Mitigation of cross-beam energy transfer in symmetric implosions on OMEGA using wavelength detuning

Abstract

The effects of frequency detuning laser beams in direct-drive symmetric implosions were investigated with a 3-D cross-beam energy transfer (CBET) model. The model shows that interactions between beams with relative angles between 45° and 90° are most significant for CBET in OMEGA direct-drive implosions. There is no net exchange in power between beams, but there is significant redistribution of power from the ingoing central portion of the beam profile to the outgoing edge as it is exiting the plasma, reducing the total absorbed power. Redistribution of laser power because of CBET increased the root-mean-square (rms) absorption nonuniformity by an order of magnitude. CBET mitigation by shifting relative wavelengths of three groups of laser beams fed by each of the different beamlines was modeled. At an on-target wavelength shift of Δλ ∼ 10 Å, the total laser absorption was maximized, and the rms absorption nonuniformity was near minimum. To completely decouple the three groups of beams from each other requires wavelength shifts Δλ > 30 Å.