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Plasma Science, IEEE Transactions on

Issue 2 • Date April 2002

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Displaying Results 1 - 22 of 22
  • Guest editorial: special issue on Z pinch plasmas

    Publication Year: 2002 , Page(s): 458 - 459
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    Freely Available from IEEE
  • X pinch plasma development as a function of wire material and current pulse parameters

    Publication Year: 2002 , Page(s): 567 - 576
    Cited by:  Papers (13)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (436 KB) |  | HTML iconHTML  

    An X pinch plasma is produced using two or more fine metal wires arranged so that they cross and touch at a single point, in the form of an "X," as the load of a pulsed power generator (pulser). X pinches produce very bright, small, short-lived sources of X-rays in the 1-10 keV range. The X-ray source size, pulse duration and photon energy spectra depend upon the wire material and diameter, and the current waveform from the pulser. Those with ∼1 μm source size can be used as point sources of X-ray radiation for point-projection radiography with high spatial resolution. We report experiments with a variety of different wire materials, including Al, Ti, Mo, W, among others, all of which produced high resolution images, and all of which have subnanosecond X-ray pulses for X-rays ≥3 keV. X pinches produced using all of these materials had common development stages. However, the wire material and the rate of rise of the current pulse influenced the details of the dynamics, leading to variation in the timing of the X-ray burst(s) relative to the start of the current pulse. For a given wire material, wire diameter had only a small effect on X-ray emission timing. Final X pinch neck implosion speeds as high as 66-75 μm/ns were estimated. View full abstract»

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  • Rayleigh-Taylor instability with a sheared flow boundary layer

    Publication Year: 2002 , Page(s): 611 - 615
    Cited by:  Papers (12)
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    S. Chandrasekhar, in his book, Hydrodynamic and Hydromagnetic Stability (New York: Dover, 1961), derives the stability criteria for a semi-infinite uniform density incompressible inviscid fluid with uniform horizontal velocity supported in a gravitational field by one of higher density and opposite velocity. A transitional layer of inviscid fluid with a density equal to the average of the upper and lower fluids, and a horizontal velocity that varies linearly with depth from that of the upper fluid at the top to that of the lower fluid at the bottom is assumed. This analysis of the Kevin-Helmholtz (K-H) instability may be transformed into a model of the effect of such a velocity sheared boundary layer on the Rayleigh-Taylor (R-T) instability of modes with wave numbers in the direction of the sheared velocity by reversing the sign of the top-bottom density differential. Orthogonal modes are unaffected by the shear in the linear limit and are, therefore, R-T unstable unless an independent mechanism for their stabilization is present, such as a magnetic field orthogonal to the sheared velocity. The combined R-T/K-H stability analysis is, therefore, expected to be most applicable for magnetically accelerated media such as a Z pinch with an axial velocity sheared outer layer orthogonal to the outer azimuthal magnetic field which drives the implosion. View full abstract»

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  • Structure of cylindrical liners compressing a magnetized D-T plasma to ignition

    Publication Year: 2002 , Page(s): 460 - 467
    Cited by:  Papers (1)
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    It has been suggested on several occasions that a fast cylindrical liner (νL≥107 cm/s) can compress a D-T plasma to volume or a spark ignition. Such a compression depends critically on the liner structure. We shall analyze this structure, first using a simple, heuristic model and subsequently comparing it with the results of one-dimensional numerical simulations. We shall then consider how an imploding liner can produce a volume or spark ignition. In the conclusion, it will be argued that the best scheme for ICF is a spark ignition, in which a liner implosion on Z pinch is synchronized with the development of an m=0 instability. View full abstract»

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  • Computational simulation of initiation and implosion of circular arrays of wires in two and three dimensions

    Publication Year: 2002 , Page(s): 593 - 603
    Cited by:  Papers (6)
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    We have performed realistic two-dimensional (2-D) r-θ resistive MHD simulations of high-current aluminum wire array initiation and implosion. These show only a moderate differentiation of wire plasma into a warm dense core and a hot diffuse corona. Wire plasmas in 28-wire array simulations implode without forming a shell; those in 56-wire simulations first merge but then separate. As both implode, thread-like plasmas settle into valleys formed across the field lines by the thread mass. Thus, shell formation does not smooth the initial wire asymmetry, because the r-θ Rayleigh-Taylor instability amplifies it. This argues against shell-formation as the primary explanation for the observed effect of increased wire number on radiation power. We have also performed three-dimensional (3-D) ideal MHD simulations that continue those 2-D simulations; they start with a fully consistent MHD state. These simulations, perturbed between the 2-D and 3-D phases, show that azimuthally uncorrelated 3-D perturbations-appropriate for wires-grow more slowly than fully azimuthally correlated 2-D r-z perturbations. Further, the uncorrelated perturbation growth rate is smaller for 56 wires than for the 28, as the magnetic field couples more plasma threads over the same distance. These 3-D effects may explain the observed radiation power improvement with increased wire number. View full abstract»

