Numerical modeling of effects of power imbalance on irradiation nonuniformities

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Abstract
We report on the importance of various sources of power imbalance of the 24 beam OMEGA laser system in contributing to laser irradiation nonuniformity. Long spatial wavelength irradiation nonuniformities on target are investigated numerically by adapting existing uniformity codes to incorporate time‐dependent density and temperature profiles. The analysis shows that power imbalance due to slightly detuned (≤100 μrad) frequency conversion crystals and small differences in the ultraviolet (UV)‐transport losses (ΔE/E≤5%) makes only a modest contribution to the overall nonuniformity. However, power imbalance from pulse shape distortion was found to create substantial irradiation nonuniformity on the target early in the implosion. The predictions of the present analysis were verified by an experiment in which a well controlled power imbalance was applied. Good agreement was found between the experimentally observed core positions and those predicted by the simulation.