Bulletin of the American Physical Society
51st Annual Meeting of the APS Division of Plasma Physics
Volume 54, Number 15
Monday–Friday, November 2–6, 2009; Atlanta, Georgia
Session GO5: Z-pinches, X-pinches, and Dense Plasma Focus |
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Chair: Mike Cuneo, Sandia National Laboratories Room: Hanover CDE |
Tuesday, November 3, 2009 9:30AM - 9:42AM |
GO5.00001: Structure of Imploding Plasma in Coiled Wire Arrays Gareth Hall, Sergey Lebedev, Simon Bland, Jeremy Chittenden, James Palmer, Francisco Suzuki-Vidal, Adam Harvey-Thompson, George Swadling, Guy Burdiak, Louisa Pickworth, Nicolas Niasse Coiled arrays, a cylindrical array in which each wire is formed into a single helix, suppress the modulation of ablation at the fundamental wavelength. Instead, ablation flow is modulated at the wavelength of the coil. Large wavelength coils produce wire beaks with sufficient axial separation that perturbations in the implosion sheath do not merge, producing an organised mode of implosion in which the global instability can be controlled and the perturbations correlated between all the wires in the array. The inductance of the current paths that may be established by an organised implosion are considered. This suggests that, for a given inductive voltage drop across the load, coiled arrays are capable of achieving significantly higher current convergence to the axis than straight arrays. These experiments were carried out on the MAGPIE generator 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] |
Tuesday, November 3, 2009 9:42AM - 9:54AM |
GO5.00002: Measurement of Linear Magneto-Rayleigh-Taylor Instability Growth in Solid Liners Daniel Sinars, Steve Slutz, Mark Herrmann, Kyle Peterson, Roger Vesey, Brent Blue The magneto-Rayleigh-Taylor (MRT) instability is ubiquitous to pinch plasmas compressed by magnetic pressure, and is an important factor in determining whether a cylindrical liner can reach the axis in a relatively intact form. While there are many RT characterization experiments, there are few well-characterized MRT experiments and none for fast ($\sim$100 ns) z-pinch implosions in which the magnetic pressure quickly dominates over material strength. We present data from an initial series of MRT growth experiments on the 20 MA Z-machine that used solid Al liners with outer radii of 3.16 mm and aspect ratios (radius/thickness) of 10. The MRT instability was seeded with sinusoidal perturbations of 200 and 400 $\mu$m and peak-to-valley amplitudes of 5 and 10 $\mu$m, respectively. Radiographs showing the evolution of the MRT instability are compared with codes being used to design magnetized liner inertial fusion loads [see S.A. Slutz, ``Magnetized liner inertial fusion,'' this conference.] [Preview Abstract] |
Tuesday, November 3, 2009 9:54AM - 10:06AM |
GO5.00003: Three dimensional simulations of DT fuel compression by an imploding Z-pinch liner J.P. Chittenden, B. Appelbe, S.A. Slutz, R.A. Vesey, K. Peterson, D.B. Sinars, M.E. Cuneo, M.C. Herrmann Z-pinch implosions provide a method of directly compressing fusion fuel to high densities and temperatures, with excellent energy coupling efficiency. The presence of large magnetic fields also provides a method of suppressing thermal conduction losses and increasing alpha particle confinement, significantly reducing the rho-R criterion required for ignition. We present three dimensional resistive magneto-hydrodynamic simulation results of one such scheme (S.A. Slutz et. al., this conference), in which a preheated, magnetised fuel is compressed by an imploding metal liner Z-pinch. Large scale parallel computing methods allowed the full liner volume to be modelled at relatively fine scale resolution. The results are used to examine how important the three dimensional structure of the magneto-Rayleigh-Taylor instability is in determining the neutron yield. Sensitivities of the fusion performance to fuel pre-heat, magnetic field configuration and current pulse shaping are also examined. This work was supported by Sandia National Laboratories. [Preview Abstract] |
Tuesday, November 3, 2009 10:06AM - 10:18AM |