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  • Measuring the gas flow from a supersonic nozzle used in a 1.5-MA gas puff Z pinch

    Publication Year: 2002 , Page(s): 482 - 487
    Cited by:  Papers (3)
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    A supersonic nozzle for a 1.5-MA gas puff Z pinch was designed and the gas flow injected out from the nozzle was measured. A miniature ionization gauge was developed to measure the density profile of the gas shell and two pressure transducers were used for determining the Mach number of the gas flow. It was found that the gas flow is not distributed symmetrically about r0, the mean radius of the exit aperture, and the radial location for the peak of the density profile moves inwards as time goes on. When the plenum pressure is 5 atm, the maximum line mass density measured was 43 μg/cm and the Mach number of the gas flow deduced from experiments is 4.3, which is close to the expectation: line mass density of about 50 μg/cm and a Mach number higher than 4. The measurements indicate that if a plenum opens into a large dead volume via a finite opening valve in a gas-puff system it will not produce a steady-state solution. Then, any agreement between the measurements and the calculations based on the steady state should not be expected. View full abstract»

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  • Development and characterization of a low current capillary discharge for X-ray laser studies

    Publication Year: 2002 , Page(s): 616 - 621
    Cited by:  Papers (25)  |  Patents (1)
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    Capillary discharge experiments were carried out for soft X-ray laser studies. A ceramic capillary, which has an inner diameter of 3 mm and a length of 150 mm, has been used for the end-on X-ray diode observation, and a Pyrex capillary has been used for side-view observation. Spike output has been observed, when operating the device with a predischarge current of 5 to 15 A, a current of 9 to 35 kA with a rise time of 55 ns in an argon gas pressure range from 100 to 800 mtorr. It is found that without a predischarge current, spike output has been hardly observed. Observation of spike output at a low discharge current of 9 kA provides us a possibility for design of a compact soft X-ray laser device. When the predischarge is turned off, the side-view observation of the capillary discharge clearly shows the growth of instabilities during the pinch process. This suggests that the predischarge is indispensable for achieving a uniform plasma, which is required by the soft X-ray lasing in a capillary discharge. View full abstract»

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  • MHD modeling of plasma compression to high pressure with capacitive-driven solid shell implosions

    Publication Year: 2002 , Page(s): 468 - 475
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    Isotropic high-pressure plasma could be used to compress targets for equation of state studies, for generating fusion reactions, for accelerating projectiles to high speed and for other high-energy density endeavors. One way to generate such high-pressure plasma is to compress a modest pressure gas via implosion of a solid shell by a pulsed electrical discharge. The pressure of an ideal gas characterized by a constant ratio of specific heats Γ that is undergoing adiabatic compression scales as ρΓ where ρ is the mass density. Whereas Γ may not be constant for realistic compressing plasmas, it is certainly always greater than unity. Since the ratio of final to initial mass density can be greater for spherical compression than for cylindrical compression, spherical compression offers an opportunity to achieve substantial pressure increase. Quasi-spherical compression of a solid shell has been achieved in the laboratory and, in this paper, we discuss in detail numerical simulations that were used to guide and interpret such experiments. The numerical simulations are performed with MACH2; an unsteady resistive-MHD code for materials that may exist in any of the solid, liquid, gas and plasma states. These simulations illustrate how Mbar pressures can be achieved in the laboratory using presently available pulsed power technology. View full abstract»

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  • A three-dimensional model of wire array instability, ablation, and jetting

    Publication Year: 2002 , Page(s): 588 - 592
    Cited by:  Papers (16)
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    A simplified algebraic model is set up to describe the principal features of the physics of wire array behavior in the early phases. Each wire is assumed to have evolved into a liquid-vapor core surrounded by a coronal plasma which carries most of the current. This plasma is unstable to m=0 instabilities, this instability being MHD or possibly electrothermal driven by inward radial heat flow on each wire. The joule heating occurs mainly in the necks of the sausage mode; flux limited heat flow transfers the energy to the core which erodes and ablates as a plasma at the sound speed. Free expansion at this velocity occurs into the lobes of the m=0 structure, and by rocket action the J_×B_ inward global force deflects the flow into inward precursor-jets. It is shown that the magnetic Reynolds' number is less than one, and for a rapidly rising current results in negligible current or magnetic field in the precursor, as found usually in experiments. In addition, it is found that in the necks the Hall parameter is much less than one, consistent with the field-free model of flux limited heat flow. View full abstract»

