2430 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 39, NO. 11, NOVEMBER 2011 Anode–Cathode Asymmetry in a Wire-Array Z -Pinch: Highly Resolved Axial-Shear-Flow Structure Observed on the Outer Edges of Ablating Wires Ryan D. McBride, Charles E. Seyler, Sergei A. Pikuz, David A. Hammer, David J. Ampleford, Tania A. Shelkovenko, and Matthew R. Martin Abstract—Presented is a laser-backlit image of a tungsten wire-array Z -pinch at 1 MA. This image shows highly resolved (to about 20–40 μm) anode–cathode-asymmetric wavelike structure on the outer edges of the ablating wires. The development of this structure implies that axial shear flow occurs in ablating wire-array Z -pinches. Index Terms—Ablation, COBRA, optical imaging, plasma di- agnostics, plasma transport processes, pulsed power, wire-array, x-ray source, Z -pinch. P RESENTED in Fig. 1 is a laser-backlit image of a wire- array Z -pinch experiment conducted on the Cornell Beam Research Accelerator (COBRA) [1]. The initial configura- tion consisted of eight 12.5-μm-diameter tungsten (W) wires, arranged as a single 16-mm-diameter 20-mm-tall cylindrical array. Due to the angle of the array relative to the laser path, only five of the eight ablating wires are visible in the full image in Fig. 1(b). However, at this angle, both the left- and rightmost wires are imaged without obstruction from the remaining six wires. The image in Fig. 1 was obtained with a frequency-doubled Nd:YAG laser (532 nm) with a 4.3-ns pulse duration. The imag- ing system and experimental setup are shown schematically in Fig. 1(a). The spatial resolution of this system was found to be about 20–40 μm by imaging well-defined objects and measuring the edge-spread function [2]. The uniformity of the imaging beam was enhanced with multipass spatial filtering. The image shown in Fig. 1 is a combination of shadowgra- phy, light-field schlieren (i.e., a limiting aperture rather than a beam stop), and time-integrated self-emission. Depending on the spatial gradients of a given scattering element, densities of Manuscript received December 2, 2010; revised July 12, 2011; accepted July 24, 2011. Date of publication September 8, 2011; date of current version November 9, 2011. This work was supported in part by the Stewardship Science Academic Alliances Program of the National Nuclear Security Administration under U.S. Department of Energy contract DE-FC03-02NA00057 and in part by Sandia National Laboratories, a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000. R. D. McBride, D. J. Ampleford, and M. R. Martin are with the Sandia National Laboratories, Albuquerque, NM 87185 USA. C. E. Seyler, S. A. Pikuz, D. A. Hammer, and T. A. Shelkovenko are with the Laboratory of Plasma Studies, Cornell University, Ithaca, NY 14853 USA. Digital Object Identifier 10.1109/TPS.2011.2163732 roughly ∼5 · 10 18 electrons/cm 3 and higher contribute to the backlit portion of the image. The image in Fig. 1 was taken approximately 180 ns into the current pulse with COBRA operating in short-pulse mode (i.e., 0–1-MA peak in 100 ns) [1]. For the array configuration tested, this image time is very late in the ablation phase of the wire-array Z -pinch process [3] and about 10–20 ns prior to the start of the main implosion of the bulk of the wire plasma. The array mass and diameter were chosen such that the im- plosion occurred well after the 1-MA peak on COBRA to reduce the implosion efficiency and resulting pinch radiation at stagnation and thus reduce the time-integrated self-emission in the image. This also extended the duration of the ablation phase (persisting for about 200 ns). As structure is known to evolve slowly during the ablation phase [3] (relative to our laser pulse duration), the image in Fig. 1 is well resolved in time and space. Interestingly, the image in Fig. 1 shows a 500-μm axial period for the plasma streams flowing from the ablating wires to the array axis. This is twice the commonly referenced 250-μm period for W [3]. The growth of this period to 500 μm is likely a result of the wire-array configuration tested and the mechanisms discussed in [4]. The image in Fig. 1 also clearly shows anode–cathode- asymmetric wavelike structure on the outer edges of the ablating wires. For this asymmetric structure to develop from initially symmetric solid W wires, an electron- and/or ion- driven axial shear flow must occur over some period of time prior to when the image was taken. This asymmetry is also reported in [3, Fig. 2(c)], where it is observed that the plasma flares extending radially outward from the wires are inclined at an angle of about 45 ◦ toward the anode. This observation appears consistent with the image shown here in Fig. 1. Ad- ditionally, the high resolution of the image in Fig. 1 shows that steep wavefronts are present and that these wavefronts, and/or the troughs just ahead of them (above them), are strongly correlated with the plasma streams flowing from the ablating wires to the array axis. In closing, we note that the anode–cathode asymmetry shown in the image in Fig. 1 is not accounted for by a single- fluid magnetohydrodynamic model. That is, with a single-fluid model, there is no mechanism to drive an asymmetric axial flow in a purely cylindrical wire-array Z -pinch geometry. At the very least, calculating this asymmetry requires a two-fluid model 0093-3813/$26.00 © 2011 IEEE