VOLUME 83, NUMBER 21 PHYSICAL REVIEW LETTERS 22 NOVEMBER 1999 Multiphase Foamlike Structure of Exploding Wire Cores S. A. Pikuz,* T. A. Shelkovenko,* D. B. Sinars, J. B. Greenly, Y. S. Dimant, and D. A. Hammer † Laboratory of Plasma Studies, Cornell University, Ithaca, New York 14853 (Received 22 April 1999) X-ray backlighter images (radiographs) of current-induced explosions of 7.5 25 mm diam metal wires show for the first time mm scale, time-resolved details of a persistent foamlike liquid-vapor structure of the expanded wire core. Experiments with refractory and highly resistive metals, with current rising to 2–5 kA per wire in 350 ns, show that a substantial portion of the wire material is not vaporized but remains in a condensed state. As the current damps out, the remaining liquid phase material coalesces into separate droplets. PACS numbers: 52.80.Qj, 51.50. + v, 68.35.Rh, 83.70.Hq Recent experiments on the Z accelerator [1] have gener- ated extremely high-power x-ray pulses from the implosion of multiwire array Z-pinch loads. In order to understand those experiments, several groups are attempting to de- velop the capability to model the whole process starting from the cold wire state [2 – 4]. Until now, however, there has been very little experimental information on the ini- tial stages of wire explosion. The existence of long-lived dense cores within lower density coronal plasmas during nanosecond explosions of single wires or multiwire arrays is well established [5]. However, the physical state of the core material and the processes occurring there have been only a matter of conjecture. Using the technique of high resolution x-ray backlighting (radiography) [6], we have found complex small-scale (typical sizes of 2 30 mm) structure of the exploding wire cores of both single-wire and multiwire loads subjected to 2–5 kA per 7.5 25 mm diam wire for 1 ms. The radiographs show features im- plying that the core has become a foamlike, liquid-vapor mixture that is a result of explosive volume boiling. Most of the energy which drives these wire explosions is deliv- ered during the initial 50 ns, when the current is only a few hundred amperes per wire. (The existence of a mul- tiphase initial stage in current-driven wire explosions, but with a very different morphology, was described with pho- tographs of microsecond discharges by Chace in 1959 [7].) The persistence of liquid-phase material is demon- strated by the eventual formation of columns of droplets along the original wire position after the discharge current decays to zero in W, Mo, NiCr, and Ti wire explosions. By contrast, Al, Au, Cu, and Ag wires, all high conduc- tivity, relatively low melting point materials, form fully vaporized, more uniform expanding columns of wire ma- terial, evidently because the energy deposited in them dur- ing the critical first 50 ns of the current pulse is sufficient to cause the multiphase condition to be short lived. Our results demonstrate that it is essential in modeling wire ex- plosions to include a whole range of material properties, as well as the formation of plasma around the wire from desorbed gases and vaporized wire material. The current waveform applied to each of these wires is similar to that at the very beginning of the pulse on the Z accelerator. The main diagnostic technique in the experiments re- ported here is pulsed point-projection radiography of the exploding wires. The x-ray sources are very short-time (,0.5 ns) x-ray bursts from collapsing 13 25 mm diam Mo-wire X pinches (Fig. 1), the characteristics of which are described in detail elsewhere [6]. In brief, Mo-wire X pinches are found experimentally to produce submicron, bright x-ray sources in the 2.5 – 10 keV x-ray energy range when the radiation is filtered by a 12.5 mm Ti foil; the spectrum consists of free-bound continuum and several L- shell lines. Radiographs are obtained of 1 to 4 wires ex- ploded in parallel by a damped sinusoidal pulse of current which reaches a peak amplitude of up to 4.5 kA in about 350 ns (the auxiliary pulser in Fig. 1) and has an e-fold damping time of about 4 ms. The Mo-wire X pinches, usually two in parallel, separated by 2 cm, are driven us- ing the 400 – 450 kA peak current XP pulser [8]. The setup gives radiographs at two times during each pulse, with the interval between the radiographs variable from near 0 to about 20 ns by changing the X -pinch wire size. A filtered film stack gives up to three radiographs of the wire(s) being tested for each X -pinch x-ray burst. The spatial resolution is 1 3 mm, depending upon the X -pinch quality and the FIG. 1. Schematic view of the experimental setup showing the x-ray backlighter source positions relative to the exploding wire array being radiographed. 0031-9007 99  83(21)  4313(4)$15.00 © 1999 The American Physical Society 4313