Development and fielding of high-speed laser shadowgraphy for electro-magnetically driven cylindrical implosions

Abstract

A laser shadowgraphy system for high-speed imaging of a convergent cylindrical shockwave generated by an electro-magnetically driven solid density liner implosion in Lucite is described. The laser shadowgraphy system utilizes an advanced high-energy, long-pulse, frequencydoubled Nd:YAG laser for target illumination and a fast framing camera for multiple frame imaging of the shockwave as it radially converges and transits the Lucite. The time window resolution is 10 ns as determined by the fastest exposure time capable with the camera. Two onaxis symmetric implosions and two off-axis antisymmetric implosion experiments were fielded at the Air Force Research Laboratory's Shiva Star 4.2 MJ capacitor bank z-pinch facility. For each experimental shot, the shadowgraphy system captured several frames of shadowgraph images as the shockwave moved through the Lucite. Analysis of the shockwave shadowgraph image shapes is done by fitting each shadowgraph image to a generic elliptical fit function and plotting the resultant 2-D image fits for comparison. For the on-axis symmetric implosion shots, a radial shock velocity is calculated. The Lucite shock speed is seen to increase monotonically from an initial velocity of 7.9 mm/µs to a near final velocity of 13.4 mm/µs as convergence effects dominate the shock speed calculated at small radii.