Qualification of a near backscattering imaging system on the National Ignition Facility

A near backscattering imaging diagnostic system has been implemented, qualified, and fielded on the first quad of beams on the National Ignition Facility [E. M. Campbell and W. J. Hogan, Plasma Phys. Controlled Fusion 41, B39 (1999)]. This diagnostic image diffusing scatter plates, placed around the final focus lenses on the NIF target chamber, to quantitatively measure the fraction of light backscattered outside of the incident cone of the focusing optics. The imaging system consists of a wide-angle lens coupled to a gated charged coupled device camera, providing 3 mm resolution over a 2 m field of view. To account for changes of the system throughput due to exposure to target debris the system was routinely calibrated in situ at 532 and 355 nm using a dedicated pulsed laser source. The diagnostic and calibration methods will be described together with recent results from the NIF early light shots