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 robust final optics in a laser inertial fusion energy (IFE) power plant. The amount of laser light the grazing incidence liquid metal mirror (GILMM) can withstand, called the damage limit is limited by the surface disturbances initiated by rapid laser heating. For 0.35-μm light, the damage limit for a sodium film 85° from normal is calculated to be 57 J/cm2 normal to the beam for a 20 ns pulse and 1.3 J/cm2 for a 10-ps pulse (2 and 90 m2 of mirror area per 100 kJ of laser energy at 20 ns and 10 ps, respectively). Feasibility relies on keeping the liquid surface flat to the required accuracy by a combination of polished substrate, adaptive (deformable) optics, surface tension and low Reynolds number, laminar flow in the film. The filmʹs substrate must be polished to ±0.015 μm. Then surface tension keeps the surface smooth over short distances (<10 mm) and low Reynolds number laminar flow keeps the surface smooth (disturbances less than ±0.01 μm) over long distances (>10 mm). Adaptive optics techniques keep the substrate flat to within ±0.06 μm over 100-mm distance and ±0.6 μm over 1000 mm distances, even after 30 years of cumulative damage via neutron 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 significant rise in temperature, forcing the use of other X-ray attenuation methods such as attenuation by xenon gas. The GILMM should be applicable to both direct and indirect drive and pulse lengths appropriate to slow compression (∼20 ns) or fast ignition (∼10 ps). For direct-drive laser beams near the poles (70°, where 90° is vertical), stable thin films become more challenging. Proof of the concept experiments are needed to verify the predicted damage limit and required smoothness.