Bulletin of the American Physical Society
49th Annual Meeting of the Division of Plasma Physics
Volume 52, Number 11
Monday–Friday, November 12–16, 2007; Orlando, Florida
Session UO6: Z Pinches and X Pinches |
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Chair: Christine Coverdale, Sandia National Labs Room: Rosen Centre Hotel Salon 5/6 |
Thursday, November 15, 2007 2:00PM - 2:12PM |
UO6.00001: Perspectives on Recent Progress in Developing Ar K-shell Z-Pinch Sources J.P. Apruzese, R.J. Commisso, B.V. Weber, D. Mosher, F.C. Young, J.W. Thornhill, J.L. Giuliani, A.L. Velikovich, J. Davis, J.S. Levine, B.H. Failor, H. Sze, N. Qi, J.W. Bannister, P.L. Coleman, C.A. Coverdale Various developments, including a three-plenum, ``pusher-stabilizer-radiator'' gas puff nozzle, have culminated in the doubling of achievable Ar K-shell yields on Sandia's Saturn generator, from 38 to 75 kJ. This was accomplished at comparatively long implosion times of $\sim $ 200 ns. This talk briefly reviews the more than 15 years of steady progress which brought this advance about. The Ar Z-pinches studied over that time have produced K-shell yields ranging from a few kJ on PITHON to $\sim $ 300 kJ on the Z generator. They have been analyzed with a consistent spectroscopic method in an effort to empirically ascertain what properties make such pinches good radiators. We also examine how much they depart from Bennett equilibrium. [Preview Abstract] |
Thursday, November 15, 2007 2:12PM - 2:24PM |
UO6.00002: The development of instabilities in wire array Z-pinches Jeremy Chittenden, Christopher Jennings The X-ray power produced by wire array Z-pinches is ultimately limited by the uniformity and the characteristic width of the imploding plasma structure. This uniformity is in turn determined by spatial variations in the ablation rate of the individual wires earlier in time. These variations exhibit a fixed and highly periodic structure with a characteristic natural wavelength. The reasons for this fixed structure and the origin of such perturbations have been the subject of much debate in recent years. Using three-dimensional MHD calculations of a single wire in an array, we show that such variations can arise as the result of an m=0 like instability growing in the plasma ablating from each wire. The structure of this instability is modified by the magnetic topology of the array and is shown to grow spontaneously from noise to adopt a fixed wavelength which is consistent with experimental observations. Using separate three-dimensional MHD simulations of the entire array volume, we then show how the separate modulations from each wire combine to form a perturbation to the array implosion as a whole. The effect of the non-linear development of this perturbation on the X-ray power pulse is then illustrated. This work was partially supported by the U.S. Department of Energy through cooperative agreement DE-FC03-02NA00057. [Preview Abstract] |
Thursday, November 15, 2007 2:24PM - 2:36PM |
UO6.00003: Bounds for Kinetic Energy and Resistance for W wire array implosions on Z Eduardo Waisman, Michael Cuneo, Daniel Sinars, William Stygar, Raymond Lemke Electrical and radiation data are analyzed for wire array z-pinches on the Z machine.The measured stack voltage and MITL current are employed to obtain the voltage at the Z vacuum convolute, and from it and the measured load current an upper bound for the wire array load inductance is constructed. Energy conservation is then employed to obtain the kinetic energy a few ns before stagnation, by observing that knowledge of the load inductance upper bound and the load current provide an upper bound for the magnetic energy downstream of the vacuum convolute, and that the internal energy is negligible at that time. An approximate lower bound for the pinch resistance is obtained at a time when the radiated X-ray power has decreased to a half of its peak value by assuming that at this point the pinch kinetic energy is negligible and by estimating the internal energy of the tungsten plasma from its apparent size and radiated power. Several single W wire array shots are analyzed using this technique. By further assuming that after the end of the ablation phase the pinch breaks up in two thin shells of prescribed mass their paths, as well as the current split between the two, are obtained self-consistently thereafter. Comparisons of the kinetic energy bound with results of 3D RMHD calculations for selected shots are provided. Insights gained by this data analysis are presented. [Preview Abstract] |
