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Flash x radiography of laser‐accelerated foils

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8 Author(s)
Whitlock, R.R. ; U. S. Naval Research Laboratory, Washington, D.C. 20375‐5000 ; Obenschain, S.P. ; Grun, J. ; Stamper, J.A.
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The successful compression of laser‐driven pellets to thermonuclear ignition depends on the stability and uniformity of the motion with which dense shells can be imploded. The motion of planar foils accelerated by the Pharos II laser has been studied by two‐dimensional, flash x radiography employing pinhole imaging and slitted crystal imaging. The acceleration was driven by a 3–5‐ns duration, 1.05‐μm laser focused to 3–6×1012 W/cm2 in a millimeter diameter spot, while a second laser beam of shorter duration produced the x‐ray flash for imaging purposes. The x‐ray images obtained clearly show that the planar foil targets are ablatively accelerated to velocities of 3×106 cm/s while maintaining a density above 3% of solid. The axial extent of the accelerated, high‐density material has been observed to be as small as 25% of the distance traveled. The sides of accelerated portions of the foil connect smoothly to the stationary regions removed from the laser illumination. This connection apparently isolates the rear surface from the hot ablation plasma and helps explain the low rear surface temperatures which have been observed. The overall appearance of the accelerated foil is localized and nearly planar at early times when it has moved distances small compared to its diameter. The x‐radiographic results are in general agreement with two‐dimensional hydrodynamic simulations and with earlier Naval Research Laboratory studies of target motion, ablation pressures, and symmetrization employing other diagnostics.

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Journal of Applied Physics  (Volume:61 ,  Issue: 1 )