Dynamics of cylindrically converging precursor plasma flow in wire-array Z-pinch experimentsS. C. Bott,1S. V. Lebedev,1D. J. Ampleford,2S. N. Bland,1J. P. Chittenden,1A. Ciardi,1M. G. Haines,1C. Jennings,2M. Sherlock,1G. Hall,1J. Rapley,1F. N. Beg,3and J. Palmer41The Blackett Laboratory, Imperial College, London SW7 2BW, United Kingdom2Sandia National Laboratory, Albuquerque, NM USA3Department of Mechanical and Aerospace Engineering, University of California San Diego, California, USA4AWE Plc., Aldermaston, RG7 4PR, United Kingdom共Received 6 April 2006; revised manuscript received 24 July 2006; published 12 October 2006兲This paper summarizes the present understanding of the processes leading to precursor column formation incylindrical wire arrays on the 1 MA MAGPIE generator at Imperial College London. Direct experimentalmeasurements of the diameter variation during the collapse and formation phase of the precursor column arepresented, along with soft x-ray emission, and quantitative radiography. In addition, data from twisted cylin-drical arrays are presented which give additional information on the behavior of coronal plasma generated inwire array z pinches. Three stages in precursor column formation are identifiable from the data: broad initialdensity profile, rapid contraction to small diameter, and slow expansion after formation. The correlation ofemission to column diameter variation indicates the contraction phase is a nonlinear collapse resulting from theincreasing on-axis density and radiative cooling rate. The variation in the minimum diameter is measured forseveral array materials, and data show good agreement with a pressure balance model. Comparison of columnexpansion rates to analytical models allows an estimate of column temperature variation, and estimates of thecurrent in the column are also made. Formation data are in good agreement with both fluid and kineticmodeling, but highlight the need to include collisionless flow in the early time behavior.DOI: 10.1103/PhysRevE.74.046403 PACS number共s兲: 52.20.Hv, 52.59.Qy, 52.70.LaI. INTRODUCTIONA wire array z-pinch experiment displays several distinctstages throughout its duration, from wire initiation to implo-sion and x-ray generation. On longer current drive共⬎100 ns兲 machines the processes occurring prior to the im-plosion of the main array mass comprise a significant pro-portion of the drive time, which presents an opportunity tostudy these in detail. One of the most interesting phenomenato develop during this period is a remarkably uniform andstable precursor plasma column on the axis of the array.Since its first identification 关1兴, precursor column structureshave been observed in a large number of experiments andwithin a wide range of array parameters 关1–3兴. The arrival ofthis mass fraction on the array axis, preimplosion, is due tothe formation of a heterogeneous plasma structure from eachwire caused by the passing of the drive current: relativelycold wire cores surrounded by a low-density hot coronalplasma 关4–7兴. The radial J ⫻B force accelerates the coronalplasma towards the axis, while the cold wire cores are con-tinuously ablated. It is the stagnation of this flow at the axiswhich forms the observed precursor column.This uniform and stable plasma object by itself deservessome attention due to an interesting combination of plasmaphysics processes occurring during its formation. These in-clude a transition from collisionless to collisional regimes ofinteraction of the plasma flow, the effect of radiation coolingon the dynamics of the plasma, and the development of ion-ization balance. In wire array z pinches, the convergingplasma flow is often sustained for a long time 共⬎100 ns兲 andin the precursor column these processes can develop in aquasistationary situation. The high degree of the azimuthalsymmetry of the plasma flow in this system makes it analmost ideal test bed for one-dimensional 共1D兲 radiation hy-drodynamic codes, and indeed is closely analogous to anexisting problem in hydrodynamics: Noh’s problem 关8兴. Theformed column is an interesting candidate for high energydensity physics 共HEDP兲 experiments, and one can also notea close analogy between the precursor formation in wire ar-ray Z pinches and stagnation of plasma on axis of symmetryin other systems with converging plasma flows, e.g., of theplasma ablated from the walls of ICF hohlraums 关9兴.The effect of the precursor plasma flow on the implosiondynamics of wire array z pinches and on parameters of thex-ray pulses generated by the implosion is actively debatedat present. Among reasons for this interest is the unexpectedremarkable performance of large diameter wire arrays, whichhave relatively large interwire separations 共⬎3mm兲关10兴.The core-corona plasma structure and the resulting inwardflow of coronal plasma is apparently important even for thearrays with very small interwire separations 共⬃0.21 mm兲,asfollows from experiments 关11兴 on the Z facility at SNL, inwhich a very early arrival of the precursor plasma on thearray axis was detected. For these arrays it is difficult toenvision formation of a plasma shell this early in the currentpulse, and hence the discrete nature of the wires should playan important role in their operation. The prefilling of theinterior of an array with the plasma should be taken intoaccount not only for understanding of x-ray pulse generation,where the accretion of mass during implosion may reduceinstabilities at the accelerating plasma front, but also in thedesign of different wire array z-pinch loads. This could beespecially important for the concepts of dynamic 关12兴andstatic wall 关13兴 hohlraums, where the precursor flow can sig-nificantly change conditions of the foam targets installed onthe array axis 关14兴. To this end, accumulation of experimen-PHYSICAL REVIEW E 74, 046403 共2006兲1539-3755/2006/74共4兲/046403共21兲 ©2006 The American Physical Society046403-1tal information on formation and behavior of the precursorplasma in wire array Z pinches is required. Experiments withrelatively low currents and long rise times can help to high-light important processes, and hence evolve a better under-standing of the corresponding physical processes throughcomparison with computer and analytical models, and shouldbe useful for establishing scaling laws.In this paper we present an experimental study of thedynamics of the precursor plasma