Thursday, November 15, 2007 2:36PM - 2:48PM |
UO6.00004: Faster, 80ns, current scaling experiments yield higher radiated x-ray power and approach quadratic dependence Michael G. Mazarakis, Michael E. Cuneo, William A. Stygar, Henry C. Harjes, Daniel B. Sinars, Brent M. Jones, Christopher Deeney, Eduardo M. Waisman, Thomas J. Nash, Kenneth W. Struve, Dillon H. McDaniel We report the results of the latest series of current scaling experiments with the Z accelerator. The novelty of this work is the shorter implosion times of 80 ns as compared with the 95ns of the previously reported work. Our results show a radiated x-ray peak power and total radiated x-ray energy current scaling closer to quadratic than the results of Stygar \textit{et al. }[Phys. Rev E \textbf{69}, 046403 (2004)]. If we include in the analysis all our thirteen shots, we find that the x-ray peak radiated power scales as the 1.57 power of the peak load current and the total x-ray radiated energy as the 1.9 power. However, we found that the power flow to the load during the shot 1608 was severely curtailed. If we eliminate that particular shot we find that the x-ray peak radiated power and total radiated energy scale respectively as 1.71 and 2.01 power. The present results are compared with the predictions of a heuristic and enhanced resistivity models. [Preview Abstract] |
Thursday, November 15, 2007 2:48PM - 3:00PM |
UO6.00005: Measurement of Doppler implosion velocity in wire array z pinches B. Jones, J.E. Bailey, G.A. Rochau, C.A. Coverdale, M.E. Cuneo, Y. Maron, V. Fisher, V. Bernshtam, A. Starobinets, C. Deeney Determination of z-pinch implosion velocity and thus coupled kinetic energy is key to understanding energy balance in wire array radiation sources. We discuss diagnosis of velocity via observation of Doppler shifts in K-shell lines for Al implosions on the 20 MA Z machine. For a small-mass load, classic oval-shaped Mg dopant lines are observed with a time- resolved, 1D-imaging (radial) spectrometer; Doppler splitting corresponding to $\sim$50 cm/$\mu$s is seen in the view through the pinch axis, while no splitting is seen in the view tangent to the imploding shell. Velocity can also be inferred from Doppler-shifted absorption lines observed in a large-mass wire array implosion when analyzed with a collisional-radiative and radiation transport model of a multi-shell plasma (emission from the hot core backlights the colder imploding mass). [Preview Abstract] |
Thursday, November 15, 2007 3:00PM - 3:12PM |
UO6.00006: Ablated Wire Material and Coronal Structure Interactions David Haas, Simon Bott, Yossof Eshaq, Utako Ueda, Farhat Beg Results are presented from experiments studying the mass ablation, propagation, and jet development exhibited in pulsed power experiments. Jets were formed from x-pinches and low wire number conical wire arrays. They were observed to form between the electrodes and propagating above the anode. All results were obtained on a compact pulsar having a maximum current of 80 kA with a rise-time of 50ns. Optical probing diagnostics included side on interferometry, shadowgraphy, and schlieren imaging. Time gated x-ray images were recorded simultaneously to trace the emission profile. Several wire materials, including aluminum, tungsten, and stainless steel, were investigated to determine the effect of radiation cooling. Jet parameters such as velocity, density, and temperature were investigated, along with dimensionless parameters to assess scalability to astro-physical regimes. Laboratory scale plasma jet interaction with side winds will be attempted. [Preview Abstract] |
