Implosion dynamics and x-ray generation in small-diameter wire-array Z pinches.

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.

Nonlinear development of stimulated Raman scattering from electrostatic modes excited by self-consistent non-Maxwellian velocity distributions.

The parametric coupling involving backward stimulated scattering of a laser and electron beam acoustic modes (BAM) is described as observed in particle-in-cell (PIC) simulations. The BAM modes evolve from Langmuir waves (LW) as the electron velocity distribution is nonlinearly modified to be non-Maxwellian by backward stimulated Raman scattering (BSRS). With a marginal damping rate, BAM can be easily excited and allow an extended chirping in frequency to occur as later SRS pulses encounter modified distributions. Coincident with the emergence of this non-Maxwellian distribution is a rapid increase in BSRS reflectivities with laser intensities. Both the reflectivity scaling with laser intensity and the observed spectral features from PIC simulations are consistent with recent Trident experiments.