The goal of the European laser fusion project, is to build an engineering facility for repetitive laser operation (HiPER 4a)
and later a fusion reactor (HiPER 4b). A key aspect for laser fusion energy is the final optics. At the moment, it is based
on silica transmission lenses located 8 m away from the chamber center. Lens lifetime depends on the irradiation
conditions. We have used a 48 MJ shock ignition target for calculations. We have studied the thermo-mechanical effects
of ions and X-rays on the lenses. Ions lead to lens melting and must therefore be mitigated. On the other hand, X-rays
(~1% of the energy) does not produce either a significant temperature rise or detrimental stresses. Finally, we calculated
the neutron flux and gamma dose rate on the lenses. Next, based on a simple model we studied the formation of color
centers in the sample, which lead to optical absorption. Calculations show that simultaneous neutron and gamma
irradiation does not significantly increase the optical absorption during the expected lifetime of the HiPER 4a facility.
Under severe conditions (HiPER 4b), operation above 800 K or lens refreshing by thermal annealing treatments seem to
assure adequate behavior.
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