Thursday, November 15, 2007 3:12PM - 3:24PM |
UO6.00007: The Implosion of Radial Wire Array Z-Pinches S.N. Bland, S.V. Lebedev, F. Suzuki-Vidal, J.P. Chittenden, G.N. Hall, A. Harvey-Thompson, J.B.A. Palmer, D.J. Ampleford, K. Chandler-Mitchell, M. Mitchell Radial wires array z-pinches - where the wires stretch radially outwards from a central cathode - offer a number of advantages over cylindrical arrays. Imploding in a similar fashion to a dense plasma focus, plasma from a radial array is projected above the electrodes, allowing experimental access over an entire hemisphere. The implosion is also compact and so is suitable for coupling to small scale hohlraums. Additionally radial arrays can be matched to low impedance generators and may enable the use of long pulsed drive currents. Here we present measurements of the dynamics of radial wire array z-pinches, examining wire ablation, implosion and the dynamics of the stagnating column. The scaling of power and yield of soft and hard X-rays with array configuration is explored. The effect of placing the radial wire array within a hohlraum is examined, as are nesting techniques both for increasing wire number and symmetry and for Xray pulse shaping. This research was supported by the NNSA under DOE Cooperative Agreement DEFC03 - 02NA00057 [Preview Abstract] |
Thursday, November 15, 2007 3:24PM - 3:36PM |
UO6.00008: The Structure of Ablated Plasma from Coiled Wire Arrays G.N. Hall, S.N. Bland, S.V. Lebedev, J.P. Chittenden, J.B.A. Palmer, F.A. Suzuki-Vidal, S.C. Bott Coiled arrays, a cylindrical array in which each wire is formed into a helix, suppress the modulation of ablation at the fundamental wavelength. Outside the vicinity of the wire cores, ablation flow from coiled arrays is modulated at the coil wavelength and has a 2- stream structure in the r,? plane. Within the vicinity of the helical wires, ablation is concentrated at positions with the greatest azimuthal displacement and plasma is axially transported from these positions such that the streams become aligned with sections of the coil furthest from the array axis. The GORGON MHD code accurately reproduces this observed ablation structure, which can be understood in terms of axial JxB forces that result from the interaction of the global magnetic field with radial components of a helical current path as well as additional current paths suggested by the simulations. These experiments were performed on MAGPIE (1MA, 240ns) at Imperial College. This research was sponsored by Sandia National Laboratories Albuquerque, the SSAA program of NNSA under DOE Cooperative Agreement DE-FC03-02NA00057. [Preview Abstract] |
Thursday, November 15, 2007 3:36PM - 3:48PM |
UO6.00009: Ablation of wires in an inverse wire array z-pinch Sergey Lebedev, G.N. Hall, S.N. Bland, F.A. Suzuki-Vidal, J.P. Chittenden, C. Jennings, A. Harvey-Thompson, Cheng Ning, J.B.A. Palmer We describe experiments on the MAGPIE facility (1MA, 250ns) with inverse wire array z-pinches, in which the wires act as a return current cage placed around a central current conductor. In this configuration the plasma ablated from the wires is pushed by the JxB force in the radially outward direction and expands into the region free of the magnetic field. This allows quantitative characterisation of the plasma ablated from an individual wire using laser interferometry, X-ray radiography and XUV imaging. The inverse z-pinch configuration also allows to measure separately the contribution to the inductance coming from the ``private'' magnetic flux of the wires and thus to evaluate the size of the current-carrying region around the wire cores. Quantitative information obtained in these experiments will be compared with results of 3-D MHD computer simulations. [Preview Abstract] |
