IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 34, NO. 5, OCTOBER 2006 2325Jet Formation and Current Transfer in X-PinchesF. N. Beg, Member, IEEE, A. Ciardi, I. Ross, Y. Zhu, A. E. Dangor, and K. KrushelnickAbstract—Observations of X -pinch discharges driven by a160-kA 80-ns pulse current are reported. X -pinches consist-ing of aluminum and molybdenum mounted at different angles(120◦and 83◦) were studied. Coronal-plasma streaming, which isperpendicular to the wires toward the midplane of the X -pinch,was observed to form jets moving toward the electrodes on the ei-ther side of the cross point. The jet velocity (2.5 × 106cm · s−1)is similar for both molybdenum and aluminum wires and isslightly larger for the narrow angle (83◦). The jets were observedto show kink (m =1mode) instability at late times, after the jetshad come into contact with the electrodes. This is due to a transferof current to the jet. The experimental results agree qualitativelywith three-dimensional resistive MHD simulations, which includeradiation. The jets produced in X -pinches may be relevant to thestudy of astrophysical jets.Index Terms—Jets, m =1instability, X -pinch.I. INTRODUCTIONX-PINCHES have been studied for many years—primarilywith regard to the application as an efficient X-ray(hυ ∼ 1−10 keV) backlighter source for diagnosing denseplasmas. However, X-pinches [1], and references therein,can also have exotic properties (up to Te∼ 1 keV, ne∼1022cm−3), and consequently, they are a convenient and low-cost medium for the study of high energy density (> 1011J/m3[2]) and radiation-dominated plasma.In an X-pinch discharge, a large current is pulsed through thetwo wires, which are arranged in the form of an “X” betweenthe electrodes. At the cross point–where the two wires makecontact, i.e., at a predetermined position–the currents combineto pinch and produce a small blob of hot dense plasma. Themagnetic field is at least a factor of two larger at the crosspoint than in the individual wires. This dense region of theplasma emits a copious amount of intense X rays in a very shortduration pulse. At the termination of the X-ray pulse, a gap isformed in the plasma at the cross point, which normally givesrise to energetic electron and ion beams.Hammer and co-workers at Cornell University have madecomprehensive studies [3]–[6] of the X-pinches and have usedthem as backlighter sources to study the complex physics of theManuscript received December 30, 2005; revised May 17, 2006. The workof F. N. Beg was supported in part by the DOE Junior Faculty under GrantDE-FG02-05ER54842.F. N. Beg is with the Plasma Physics Group, Blackett Laboratory, ImperialCollege, London, U.K., and also with the Department of Mechanical andAerospace Engineering, University of California, San Diego, CA 92093 USA(e-mail: [email protected]).A. Ciardi, I. Ross, Y. Zhu, A. E. Dangor, and K. Krushelnick are with thePlasma Physics Group, Blackett Laboratory, Imperial College, London, U.K.Color versions of Figs. 1, 3, and 7 are available at http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/TPS.2006.878360initial stages of wire-array Z-pinch implosions [7]. In addition,it was determined that the bright X-ray emitting region can havea size less than 1 µm, an electron temperature of 1 keV, andan electron density, which can approach that of a solid. Thepulse duration of the X-ray emission at energies greater than3 keV has been observed to be less than 100 ps. Jets of theplasma are observed on either side of the cross point movingtoward the electrodes. These jets have been studied by an X-raybacklighting with another adjacent X-pinch [8] and recently byan optical probing [9], [10]. These latter observations indicatedqualitatively that the jets were formed by the coronal plasma,which flow toward the midplane early in the discharge and latein discharge by a compression and ejection of the plasma by amagnetic pressure at the cross point.Supersonic plasma jets produced in the laboratory have beenof particular interest recently, due to their potential similaritiesto the astrophysical phenomena. Highly collimated radiativejets have been observed using conical wire arrays [11]. In suchconfigurations, the ablated plasma from the wires is acceleratedtoward the axis via the magnetic field (J × B forces). Thisresults in a convergent flow perpendicular to wires, which formsa plasma column with an axial momentum, thus producing ahigh-velocity low-divergence jet due to a radiative cooling.In this paper, we present further investigations of the jetformation in the X-pinches. The results show that the su-personic jets are indeed formed by streaming of the coronalplasma, which are produced by the discharge in the wires to themidplane of the X-pinch, where there is a line of zero magneticfield, and where the plasma converges to form jets, whichadvance toward the electrodes. The jet formation is strongerand much more pronounced with narrow X-pinch angles (83◦as compared to 120◦). The jets in the narrow-angle X-pinchesare observed to be m =1unstable after the jets make contactwith the electrodes. The experimental results are compared withthree-dimensional (3-D) MHD simulations, which include aradiation and agree well qualitatively. According to our knowl-edge, this is the first such comparison of the X-pinch dynamics.II. EXPERIMENTThe experiments were carried out on the IMP generator atImperial College London, which is described in detail in [12].It consists of a Marx generator (eight stages, each stage havinga0.7-µF capacitor charged to a maximum of 100 kV), in whichthe pulse charges a water dielectric coaxial pulse-forming line(4Ω, 40-ns single transit time). A self-breaking SF6gas pressur-ized switch connects the line to the Z-pinch load via a transferline with a matched impedance and identical transit time. The90◦elbow in the transfer line was to accommodate a verticalX-pinch load. A schematic of the generator is in Fig. 1. The0093-3813/$20.00 © 2006 IEEE2326 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 34, NO. 5, OCTOBER 2006Fig. 1. Schematic of IMP current generator. Inlet shows the electrode configuration with an X-pinch angle.X-pinch was positioned between the electrodes separated by20 mm. The chamber was evacuated to below 10−2Pascal. Inmost shots, a surface breakdown occurred late in time alongthe Lucite (polymethylmethacrylate) insulator in the vacuumon the X-pinch side. This crowbarred the current in the pinch