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  • Mixed gas Z pinch experiments using a shell-on-shell nozzle on Double-EAGLE

    Publication Year: 2002 , Page(s): 512 - 516
    Cited by:  Papers (4)
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    A series of Z pinch experiments using argon and krypton was conducted on the Double-EAGLE pulse power driver at 3.5 to 4.0 MA peak current and 170-190 ns implosion time. A shell-on-shell nozzle provided the opportunity to separate the two gases and to control which was driven more strongly (by virtue of being in the inner plenum). With argon in the inner plenum, 12 to 16±3 kJ krypton L-shell and 8 to 10±3 kJ of argon K-shell radiation was produced. With krypton in the inner plenum, 23.6 ±2.5 kJ of krypton L-shell and 2.5±2.3 kJ of argon K-shell radiation were produced. Since the optimum implosion times for the two gases were different, changing the mass of the z pinch varied the ratio of the yields. Using a streak spectrograph and PCDs with a Ross filter pair, the time history of the krypton and the argon radiation could be distinguished and seen to be simultaneous. Spectroscopic measurements of a chlorine dopant in the inner shell gas demonstrated that the implosions with krypton achieved lower density and electron temperature than with argon whether the krypton was in the inner or outer plenum. View full abstract»

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  • Free surface boundary conditions for diffusive radiation transport in PRS simulations

    Publication Year: 2002 , Page(s): 604 - 610
    Cited by:  Papers (4)
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    The use of a free surface boundary condition for radiation streaming into an optically thin exterior region of a plasma radiation source (PRS) is developed and tested within the Mach2 code. The matching problem for diffusive and free streaming radiation flux at an interface is solved in the presence of energy exchange between the diffusing radiation and the ambient electron fluid. It is found that the ratio of Planck to Rosseland opacity is a strong modifier of the radiation temperature profile required to meet the match condition. Moreover, as compared to an external radiation temperature boundary condition, a free surface boundary condition predicts a radiation fluence more compatible with a semithin emission regime assessed by the tabular CRE model. The free surface condition also removes the need to assess the radiation temperature at simulation boundaries far away from the PRS. View full abstract»

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  • Two-dimensional MHD simulations of a neon Z pinch on Hawk

    Publication Year: 2002 , Page(s): 488 - 497
    Cited by:  Papers (4)
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    Two-dimensional magnetohydrodynamic (MHD) simulations using MACH2 are benchmarked against laser shearing interferometer (LSI) images of the evolving electron-density sheath during 250-ns neon gas-puff Z pinch implosions on the Naval Research Laboratory Hawk generator. The initial density distribution for the MHD simulations is calculated using a ballistic-flow-model fit to the measured initial gas-density distributions. The implosion is modeled using an applied current profile, single-temperature energy equations, and black-body-limited optically thin radiation. For consistency with the radiation model, neon ionization and equation-of-state models have been added to MACH2. Computed MHD ion-density distributions compare well with LSI images as the snowplowed plasma channel evolves during implosion. The MHD results also show features that may be helpful for understanding early and weak K-shell radiation observed near the nozzle in Double-Eagle gas-puff experiments. Current-channel evolution derived from one-dimensional snowplow calculations compare well to the MHD results for the same current history and initial density distribution, indicating that axial mass flow does not strongly impact the implosion dynamics. View full abstract»

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  • K-shell radiation from nickel wire arrays at 18 MA

    Publication Year: 2002 , Page(s): 532 - 537
    Cited by:  Papers (5)
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    A series of nickel z pinch experiments was conducted at 18 MA current on the Z accelerator producing up to 13 kJ of K-shell radiation at 7.8 keV and above. Double-shell wire arrays were used, with the diameter of the outer array of the nested structure varied from 55 to 70 mm. By using Cr and Mn dopants in conjunction with a streak spectrograph, the interaction of the outer and inner array could be investigated. The radiation from the outer array E array started slightly after the inner array and lasted longer, producing comparable energy per atom at lower power. This indicates that the arrays were well mixed for most of the radiation pulse, in contrast to the observed behavior of a double-shell gas puff. View full abstract»

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  • The dynamics of single and nested nickel wire array Z pinch implosions