Thursday, November 15, 2007 3:48PM - 4:00PM |
UO6.00010: Investigation of implosions in star-like wire arrays on the 1-MA Zebra generator V.V. Ivanov, V.I. Sotnikov, A. Haboub, A.L. Astanovitskiy, A. Morozov, S.D. Altemara, C. Thomas, S. Batie, V. Nalajala, A.P. Shevelko, E.D. Kazakov Implosions in star-like wire arrays, consisting of multiple rays of linear wire rows extending from the central axis, were investigated in the 1-MA Zebra generator. Arrays with 12-24 wires placed on 3-8 rays and 3-6 cylinders were tested. In star-like arrays the imploding plasma cascades to the center from wire to wire along rays of the star and forms plasma columns with a smooth leading edge. Shadowgraphy shows improved plasma homogeneity and mitigation of instabilities. Despite the low azimuthal symmetry, star-like wire arrays produce a stable x-ray pulse with the enhanced peak power of 0.4 TW and the shortest duration of 8-12-ns among other loads in this series of shots. Dynamics of the implosion in star-like wire arrays are compared with cylindrical and nested arrays. Work was supported by the DOE/NNSA under UNR grant DE-FC52-06NA27616. [Preview Abstract] |
Thursday, November 15, 2007 4:00PM - 4:12PM |
UO6.00011: Analysis of implosion and spectroscopic characteristics of combined planar wire arrays composed from low- and mid-z wire materials on the 1MA pulsed power generator at UNR A.S. Safronova, V.L. Kantsyrev, N.D. Ouart, M.F. Yilmaz, K. Williamson, V. Shlyaptseva, I. Shrestha, G. Osborne, C.A. Coverdale, C. Deeney Analysis of single combined planar wire array experiments is presented. In these experiments, which were conducted at the 1 MA pulsed power generator at UNR, the Z-pinch load consisted of two Al and several Cu alloyed wires mounted in a single linear row with a 1 mm gap. Two wires of the primary Cu material were replaced by Al wires with the similar mass at three different locations. Implosion and radiative characteristics of such arrays were studied in connection with the particular location of two Al wires using filtered PCD and XRD detectors, a bare bolometer, time-gated and time-integrated pinhole cameras, and time-integrated, spatially resolved and time-gated spatially-integrated spectrometers. Non-LTE kinetic models were used to provide plasma parameters and to estimate opacity effects. This work was supported by NNSA under DOE Coop. Agr. DE-FC52-06NA27588, DE-FC52-06NA27586, and in part by DE-FC52-06NA27616. Sandia is a multi-program laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States department of Energy under Contract DE-AC04-94AL85000. [Preview Abstract] |
Thursday, November 15, 2007 4:12PM - 4:24PM |
UO6.00012: Implosion dynamics and radiation features of planar, compact cylindrical, and nested wire arrays on 1 MA, 100 ns z-pinch generators V.L. Kantsyrev, A.S. Safronova, A.A. Esaulov, K. Williamson, I. Shrestha, G. Osborne, N.D. Ouart, M.F. Yilmaz, V. Shlyaptseva, J.B. Greenly, J.D. Douglass, R.D. McBride, D.A. Chalenski, D.A. Hammer, B.R. Kusse, L.I. Rudakov, A.S. Chuvatin Plasma formation and implosion features of planar wire arrays (PWA), compact cylindrical wire arrays (CWA), and low-wire number nested wire arrays (NWA) of the small size (6-15 mm) were studied on the 1 MA, 100 ns UNR Zebra and Cornell COBRA generators. The powers and yields were maximum for Mo double PWA, followed by W compact CWA and PWA, Mo single PWA and compact CWA, stainless steel (SS304) and Al compact CWAs, and Al/SS304 and SS304/SS304 NWAs. Despite different implosion dynamics of PWAs and compact CWAs they formed plasma that radiated similar yields and powers. The possibility of radiation pulse shaping by varying geometry and materials of PWAs and NWAs will be discussed. Simulations with Wire Dynamics Model and 2D MHD model with enhanced resistivity will be presented. Work was supported by DOE/NNSA under Coop. Agr. DE-FC52-06NA27586, DE-FC52-06NA27588, DE-F03-02NA00057, and in part by DE-FC52 06NA27616. [Preview Abstract] |
