Damage threats and response of final optics for laser-fusion power plants

M. S. Tillack, S. A. Payne, and N. M. Ghoniem

The final beam steering optics for laser-IFE power plants will be located in the direct line-of-sight of the target emissions. Damage threats include high-energy neutrons and gamma rays, x-rays and ionic debris, and contamination by condensable species and particulate. Experiments and modeling are being performed in order to understand the mechanisms for damage, to characterize the design and performance limitations in a fusion environment, and to offer solutions to the most challenging problems.

For transmissive optics, radiation-induced darkening is a particularly serious concern. Highly pure SiO2 and CaF2 have been irradiated and the increased absorption coefficients compared with known color centers. The possibility of reducing the damage through annealing at elevated temperature has been evaluated. In addition, molecular dynamics calculations of 14 MeV neutron collisional cascades and their resulting defects will be discussed and the results compared with experiments.

For reflective optics, the primary design option considered is grazing-incidence metal mirrors. For these, the most serious concern is reduction in laser damage threshold due to long-term exposure and contamination. Unstable surface deformations due to operation at grazing incidence angles are possible and will be evaluated. Both experiments on LIDT and modeling of defects will be presented.

Future R&D needs and prospects for extrapolation to IFE power plants will be discussed.