Use of Incoherence to Produce Smooth and Controllable Irradiation Profiles with KrF Fusion Lasers

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Abstract
A technique called echelon-free induced spatial incoherence is proposed for producing smooth, controllable target beam profiles with large KrF fusion lasers. The idea is basically an image projection technique that projects the desired time-averaged spatial profile F(x) onto the target via the laser system, using partially coherent broadband light. The information needed to reproduce F(x) is transported through the system by a multitude of independent coherence zones, whose diameters are small compared to scalelengths of linear aberration and gain nonuniformities; as a result, F(x) remains relatively insensitive to these effects. This concept is closely related to the induced spatial incoherence technique used with glass lasers, except that it does not require echelons at the output of the system. An analysis is carried out to evaluate the perturbations of F(x) due to linear aberration, self-focusing, gain saturation, and diffraction. It shows that under conditions applicable to large KrF lasers, the perturbations will result in a small broadening and smoothing of F(x), whose functional form should be controllable to within a few percent. The ability of this technique to generate smooth focal profiles is demonstrated using a small KrF discharge oscillator-preamplifier system.