Mitigation of a novel phase-defect-induced laser damage mechanism on NIF final optics

Operating the National Ignition Facility (NIF) near its power and energy performance limits has revealed a new damage initiation mechanism in the final UV optics. The typical damage event involves the last three optics in the NIF beamline: the final focusing lens, the grating debris shield, and the target debris shield. It occurs on high power shots from intensifications from small phase defects (pits) on the exit surface of the focusing lens that travel through the grating debris shield before reflecting off the AR-coated target debris shield about 75 cm downstream, then propagate back upstream and damage the input surface of the grating debris shield optic which is 15 cm downstream of the focusing lens. Ray tracing has firmly established the direct relationship between the phase defects on the final focusing lens and the damage on grating debris via the reflection from the target debris shield. In some cases, bulk filamentary damage is also observed in the 1-cm thick fused silica grating debris shield. It is not fully understood at this point how there can be enough energy from the reflected beam to cause damage where the forward-going beam did not. It does not appear that interaction between the forward-going beam and the backward-going reflected beam is necessary for damage to occur. It does appear necessary that the target debris shield be previously exposed to laser shots and/or target debris. Furthermore, there is no evidence of damage imparted to the target debris shield or the final focusing lens. We will describe all the conditions under which we have (and have not) observed these relatively rare events, and the steps we have taken to mitigate their occurrence, including identification and elimination of the source phase defects.