Thursday, November 15, 2007 4:24PM - 4:36PM |
UO6.00013: Plasma Density Measurements within Tungsten Wire-Array Z-Pinches on the COBRA Accelerator Jon Douglass, Ryan McBride, Kate Bell, Patrick Knapp, John Greenly, Sergei Pikuz, Tanya Shelkovenko, David Hammer The COBRA pulsed-power generator, with a nominal peak current of 1.1 MA and a minimum zero-to-peak rise-time of about 100ns, is being used to study the early phases of wire-array z-pinch development with a variety of diagnostics. Here we present the results of applying point-projection x-ray radiography to make accurate, high-resolution spatial and temporal measurements of the plasma density distributions in tungsten (W) wire-array z-pinch implosions. Density measurements are quantified by comparing x-ray transmission recorded on photographic films to transmission through W calibration steps of known thicknesses. Plasma density distributions as a function of time are presented for the coronal (10$^{18}$-10$^{20}$/cm$^{3})$, ablation ($<$10$^{18}$/cm$^{3})$ and on-axis ($<$10$^{19}$/cm$^{3})$ plasmas during the pre-stagnation phases of z-pinch dynamics (70-170 ns after the start of the current pulse). With this data set the time dependence of ablation velocity and corresponding mass ablation rate are addressed. [Preview Abstract] |
Thursday, November 15, 2007 4:36PM - 4:48PM |
UO6.00014: Implosion dynamics of wire-array z-pinches on the COBRA accelerator R.D. McBride, K.S. Bell, I.C. Blesener, D.A. Chalenski, J.D. Douglass, J.B. Greenly, P.F. Knapp, S.A. Pikuz, T.A. Shelkovenko, T. Blanchard, H. Wilhelm, D.A. Hammer, B.R. Kusse Experimental results characterizing wire-array z-pinch implosion dynamics on the 1-MA, 100-ns rise time COBRA pulsed power generator are presented. Diagnostics fielded include an optical streak camera, a time-gated XUV framing camera, a laser shadowgraph system, filtered time-integrated pinhole cameras, a focusing x-ray spectrometer with spatial resolution (FSSR), a load voltage monitor, a faraday cup, a bolometer, silicon diodes and diamond photoconducting detectors (PCDs). The load geometries investigated in this set of experiments include cylindrical arrays ranging from 6 to 16 mm in diameter, and consisting of 8, 16, or 32 wires of either aluminum (Al) or tungsten (W). The data produced by the entire suite of diagnostics are analyzed and presented to provide an overall picture of implosion dynamics and timing on COBRA. In particular, data fitting to various implosion trajectory models, as well as x-ray pulse shape dependencies on various loads and implosion characteristics are presented and discussed. [Preview Abstract] |
Thursday, November 15, 2007 4:48PM - 5:00PM |
UO6.00015: Soldered contact effect on wire array Z-pinches David Chalenski, Bruce Kusse, John Greenly The Cornell University COBRA pulser is a nominal 1MA machine, capable of driving up to 16 wire cylindrical Z-pinch arrays. COBRA can operate with variable current risetimes ranging from 100ns to 250ns. Wires are typically strung with a ``press'' contact to the electrode hardware, where the wire is loosely pulled against the hardware and held there to form an electrical contact. Previous research with single wires on a 1-5kA pulser has shown that soldering the wire, thereby improving the wire/electrode contact, can improve the expansion of and energy deposition into the wire. Previous experiments on Z (T.W.L. Sanford et al., Phys. Plasmas 12, 122701 (2005)) have shown that improving the contact improves the X-ray yield. Data were collected on 16- and 8-wire, Aluminum, Copper and Tungsten Z-pinch arrays, with radii ranging from 16mm to 8mm. Experiments were conducted with both slow and fast risetimes (100ns and 200ns). The initiation, ablation, implosion and stagnation phases were compared for soldered and unsoldered arrays. Soldering appeared to produce a smaller radius pinch and improve X-ray yield in properly massed arrays. [Preview Abstract] |
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