Search

Implosion characteristics of cylindrical arrays of aluminum (Al) or tungsten (W) wires with low number of wires were studied on the 1-MA 100-150-ns current rise time COBRA generator at Cornell University using X-ray/extreme ultraviolet detectors, time-gated cameras, spectrometers, and electrical diagnostics. Total radiation yields [E.sub.T] of 2.8 and 4.1 kJ were measured for Al and W, respectively. The yield above 0.75 keV for W arrays was lower than for Al. Al spectra imply [T.sub.e] of 250-280 eV. X-ray spectra in the 2-3-keV range from W arrays showed only very weak spectral lines. A relatively uniform precursor plasma column was observed on time-gated images in the initial implosion phase for both Al and W. Rapid radiation cooling of W plasmas after precursor plasma formation leads to a decrease of plasma [T.sub.e] before the stagnation phase of the implosion. No evidence of the increased energy deposition that could be associated with the single-shell or closed toroidal magnetic structures models was found. Index Terms--Wire arrays, X-ray imaging, X-ray source, X-ray spectroscopy, Z-pinch.

The radiative performance of Al, Ni, and W planar wire arrays, to which little energy could be coupled via the conventional magnetic-to-kinetic conversion mechanism, is investigated. However, the planar wire arrays were shown to radiate much more energy in a short intense peak than possible due to dissipation of the kinetic energy. The planar array gives the unique possibility of seeing the evolution of the small-scale inhomogeneity of wire-array plasmas during wire ablation and implosion phases and highlights the importance of the Hall plasma phenomena and their impact on the dynamics, energy coupling, and radiation performance of wire-array Z-pinches. Index Terms--Planar wire array, X-ray diagnostics, X-ray radiation source, Z-pinch.

A Faraday rotation diagnostic was applied for the investigation of magnetic fields in plasma of 1-MA wire arrays and X-pinches. Laser-probing diagnostics at the Zebra generator include a four-channel polarointerferometer and a four-frame shadowgraphy. The Faraday rotation diagnostic consists of shadow and Faraday channels, shearing air-wedge interferometer, and an additional schlieren channel. The implosion dynamics of the wire arrays were studied. A current in the plasma column of AI low-wire number arrays was found by the Faraday rotation diagnostic. Optical diagnostics showed a turbulent plasma and bubblelike objects in the plasma column of Al wire arrays. The Faraday rotation diagnostic demonstrated a complicated structure of magnetic fields in X-pinch plasma. Index Terms--Faraday effect, laser probing, plasma turbulence, wire-array Z-pinch, Z-pinch plasmas.

Experiments with X-pinches made with both Mo and W wires have been performed on 1-MA pulsed power generators at Cornell University and University of Nevada, Reno. X-ray images and spectra have been studied and compared for three different configurations of X-pinch loads with Mo and W wires. For all X-pinches, the image size decreases with decreasing wavelength and photoconducting diode (PCD) signals show multiple bursts except for one variant of the mixed Mo and W configurations. Time-gated, as well as time-integrated, images indicate the presence of radiation from energetic electrons. Previous experience with application of L-shell Mo modeling to various Z- and X-pinch experiments helped to determine plasma parameters in the X-pinches studied here, and permitted identification of M-shell W spectral features useful for plasma parameter estimation. Index Terms--L-shell Mo radiation, modeling, M-shell W radiation, X-pinch, X-ray imaging, X-ray spectroscopy.

In the following experiments, we studied implosions of different wire arrays and X-pinches produced on the 1-MA Zebra generator at the University of Nevada, Reno. Diagnostics included both spatially-resolved and time-gated X-ray imaging and spectroscopy, and laser probing. In particular, we compared planar wire arrays, to which little energy could be coupled via the conventional magnetic-to-kinetic conversion mechanism, to cylindrical wire arrays of comparable dimensions and mass. The planar wire arrays were shown to radiate much higher peak power and more energy in subkiloelectronvolt and kiloelectronvolt spectral ranges than cylindrical wire arrays. We tested the theoretical conjecture that enhanced resistivity due to the small-scale inhomogeneity of wire-array plasmas has a major effect on dynamics, energy coupling and radiation performance of wire-array Z-pinches. The study of Al, Alumel, and W cylindrical wire arrays shows a wide variety of characteristic behaviors in plasma implosions discussed hereinafter. Additional experimental results for symmetric and asymmetric, uniform stainless steel, Cu, Mo, combined Al/Mo, Mo/Al, Al/W, W/Al, and Mo/W X-pinches are also presented. New data for the total radiation yield are obtained. The planar structures of X-pinch plasma and the corresponding electron beam was observed for most of X-pinches. The generation of hot spots along original wires positions--cooler than those from the cross-wire region--and arc structures with hot spots between wires were found for X-pinches composed from Al, Cu, and W wires. Index Terms--Wire arrays, X-pinches, X-ray imaging, X-ray spectroscopy, Z-pinch.