Design, Fabrication, and Testing of an f/0.3 Achromatic Focusing System

One of the crucial problems in laser fusion research is the efficient delivery of enormous amounts of power (typically several terawatts) to irradiate pellets of approximately 100 micrometer diameter. An illumination system consisting of a pair of ellipsoidal mirrors and aspheric single element lenses has been used with a two beam laser for several years at KMSF and has been very successful. 1. The lens surfaces are so steep that it is very difficult to apply anti-reflective coatings. 2. The lens must be very thick to accomodate short radii of curvature. The deleterious effects of high power beams propagating through refractive media are well known. 3. The lens also causes the system to be stigmatic for only one wavelength. The last item is especially important as recent experience has indicated that pellet energy absorption is increased by harmonically generated short wavelength and/or broadband laser light.