Session CO2: Z Pinches

Measurements of three-dimensional instability growth of the imploding mass distribution for single and nested wire arrays

Radiographic measurements at 6.151 keV show that the imploding mass distribution of a wire array z-pinch has the character of the magneto-Rayleigh Taylor instability [1]. Although the instability wavelength $\lambda $ and amplitude $\Delta $ are similar on either side of the z-pinch axis, detailed instability features (individual bubbles and spikes) are misaligned, which indicates a partly three-dimensional character, with some azimuthal incoherence. The temporal characteristics of the x-ray power are correlated with the instability amplitude near the axis at stagnation. Tailoring of the radial mass density profile of the z-pinches using current-transfer nested-arrays [2] and axial CH$_{2}$ foams leads to improved pulse compression, smaller amplitude $\Delta $ and x-ray powers of up to 220-240 TW. The data may support a self-similar spatio-temporal evolution with $\lambda \sim \Delta $ during part of the implosion. [1] D. B. Sinars et al., Phys. Plasmas [2005], [2] M. E. Cuneo, et al., Phys. Plasmas [2006]   

Variations on Nested Stainless Steel Wire Arrays

K-shell x-ray production requires high plasma temperatures ($>$3 keV for stainless steel), which ultimately dictates load designs with large initial radii and low mass. These loads are susceptible to Rayleigh-Taylor implosion instability. To mitigate this, we have explored nested wire arrays (55 mm and 75 mm outer diameter) with a variable inner wire array configuration. Nested arrays with 55 on 27.5 mm diameter, 2:1 mass ratio (outer:inner) retain their total and K-shell radiation output with a smaller, lighter inner array, but care must be taken to ensure simultaneous stagnation of the two arrays, accounting for transparency of the inner array to the imploding outer material. The addition of a third 13.75 mm array to the 55 on 27.5 mm Z load increased the total x-ray power while keeping the yield fixed and resulted in an enhanced x-ray foot prior to the main emission peak.   

Wire-array on foil implosions for radiation pulse shaping

Previous experiments used nested wire-array z-pinches in a current-transfer (i.e., transparent) mode to produce shaped radiation pulses consistent with three-shock low-adiabat compression of high-yield inertial confinement fusion capsules [1-2]. Here we discuss experiments where the inner wire array is replaced with a thin foil of comparable mass. The interaction with the foil occurs at a larger radius and has a slightly larger power than tests using an inner wire array. Experiments with foams inside of the foil do not show an interaction pulse with the foam, and the main radiation pulse is broadened so that the peak power is lower when using inner foils (60 TW versus 100 TW for similar nested arrays). We will present comparisons of nested-array and array-on-foil implosions. [1] M.E. Cuneo et al., Phys. Rev. Lett. [2005], [2] M. E. Cuneo et al., Phys. Plasmas [2006]   

Investigation of the interaction pulse in nested wire array z-pinches

Radiation pulse shaping is vital for z-pinch driven ICF concepts. One method to achieve such pulse shaping is to use the foot pulse generated by the interaction between two nested wire arrays. We present data from experiments investigating this interaction pulse on the MAGPIE generator (1MA, 240ns) at Imperial College London. On MAGPIE, the current through the inner array is suppressed by enhancing its inductance, leading to a similar current fraction to that observed on the Z-generator. In these experiments time gated imaging of photon energies $>$30eV indicates that radiation is emitted as leading bubbles of the imploding outer array reach and pass the inner array. Experiments using novel configurations to eliminate possible heating mechanisms will also be discussed, and data will be compared to simulations from the Gorgon 3D MHD code.   

Spectroscopy and implosion dynamics of nested wire arrays produced on the 1 MA z-pinch generator at Cornell University

Experiments with low wire number nested wire arrays from Al, Stainless steel, and combinations of these two materials have been performed on the 1 MA Cobra generator at Cornell University. The diagnostic complex included fast x-ray and EUV detectors, both time-gated and integrated x-ray pinhole cameras, x-ray and EUV spectrometers, and laser probing imaging. Modeling of time-gated spectra indicates that the electron temperature gradually increases with time in the nested wire array experiments that were analyzed, even after the current maximum, whereas the electron density shows more non-monotonic behavior. In addition, for spatially resolved, time integrated spectra from combination arrays, the results of modeling of radiation from outer and inner wires were compared. Modeling of K-shell Fe indicates the highest electron temperature, Te $\sim $800eV, which was reached with the pure SS304 nested array. This work was supported by NNSA/SSAA under DOE Cooperative Agreement DE-F03-02NA00057, by the NNSA under UNR grant DE-FC52 01NV14050, and by Sandia National Laboratories under DOE contract DE-AC04-94AL85000.   

