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 et al., Phys. Rev. A 44, 6762 ͑1991͔͒ but possible mechanisms of the enhanced x-ray production are still being discussed. Enhanced plasma heating in smalldiameter 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 smalldiameter wire arrays are discussed to identify mechanisms of energy dissipation.
A study of the contribution of refluxing electrons in the production of K-alpha radiation from high-intensity laser irradiated thin targets has been performed. Thin copper foils both freestanding, and backed by a thick substrate were irradiated with laser pulses of energies around 100 J at intensities ranging from below 1017 to above 1019 W/cm2. At high laser intensities we find a strong reduction in the K-alpha yield from targets backed by the substrate. The observed yield reduction is in good agreement with a simple model using hot electron spectra from particle-in-cell simulations or directly inferred from the measured bremsstrahlung emission and can therefore be interpreted as due to the suppression of hot electron refluxing. The study shows that refluxing electrons play a dominant role in high-intensity laser driven K- alpha generation and have to be taken into account in designing targets for laser driven high-flux K-alpha sources.
Deuterium gas puff experiments were carried out on the S-300 Z-pinch at the Kurchatov Institute in Moscow. Gas puffs imploded onto the axis before a current peak at about 100 ns. Fusion neutrons were generated after the gas puff implosion during global expansion of a plasma column. Neutron emission lasted on average 35 ± 5 ns (full width half maximum, FWHM). In the downstream direction (on the Z-pinch axis behind the cathode), a mean neutron energy was 2.6 ± 0.1 MeV. Side-on neutron energy spectra peaked at 2.40 ± 0.05 MeV with about 600 ± 150 keV FWHM. A broad width of sideon neutron spectra implied a high radial component of deuteron velocities. An average kinetic energy of fast deuterons, which produced fusion neutrons, was 150 keV. A peak neutron yield reached a value of 6 × 10 10 on a current level of 1.5 MA. It was by one order higher in comparison with other deuterated loads used on the same current generator. On the basis of experimental observations, we concluded that a total energy of deuterons accelerated to fusion energies was above 1.5 kJ. It is more than 15% of the energy input into a plasma. Therefore gas puff Z-pinches seem to be not only powerful sources of x-ray radiation but also efficient sources of 100 keV deuterons. Such a result is consistent with high Plasma Phys. Control. Fusion 52 (2010) 065013 D Klir et al neutron yields observed on the Angara Z-pinch and plasma foci with similar currents.
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