Optimal X-Ray Pulse Compression with Compact Nested Wire Arrays On Z

Abstract

Summary form only given. Compact nested tungsten wire arrays operating in current transfer mode produce peak soft x-ray powers of 200 to 250 TW on the Z accelerator. Such arrays use outer (inner) array diameters of 20 (12) mm and also employ low density, 4 to 6 mm diameter CH2 foam pulse shaping targets on the axis. These are among the highest powers produced on Z, at any array diameter. Optimized single tungsten wire arrays produce peak powers of 160 TW at 20 mm diameter. These nested arrays also produce the narrowest x-ray bursts of any arrays on Z (1.9 to 2.5 ns) compared to 4 to 7 ns with single arrays. X-ray radiography at 6.151 keV is used to determine the mass density profile as a function of space and time. The density profiles are obtained from the radiographs via Abel inversion. These data indicate that the narrowing of the x-ray pulse of nested arrays is correlated to a narrowing of the measured spatial width of the mass distribution at the axis near stagnation. The nested mass distribution narrows as a result of three factors: (1) Current transfer mode operation allows the Magneto-Rayleigh-Taylor (MRT) perturbation and wavelength to be reset to smaller values at the time of current transfer to the inner array, consistent with an ablation phase of the inner wire array. The wavelength and amplitude of the outer array is not impressed on the inner at switching. The MRT wavelength and perturbation is reduced by a factor of ~3.4 compared to a single wire array when compared at the same implosion radius. (2) Current transfer mode allows the outer array mass to fill the interior of the inner wire array. Thus the MRT growth on the inner wire array is stabilized by mass accretion. (3) Finally, low-density axial foam targets also change the mass profile near the axis and further narrow the pulse through mass accretion. Such optimized nested arrays may permit a peak power of 100 TW in 5 ns width from a 10-mm diameter array. This more compact array would be compatible with driving a scale-0.6 double-ended hohlraum ICF concept, which would permit pulsed-power ignition with fusion yields of 50 MJ at half of the required facility energy storage of the scale-1.0 double-ended hohlraum design (400 MJ yields).