Unabsorbed light beamlets for diagnosing cross-beam energy transfer.

A new diagnostic has been fielded on OMEGA to diagnose cross-beam energy transfer (CBET) during direct-drive implosions. Unabsorbed light from each OMEGA laser beam is imaged as a distinct "spot" onto a gated optical imager. Each spot is in essence the endpoint of a beamlet of light that originates from different regions of each beam profile and follows a path determined by refraction. The intensity of light in the beamlet varies along its path as a result of absorption and CBET with other beamlets. This diagnostic allows the investigation of the effects of CBET on laser energy from specific locations of the beam profile. The diagnostic records images in two 200-ps time windows and includes a Wollaston prism to split each beamlet into two orthogonal polarizations recorded on separate images, allowing the absolute polarization of each beamlet to be determined. This diagnostic has provided the first evidence of polarization rotation caused by CBET during direct-drive implosions.