GO5.00004: Nonlinear Rayleigh-Taylor instabilities in fast z-pinches Aaron Miles Simplified analytic models are used to describe the implosion of a cylindrical plasma by an azimuthal magnetic field of sufficient magnitude to drive a strong shock wave into the plasma. These models are employed together with buoyancy-drag-based models of nonlinear single-mode and turbulent multimode Rayleigh-Taylor (RT) growth to investigate the mixing process in such fast z-pinches. Together, the implosion and instability models give predictions that characterize limitations that instabilities can impose on the implosion in terms of maximum convergence ratios (CR) attainable for an axially coherent pinch. Results from high-resolution numerical simulations are used to validate both implosion and instability models. [Preview Abstract] |
Tuesday, November 3, 2009 10:18AM - 10:30AM |
GO5.00005: Modifying implosion dynamics of a wire array by using an inverse wire array z-pinch as a current switch Sergey Lebedev, A. Harvey-Thomson, G.N. Hall, S.N. Bland, G. Burdiak, F.A. Suzuki-Vidal, G. Swadling, E. Khoory, J.P. Chittenden, L. Pickworth We will describe experiments on the MAGPIE facility (1.5MA, 250ns) in which an inverse wire array (with the wires acting as a return current cage placed around a central current conductor) operated as a fast current switch. This allowed to significantly reduce the rise-time of the current pulse ($<$100ns) delivered to a separate, standard imploding wire array z-pinch load. It was found that this arrangement generates a short current pre-pulse ($\sim $5kA, $<$15ns) through the imploding array, followed by a $\sim $140ns interval with zero current, before the main fast rising current pulse is switched into the array. This led to a significant change in the implosion dynamics, suppressing the ablation phase, introducing a 0-D-like implosion and reducing the level of trailing mass. [Preview Abstract] |
Tuesday, November 3, 2009 10:30AM - 10:42AM |
GO5.00006: Influence of ion temperature gradient effects on magnetic-curvature-driven flute instability V.I. Sotnikov, J.M. Kindel, O.G. Onishchenko, E. Yasin, J.N. Leboeuf, B.V. Oliver, T.A. Mehlhorn, C. Deeney In order to study flute mode instability in the presence of ion temperature gradient effects, a nonlinear system of equations in two fluid approximation applicable for spatial scales comparable with the ion Larmor radius was derived. Linear analysis of this system shows that the range of unstable wavelengths in a plasma with large ion charge numbers extends into the region of spatial scales of the order of the ion Larmor radius. Nonlinear stage of the instability was analyzed with the modified version of the numerical code FLUTE. Mixing length estimates of nonlinear saturation levels are in good agreement with simulation results. [Preview Abstract] |
Tuesday, November 3, 2009 10:42AM - 10:54AM |
GO5.00007: Polarity Contact and Risetime Effects on Wire Array Z-Pinches David Chalenski, Bruce Kusse The Cornell COBRA pulser is a 1MA, negative polarity machine. COBRA can operate with 100 to 200 ns current risetimes. 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 establish electrical contact. The machine is normally negative, but a bolt-on convolute can be used to modify the current path and change the electric field at the wires, thereby producing positive polarity with respect to ground at the load. Data presented are the culmination of experiments studying the combined effects of contact, polarity and risetime on wire array z-pinches. Data were collected on 16-wire Aluminum z-pinch arrays in both negative and positive polarity, with solder and non-solder contacts, and with slow and fast risetimes (100ns and 200ns). Five shots were collected for each case. The initiation, ablation, implosion and stagnation phases were compared for the various cases. [Preview Abstract] |
Tuesday, November 3, 2009 10:54AM - 11:06AM |
GO5.00008: Precursor focusing by conical electrodes inside wire array Z-pinches J.B. Greenly, D.A. Chalenski, P.F. Knapp, T.A. Shelkovenko, S.A. Pikuz The addition of conical extensions of the anode and cathode electrodes into the interior of wire array Z-pinches is being studied. So far, 30 and 45 degree simple solid cones have been added to both electrodes, for instance, 16 mm base diameter, 8 mm tall cones, centered on axis, projecting from both electrodes into a 20 mm diameter (16 x 12 micron dia. Aluminum) wire array (thus leaving a 4 mm gap between the points of the cones on axis).The inward flow of ablation streams from the wires is strongly modified by the cones, especially on the anode side: the effects are not symmetrical at anode and cathode. At the anode, the streams are redirected to form a well-defined layer of flow along the cone and come to a focus $\sim $1.5 mm off the tip of the cone, forming a hot, high-density spot $\sim $1.5 mm diameter which persists in a nearly steady state for 10's of ns during the ablation phase. Images and x-ray spectra will be presented to quantify the characteristics of this focal spot and other phenomena of the ablation flows with cones. [Preview Abstract] |
