Low-Mode Asymmetries in Direct-Drive Implosion Prediction and Correction Using 3-D Modeling of Beam Balance, Beam Pointing, and Beam-Polarization Cross-Beam Energy Transfer Effects*

A growing body of evidence suggests OMEGA implosions are more asymmetric than predictions. In particular, the low-mode-number asymmetries of core flow (mode 1) and a prolate/oblate shape to the core (mode 2) are commonly observed in the compressed core during cryogenic implosions. The source of these persistent drive nonuniformities has been studied using warm implosions that remove some of target-related uncertainties of cryogenic implosions. Fully 3-D cross-beam energy transfer modeling of implosions beam balance, beam pointing, and beam polarization is shown to predict mode‑1 absorption asymmetries that correspond well to the observed core flow direction. Modeling of implosions also predict absorption mode-2 asymmetries that correlate to observed compressed core shapes. The low-mode effects of beam balance, pointing, and polarization are found to be systematic and persistent from shot to shot, raising the possibility of correcting low-mode absorption nonuniformity by adjusting the beam balance to zero the modes. This may have advantages over the current procedure of using target offset to attempt to zero the core flow during cryogenic implosions.