IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 36, NO. 5, OCTOBER 2008 2759 Quantitative Measurements of Wire Ablation in Tungsten X -pinches at 80 kA Simon C. Bott, Member, IEEE, David M. Haas, Yossof Eshaq, Utako Ueda, Sergey V. Lebedev, Member, IEEE, Jeremy P. Chittenden, Member, IEEE, James B. A. Palmer, Simon N. Bland, Member, IEEE, Gareth N. Hall, Member, IEEE, David J. Ampleford, Member, IEEE, and Farhat N. Beg, Member, IEEE Abstract—This paper investigates the ablation of wires in two-wire tungsten X-pinches driven by an 80-kA current over 50 ns. High-resolution imaging using a Nomarski interferometer allows measurements close to the X-pinch cross point, where the ablation “flare” structure is observed to clearly develop during the drive-current rise time. Electron density profiles are recovered as a function of both distance normal to the wire and of time. Results compare favorably to the rocket model of wire ablation. In addition, the density contrast over the ablation “stream” and “gap” structure is measured and compared to similar measure- ments made using quantitative radiography on the 1-MA 250-ns MAGPIE generator at Imperial College London, London, U.K. Index Terms—Precursor plasma, wire ablation, X-pinch. I. I NTRODUCTION T HE UNDERSTANDING of the ablation phase of explod- ing wire experiments is of fundamental importance to their continued development. In cylindrical wire arrays, this phase comprises up to 80% of the experiment, and the mass redistrib- ution resulting from wire ablation is crucial to the generation of impressive X-ray powers measured from imploding wire- array Z -pinches [1] and, hence, their application to inertial confinement fusion research. When a fast-rising current is passed through fine wires, a heterogeneous plasma structure is formed: A cold dense core is surrounded by a low-density hot corona which carries much of the drive current [2]–[4]. Where a global magnetic field is Manuscript received September 30, 2007; revised November 9, 2007. First published October 24, 2008; current version published November 14, 2008. This work was supported by U.S. Department of Energy Junior Faculty Grant DE-FG02-05ER54842. S. C. Bott is with the Center for Energy Research, University of California, San Diego, La Jolla, CA 92093 USA (e-mail: sbott@ucsd.edu). D. M. Haas, Y. Eshaq, U. Ueda, and F. N. Beg are with the Department of Mechanical and Aerospace Engineering, University of California, San Diego, La Jolla, CA 93093 USA (e-mail: fbeg@ucsd.edu; dmhaas@ucsd.edu; yeshaq@ucsd.edu; uueda@ucsd.edu). S. V. Lebedev is with the Plasma Physics Group, Blackett Labora- tory, Imperial College London, London SW7 2AZ, U.K., and also with Budker Institute of Nuclear Physics, Novosibirsk 630090, Russia (e-mail: S.Lebedev@imperial.ac.uk). J. P. Chittenden, S. N. Bland, and G. N. Hall are with the Plasma Physics Group, Blackett Laboratory, Imperial College London, London SW7 2AZ, U.K. (e-mail: J.Chittenden@imperial.ac.uk; SN.Bland@imperial.ac.uk; Gareth.Hall@imperial.ac.uk). J. B. A. Palmer is with the Plasma Physics Department, AWE Plc, Aldermaston RG7 4PR, U.K. (e-mail: James.Palmer@imperial.ac.uk). D. J. Ampleford is with Sandia National Laboratories, Albuquerque, NM 87185-1194 USA (e-mail: DAMPLEF@Sandia.gov). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TPS.2008.2003964 present, the low-density corona is swept to the system axis by the J × B global force. The rate at which mass is ablated from the wire cores to replenish the corona is, in general, well approximated by a rocket model, assuming a fixed velocity of the ablated material [5] V abl dm dt = μ 0 I 2 4πR 0 (1) where V abl is the fixed “ablation” velocity, dm/dt is the mass ablation rate per unit length, I is the drive current, and R 0 is the array radius. The acceleration of material from wires is not axially uniform, however, and all systems with a global field demonstrate a periodic radial flaring structure. This has been observed by both laser imaging and radiography at different current levels for many different experiments, including cylin- drical [6], [7] and conical [8] wire arrays and X-pinches [9]. The cause of this structure is currently not clear. A modified m =0 magnetohydrodynamic (MHD) instability [10] and an electrothermal instability [11] are two of the several possible candidates, and experimental information is needed to define both the underlying mechanism and its likely scaling with driver current. In an X-pinch, the global magnetic field changes along the Z -axis as wire separation increases, and therefore offers an op- portunity to study the variation of the ablation rate with this pa- rameter and to determine whether the rocket model provides an adequate description in this case. Measurements of the global ablation rate and flare wavelength have been made for conical wire arrays at larger diameters and higher drive currents [8], but this paper is the first study of these phenomena for X-pinch experiments. It should be noted that laser interferometry has been used previously to study X-pinch evolution, notably in [12], but this work focuses on the quantitative measurement of the ablation structure close to the wire core. Mass ablation rates of X-pinches at 80 kA are then compared to cylindrical wire ar- rays at the 1-MA MAGPIE facility at Imperial College London. II. EXPERIMENTAL SETUP The X-pinch pulser at UCSD comprises a Marx bank (4 × 0.2-μF capacitors charged to 50 kV), a coaxial discharge line, a water-filled pulse-forming line, and a self-breaking switch (SF 6 at 18 lbf/in 2 ). This typically delivers 80 kA to a load with a rise time of 50 ns. The load is formed from two wires of 7.5-μm tungsten. These are hung initially parallel between two electrodes, which 0093-3813/$25.00 © 2008 IEEE Authorized licensed use limited to: Univ of Calif San Diego. Downloaded on November 19, 2008 at 19:05 from IEEE Xplore. Restrictions apply.