Tuesday, November 3, 2009 11:06AM - 11:18AM |
GO5.00009: High energy density plasma experiments using radial foils P.-A. Gourdain, I.C. Blesener, J.B. Greenly, D.A. Hammer, P.F. Knapp, B.R. Kusse, P.C. Schrafel A novel technique involving radial foil explosions can produce high energy density plasmas (HEDP). A current flows radially inward in a 5-micron thick aluminum foil from a circular anode, which contacts the foil on its outer rim, to the cathode, which connects to the foil at its geometrical center. JxB forces lift the foil upward, forming a plasma shell, with electron density above 5x10$^{19}$ cm$^{-3}$, around a central plasma column, carrying most of the current. When using very small ``pin'' cathodes ($\sim $ 1 mm in diameter) on a medium size pulsed-current generator such as the COrnell Beam Research Accelerator (COBRA), central magnetic fields approach 2 Mgauss and local JxB force densities are on the order of a mega-newton per cubic millimeter. Combining well-established diagnostics, such as laser backlighting, interferometry and fast camera imaging, with new measurements techniques, such as micro B-dot probes, we will present the magneto-hydrodynamics properties of such plasmas. Particular attention will be given to flow and magnetic field measurements. [Preview Abstract] |
Tuesday, November 3, 2009 11:18AM - 11:30AM |
GO5.00010: Recent Developments and Future Research at the Nevada Terawatt Facility Joseph Kindel High Energy Density Physics is the focus of research at the Nevada Terawatt Facility (NTF). The NTF consists of a 1 MA, 2 TW Z-pinch, achieving 400 shots a year, and a 50 TW, 400ps, one micron glass laser. Experiments are undertaken in three ways: Z-pinch only, Z-pinch coupled with laser and laser only. Our facility generates Terawatt x-ray bursts, Megagauss magnetic fields, and investigates magnetized laser plasmas, including astrophysical like plasmas. Plasma diagnostics include a multiframe laser probing, optical and x-ray imaging, particle diagnostics, and x-ray spectroscopy. Wire array pinch experiments show significant structure due to the magnetic Rayleigh-Taylor instability. The magnetic Rayleigh-Taylor instabilities are modeled with nonlinear hydrodynamic modeling. This fluid modeling is capable of describing a wide range of instabilities in high beta plasmas on scales of an ion Larmor radius. [Preview Abstract] |
Tuesday, November 3, 2009 11:30AM - 11:42AM |
GO5.00011: Multi-Planar Wire Array Precursor Formation on the 1MA Zebra Generator Kenneth Williamson, Victor Kantsyrev, Andre Esaulov, Alla Safronova, Ishor Shrestha, Glenn Osborne, Michael Weller, Veronica Shlyaptseva The double-planar wire array (DPWA) is the most promising z-pinch load studied on the 1MA Zebra generator for application to radiation physics and ICF studies. Initial investigation of this load revealed unique implosion dynamics: each plane implodes independently into two off-axis columns before coalescing into the final pinch.\footnote{V. L. Kantsyrev, et. al., PoP, \textbf{15}, 030704 (2008)} New results are presented exploring the formation of these off-axis plasma columns with laser shadowgraphy and time-gated x-ray imaging, each fielded parallel to wire planes 6mm apart. Simulated radial current distribution and ablation effects\footnote{A. A. Esaulov, et. al., HEDP, \textbf{5} (2009), in press} along with simple 3-D calculations were used to establish the processes that created the observed structures. These dynamics are then compared with triple-planar wire array (TPWA) loads. [Preview Abstract] |
Tuesday, November 3, 2009 11:42AM - 11:54AM |
