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. 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.