Modifying Wire-Array Z-pinch Ablation Structure and Implosion Dynamics Using Coiled Arrays

Coiled arrays, a cylindrical array in which each wire is formed into a single helix, have been used to suppress the modulation of ablation at the fundamental wavelength for the first time. Ablation flow in coiled arrays is modulated at the wavelength of the coil. With this ability to control where ablation streamers occur, large wavelength coils were constructed such that wire beaks had sufficient axial separation that perturbations in the implosion sheath did not merge. This produced a new, organised mode of implosion in which the global instability can be controlled and the perturbations correlated between all the wires in the array. For large wavelength 8-wire coiled arrays, this produced a dramatic increase in x-ray power, equaling the x-ray power of a 32-wire straight array. These experiments were carried out on the MAGPIE generator (1MA, 240ns) at Imperial College, London. This research was sponsored by Sandia National Laboratories Albuquerque, the SSAA program of NNSA under DOE Cooperative Agreement DE-FC03-02NA00057.   

Implosion dynamics and radiation characteristics of planar wire arrays and their comparison with low-wire number cylindrical wire arrays features on the 1 MA z-pinch generator at UNR

Planar wire arrays$^{ }$ [1] were shown to produce $>$ 17-18 kJ EUV/x-ray radiation yields for Cu and Mo in a ns-scale high-power pulse. The measurements of the T$_{e}$ and n$_{e}$, yields, time-gated x-ray spectra and images, laser probing images from the single and double planar arrays showed that their plasma formation and implosion are different from cylindrical arrays. The explanation why the current is nearly equally redistributed over all wires and specific of plasma implosion dynamics and radiation is given. Results were compared with low-wire number cylindrical arrays with implosion time close to planar arrays one. The modeling has shown that the enhanced resistivity of the inhomogeneous plasma plays a significant role in the presented planar wire arrays experiments. [1] Kantsyrev \textit{et al}., IEEE Trans. Plasma Sci., v. 34,194 (2006).   

Simulations of high current wire array Z-pinches using a parallel 3D resistive MHD

We present calculations of the implosion and stagnation phases of wire array Z-pinches at Sandia National Laboratory which model the full 3D plasma volume. Modelling the full volume in 3D is found to be necessary in order to accommodate all possible mechanisms for broadening the width of the imploding plasma and for modelling all modes of instability in the stagnated pinch. The width of the imploding plasma is shown to arise from the evolution of the uncorrelated modulations present on each wire in the array early in time into a globally correlated 3D instability structure. The 3D nature of the collision of two nested arrays is highlighted and the implications for radiation pulse shaping are discussed. The addition of a simple circuit model to model the Z generator allows the pinch energetics during stagnation to be treated more accurately and provides another point of comparison to experimental data. The implications of these results for improved X-ray production are discussed both for the keV range and for soft X-ray radiation sources used in inertial confinement fusion research. This work was partially supported by the U.S. Department of Energy through cooperative agreement DE-FC03-02NA00057.   

Simulation of Wire-Array Z Pinches with ALEGRA

Wire-array z pinches provide the x-ray radiation drive for Inertial Confinement Fusion Experiments at Sandia National Laboratories. A physical understanding of the physics of wire-array z pinches is important in providing a future radiation source capable of driving high-yield fusion capsules. Modeling of wire-array implosions on the Z machine were performed using the 2-D radiation MHD code Alegra. These new calculations use more accurate initial conditions that are more representative of the experimental data, allowing us to model the implosion through stagnation, to avoid radiation collapse, and to generate a radiation pulse that compares well with data. Code predictions will be compared with tungsten {\&} aluminum wire-array data from Z. \newline \newline Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the U.S. Department of Energy under Contract No. DE-AC04- 94AL85000. a Ktech Corporation, 1300 Eubank Blvd. S.E., Albuquerque, NM 87123-3336   

Z-pinch stagnation and radiation production using a MHD turbulence model

The physics of a Z-pinch stagnation is not clearly understood. Most simulations of z-pinches tend to significantly overpredict radiated power and underpredict the amount of radiated energy. We will present 1D and 2D simulations of tungsten wire array Z- pinches that incorporate a new K-epsilon MHD turbulence model. In this model, plasma stagnation is dominated by turbulent kinetic energy. The turbulent energy relatively slowly cascades into the ions, equilibrates with the electrons and radiates. In contrast to simulations without the turbulence model, simulated radiated energy, pulse width, and pulse power are found to be in reasonable agreement with experiment. Sensitivity to initial conditions will also be discussed.   

