NIF is a glass laser at 1.05 µm with frequency conversion crystals that frequency triple the light to 0.35 µm. As a result of the frequency conversion process, the primary source of prepulse is 1.05-µm light. This is controlled at the 3ω focus by the dispersion of the WFL. There is an offset of the footprint of 1ω unconverted light from the 3ω focus with a 4.8-mm clearance.
The effect of prepulse on a target depends on the composition and orientation of the target surfaces. As a general guideline, intensities above about 108 W/cm2 will begin to form weakly ionized plasma on a metal surface [see Spaeth et al. NIF Performance (2016)]. The NIF prepulse will exceed 108 W/cm2 of 1.05-µm light approximately 3–5 ns before the nominal start of the laser pulse shape.
