RESUMEN
High-resolution laser diagnostics at the wavelength of 266 nm were applied for the investigation of Z pinches at the 1-MA generator. The internal structure of the stagnated Z pinches was observed in unprecedented detail. A dense pinch with strong instabilities was seen inside the column of the trailing plasma. Kink instability, disruptions, and micropinches were seen at the peak of the x-ray pulse and later in time. The three-dimensional structure of the stagnated Z pinch depends on the initial wire-array configuration and implosion scenario. Small-scale density perturbations were found in the precursor plasma and in the stagnated Z pinch. Development of instabilities is in agreement with three-dimensional magnetohydrodynamic simulations.
RESUMEN
Wire-array plasmas were investigated in the nonradiative ablation stage via x-ray absorption spectroscopy. A laser-produced Sm plasma was used to backlight Al wire arrays. The Sm spectrum was simultaneously observed by two spectrometers: one recorded the unattenuated spectrum and the other the transmission spectrum with 1.45-1.55 keV K-shell absorption lines. Analysis of absorption spectra revealed electron temperature in the range of 10-30 eV and the presence of F-, O-, N- and C-like Al ions in the absorbing plasma. A comparison of this electron temperature with the postprocessed absorption spectra of a 2D MHD simulation yields results in general agreement with the data analysis.
RESUMEN
It is known from experiments that the radiated x-ray energy appears to exceed the calculated implosion kinetic energy and Spitzer resistive heating [C. Deeney, Phys. Rev. A 44, 6762 (1991)] but possible mechanisms of the enhanced x-ray production are still being discussed. Enhanced plasma heating in small-diameter wire arrays with decreased calculated kinetic energy was investigated, and a review of experiments with cylindrical arrays of 1-16 mm in diameter on the 1 MA Zebra generator is presented in this paper. The implosion and x-ray generation in cylindrical wire arrays with different diameters were compared to find a transition from a regime where thermalization of the kinetic energy is the prevailing heating mechanism to regimes with other dominant mechanisms of plasma heating. Loads of 3-8 mm in diameter generate the highest x-ray power at the Zebra generator. The x-ray power falls in 1-2 mm loads which can be linked to the lower efficiency of plasma heating with the lack of kinetic energy. The electron temperature and density of the pinches also depend on the array diameter. In small-diameter arrays, 1-3 mm in diameter, ablating plasma accumulates in the inner volume much faster than in loads of 12-16 mm in diameter. Correlated bubblelike implosions were observed with multiframe shadowgraphy. Investigation of energy balance provides evidence for mechanisms of nonkinetic plasma heating in Z pinches. Formation and evolution of bright spots in Z pinches were studied with a time-gated pinhole camera. A comparison of x-ray images with shadowgrams shows that implosion bubbles can initiate bright spots in the pinch. Features of the implosions in small-diameter wire arrays are discussed to identify mechanisms of energy dissipation.
RESUMEN
High-resolution laser probing diagnostics at a wavelength of 266 nm allow observation of the internal structure and instabilities in dense stagnated Z pinches, typically hidden by trailing material. The internal structure of the 1-MA Z pinch includes strong kink and sausage instabilities, loops, flares, and disruptions. Mid- and small-scale density perturbations develop in the precursor and main pinch. The three-dimensional shape and dynamics of the wire-array Z pinch are predetermined by the initial configuration of the wire array. Cylindrical, linear, and star wire-array Z pinches present different sets of instabilities seeded to the pinch at the implosion stage. Prolonged implosion of trailing mass can enhance x-ray production in wire arrays. Fast plasma motion with a velocity >100 km/s was observed in the Z pinch at stagnation with two-frame shadowgraphy. Development of instabilities in wire arrays is in agreement with three-dimensional magnetohydrodynamic simulations.
RESUMEN
Implosions in starlike triple and quadruple wire arrays were investigated in a 1 MA Zebra generator. Implosion in these loads is directed along the rays of the star and cascades from wire to wire to the center. Shadowgraphy shows improved homogeneity of imploding plasma and mitigation of instabilities. Despite the low azimuthal symmetry, starlike wire arrays produce a stable x-ray pulse with the highest peak power of >0.4 TW and the shortest duration of 8-12 ns among different types of tested loads. This can be linked to stabilization of instabilities due to the multiple nesting.