Wire initiation critical for radiation symmetry in Z-pinch driven Dynamic Hohlraums

A high degree of large-scale axial symmetry in soft X-ray radiation and hence also in the plasma implosion in multi-wire array Z-pinches is very important for the creation of Dynamic Hohlraums of high quality and utility. Such hohlraums are used for the compression of inertial confinement fusion capsules and for driving radiation flow experiments. We present the first experimental evidences that this symmetry is controlled in large measure by subtle details of the initiation of the wires in the arrays. Important parameters of the wire initiation are examined and mechanisms of how these parameters may affect the quality of the plasma implosion are considered.   

Effects of Wire Ablation on Foam Targets in Wire Array Z-pinches

Cylindrical foam targets can be placed on the axis of wire array z-pinches, in particular Dynamic Hohlraum (DH) and Double Ended Drive Hohlraum (DEDH); Z-pinch configurations used for ICF experiments on Z, Sandia National Laboratory. Precursor plasma produced by array wires prior to array implosion is accelerated to the axis and impacts the foam target. This can modify the target prior to the implosion of the wire array. Kinetic pressure and thermal heating of the foam are two mechanisms that will affect the foam. Experiments have been preformed with non-imploding wire arrays on the 1 MA MAGPIE generator at Imperial College London. Over-massed arrays injected precursor plasma into the array for the duration of the current pulse; absence of the x-ray pulse an imploding array emits permits radiography of the array axis. The entire width of the array has been radiographed for the first time. Diagnostics included point-projection radiography with x-pinches and x-ray emission framing cameras. Results show ablation of low-density plasma from the foam surface and compression of the foam by precursor pressure.   

Experimental Radial Density Profiles in Wire Array Z-Pinches

The ablation processes occurring in wire arrays are fundamental to the evolution of the radial density profile, the development of precursor structures, and the trajectory of the final implosion of the main array mass. During the flow of plasma to the array axis, a quasi-periodic axial modulation in the density is observed experimentally for all array materials. The wavelength of this `flare' structure varies with material, but the mechanism which seeds the development of this axial modulation axis remains unresolved. This is often introduced into numerical simulations by artificially by use of either an initial axial mass or temperature variation at the appropriate periodicity in order to emulate the experiment. Experimental characterization of the evolution of the radial density profile will provide a constraint for the initial conditions of the simulation. This paper describes radiographic and interferometric data from wire array experiments on the MAGPIE generator at Imperial College London. In particular, two-frame radiography is applied to image a single wire core at multiple times to study the development of the radial flare structure, and these data are compared to results from the Gorgon 3D MHD code. This research was sponsored by the NNSA under DOE Cooperative Agreement DE-F03-02NA00057.   

Nonlinear Interaction Between the Radiation and Multidimensional Plasma in Large Diameter Structured Argon Gas Puff Z-Pinch Loads

It has recently been demonstrated that one can efficiently produce K-shell x-ray radiation with z pinches imploded from larger initial diameters, by longer current pulses than previously thought possible, using a ``pusher-stabilizer-radiator'' load formed by a supersonic nozzle injecting outer and inner annular shells and a high-density central jet between the cathode and the anode [1]. We present a detailed numerical investigation of the implosions of such loads. Our simulations were performed with the 2D RMHD Mach2 code incorporating the dynamical domain tabular collisional radiative equilibrium (DDTCRE) radiation transport model [2]. This model provides a realistic description of the self-consistent multidimensional non-local non-LTE ionization dynamics and radiation transport physics in a computationally efficient manner. A numerical simulation analysis of various nozzle load configurations, with/without the central jet and/or shells, is used to further assess and validate the physical model by simulating the experimental spectra, plasma images and radiation emission characteristics. [1] H. Sze et al., PRL \textbf{95}, 105001(2005). [2] Y. K. Chong et al., ICOPS 2005, Monterey, CA.