Characterization of Thick Cryogenic Fuel Layers: Compensation for the Lens Effect Using Convergent Beam Interferometry

Historically, the fuel content and fuel-layer uniformity of cryogenic targets have been characterized interferometrically using plane-wave i l l~mination.l-~ This technique has the sensitivity necessary to detect a deviation from sphericity of the fuel layer's inside surface as small as a few percent of its total thickness. In the past at LLE, the targets examined were typically 250-pm-diam glass capsules with wall thicknesses of a few micrometers that were filled with enough fuel to produce a condensed fuel layer less than 10 pm in thickness. Future OMEGA Upgrade cryogenic targets will consist of polymer capsules several tens of micrometers thick with diameters ranging from 700-1 100 pm. These will be filled with condensed D2 or DT fuel with a thickness of up to 100 pm. A capsule with a thick cryogenic layer condensed on its interior behaves as a strong negative lens, which has several adverse effects on its interferogram when created with planewave illumination. Computer simulations of typical interferograms are shown in Fig. 58.24. The highly divergent and spherically aberrated wavefront created by the target cannot be effectively collected and imaged using optics with convenient numerical apertures, resulting in loss of information near the perimeter of the target's image. In addition, when this highly curved wavefront interferes with a planar reference wavefront, an interferogram with a fringe spatial frequency that increases radially to very high values near the perimeter of the target's image is produced. Since the phase, and therefore