GO5.00012: Z-pinch stability in the presence of an axial flow R. Presura, D. Martinez, S. Stein, C. Plechaty, S. Neff In axially symmetric wire array z-pinches, axial flow can be produced by tilting the wires radially. Adding a wire on the axis of the resulting conical wire array introduces a radial profile in the axial velocity of the flow. For certain conditions, such a shear flow has been predicted theoretically to have a mitigating effect on z-pinch instabilities, but experimental support for this stabilization mechanism is scarce. At the Nevada Terawatt Facility, experiments with conical wire arrays driven by the 1 MA Zebra z-pinch showed improved stability both in the precursor and the stagnation phases of the z-pinch. To focus on the kink (m=1) instability, the study included central wires with helical perturbations. Strong experimental evidence supports the formation of shear flow in the presence of a central wire. The stabilizing effect of the shear flow was assessed by comparing the evolution of the pre-imposed perturbation in equivalent conical and cylindrical arrays. [Preview Abstract] |
Tuesday, November 3, 2009 11:54AM - 12:06PM |
GO5.00013: Impact of load geometry on plasma formation and radiative properties of Z-pinches at stagnation V.L. Kantsyrev, A.S. Safronova, A.A. Esaulov, A.L. Velikovich, L.I. Rudakov, A.S. Chuvatin, K.M. Williamson, M.F. Yilmaz, G.C. Osborne, M.E. Weller, I. Shrestha, V.V. Shlyaptseva The double planar wire array (DPWA) is the best x-ray radiator at 1 MA [V. Kantsyrev \textit{et al}., HEDP 5 (2009), in press]. To improve its radiative performance by reducing the MRT instability growth rate, Al, brass and W DPWAs were skewed to produce initial axial magnetic field Bz. The diagnostics included x-ray devices and laser shadowgraphy. Experiments on 1.6 MA Zebra generator at UNR and MHD modeling have shown that Bz mitigated the MRT instability in the precursor. The stagnation starts in the middle of the A-K gap, and more uniform plasma column with a higher temperature Te is formed compared to a standard DPWA. The yield and power were comparable with standard DPWA. Highest yield and power were for W and brass, respectively. Feasibility of the x-ray pulse shaping was demonstrated in experiments. Research plans are discussed. [Preview Abstract] |
Tuesday, November 3, 2009 12:06PM - 12:18PM |
GO5.00014: A Low Voltage, High Current Dense Plasma Focus using an Impedance Transformer Brian Bures, Mahadevan Krishnan, Kristi Wilson-Elliot, Robert Madden, Florian Blobner The dense plasma focus is a high voltage, high current device. In principle, high voltage is unnecessary as the dense plasma focus (DPF) is a low impedance load. The impedance of the driver usually dominates the impedance of the circuit. An alternative approach is to use a step down transformer to increase the current and reduce the voltage at the load. Additional benefits of this approach are: reducing driver inductance to less than 5nH from a $>$30nH driver; reducing stored energy to achieve a large current; minimizing consumable switches; and reducing anode heating. A module capable of producing 33 kA with a quarter period of 350 ns was demonstrated using a 3:1 current step-up transformer. The source stored only 40 J and produced $\sim $1E4 n/pulse using deuterium gas, while using a single Thyratron switch. The new module is a 6:1 current step up transformer which is projected to produce 60-66kA with 50J stored. Key performance metrics for the DPF including peak current, energy stored, and neutron yield will be described. [Preview Abstract] |
Tuesday, November 3, 2009 12:18PM - 12:30PM |
GO5.00015: Deuterium Gas-Puff Experiments on the S-300 Z-pinch Daniel Klir, Jozef Kravarik, Pavel Kubes, Karel Rezac The neutron production from a deuterium gas-puff Z-pinch has been studied on the S-300 Z-pinch (Kurchatov Institute, Moscow). Neutron time-of-flight diagnostics was used to give an insight into the acceleration of fast deuterons. The emphasis was put on the finding of (i) the energy distribution of deuterons which produced fusion neutrons, (ii) the anisotropy of neutron emission, and (iii) the time and duration of neutron production with respect to the general Z-pinch dynamics. The peak neutron yield above 10$^{10}$ was achieved on the current level of 2 MA. The fusion neutrons were generated for 30 ns during the stagnation and at the beginning of the expansion of a plasma column. The side-on neutron energy spectra peaked at 2.42$\pm$0.04~MeV with about 400 keV FWHM. In the downstream direction (i.e. the direction of the current flow from the anode towards the cathode), the peak neutron energy was 2.6 $\pm$0.1~MeV. The average kinetic energy of reacting deuterons was about 100 keV. The broad width of radial neutron spectra implied a high radial component of deuteron velocity. On the basis of these experimental data, it is possible to discuss acceleration of deuterons and fusion neutron production. [Preview Abstract] |
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