface smooth over short distances (< 10 mm) and low Reynolds nu mber laminar flow keeps the surface smooth (disturbances less than +/- 0.01 <mu>m) over long distances (> 10 mm). Adaptive optics techniques keep the substrate flat to within +/- 0.06 mum over 100-mm distance and +/- 0.6 mum over 1000 mm distances, even after 30 years of cumulative damage via neutro n irradiation. The mirror can stand the X-ray pulse when located 30 m away from the microexplosions of nominal yield of 400 MJ (50 MJ of X-rays) when Li is used, but for higher atomic number liquids like Na there may be a sig nificant rise in temperature, forcing the use of other X-ray attenuation me thods such as attenuation by xenon gas. The GILMM should be applicable to b oth direct and indirect drive and pulse lengths appropriate to slow compres sion (similar to 20 ns) or fast ignition (similar to 10 ps). For direct-dri ve laser beams near the poles (70 degrees, where 90 degrees is vertical), s table thin films become more challenging. Proof of the concept experiments are needed to verify the predicted damage limit and required smoothness. (C ) 2000 Elsevier Science B.V. All rights reserved.