    Publication Year: 2002 , Page(s): 552 - 558
    Cited by:  Papers (14)
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    Experiments on the implosion dynamics of single and nested nickel wire arrays driven by the MAGPIE generator with a 1-MA, 240-ns current pulse are reported. Like aluminum and tungsten wire discharges, the plasma formed from individual wires has a core-corona structure. In a single nickel array, a wire core size of 75 μm before the implosion was measured by X-ray radiography. The precursor plasma is formed on axis and is m=1 unstable. The implosion of the array occurs later than predicted by the 0-D model, indicating that a fraction of current flows through the precursor plasma. This is in contrast with copper arrays (copper has the similar radiation properties as nickel, Znickel=28, Zcopper=29) where the precursor plasma is uniform and implosion time is as predicted by 0-D model. Experiments with nickel nested arrays show that the expansion of the wires in the outer and the inner array is similar and a precursor plasma is formed on the axis earlier than in single arrays. It appears from the experimental data that a significantly larger fraction of the current is flowing through the inner array. View full abstract»

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  • The relationship between exploding wire expansion rates and wire material properties near the boiling temperature

    Publication Year: 2002 , Page(s): 577 - 587
    Cited by:  Papers (16)
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    The energy deposited by a submicrosecond, ∼1-kA current pulse in a 25-μm diameter metal wire prior to its explosion, correlates directly with the expansion rate of the wire after the explosion. Energy deposition by resistive heating is terminated by the formation of plasma around the wire and a collapse of the voltage along the wire, and is evidently facilitated by the desorption of gases from the wire and/or the evaporation of the wire material (or impurities within it) as it heats up. Data presented here implies that the relationship between materials with the lowest resistivities and high exploding wire expansion rates found in earlier work (D. B. Sinars et al. 2000) is a result of the reduced voltage delaying the gas breakdown along such wires. This, in turn, increases the energy deposited resistively in the wire before the current shifts to the surrounding plasma. If gas breakdown does not occur until close to the full vaporization energy is deposited in the wire, the expansion rate will be more rapid than if a small fraction of the vaporization energy is deposited before voltage collapse. View full abstract»

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  • An experimental study of the effect of Rayleigh-Taylor instabilities on the energy deposition into the plasma of a Z pinch

    Publication Year: 2002 , Page(s): 524 - 531
    Cited by:  Papers (8)
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    The mechanism for the heating of the plasma of a Z pinch due to the generation of toroidal magnetic structures (magnetic bubbles) which are formed in the plasma as a result of the penetration of the azimuthal magnetic field into the gas puff plasma was investigated experimentally. The experiments were performed with single-shell and double-shell gas puffs (60/30 mm in diameter) on the IMRI-4 generator (Imax=350 kA, T/4=1.1 μs). The gases used for the gas puff material were neon, argon, and krypton. Electrical investigations have shown that the final resistance of the plasma depends on the linear mass of the gas puff and equals to ∼0.06÷0.1 Ω, which coincides in the order of magnitude with the prediction of the theory of an enhanced energy deposition into the plasma of a Z pinch. Probing of the plasma was carried out with a YAG:Nd3+ laser with a wavelength of 532 nm, a pulse energy of the order of 30 mJ, and a pulse full-width at half-maximum of ∼5 ns. Polarimetry has shown that at the stage of stagnation of a Z pinch, there are regions inside the plasma column where the radial distribution of the electron density has a local minimum. The rotation of the polarization plane of the electromagnetic wave probing the plasma suggests that some portion of the azimuthal magnetic field of the Z pinch is captured by the current loop (a magnetic bubble is formed). The magnetic field inside the magnetic bubble is, on the average, 600÷800 kG and coincides in the order of magnitude with the magnetic field near the pinch boundary. View full abstract»

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  • Preliminary neutron experiments with the PF-1000 plasma-focus facility

    Publication Year: 2002 , Page(s): 476 - 481
    Cited by:  Papers (9)
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    The paper reports on first neutron-emission experiments carried out with the modernized PF-1000 facility. Current sheath dynamics was investigated by means of high-speed cameras. Based on smear pictures, which were taken through a narrow radial slit, the compression velocity was determined. The total neutron yield and angular distribution anisotropy were measured with four silver-activation counters placed at different angles to the electrode axis. It was found that for each initial D2-filling pressure value, with an increase in the charging voltage (V0), the neutron yield increases to some maximum and with further increase in V0 the neutron emission decreases. The highest neutron yield registered so far has been 2·1011 per shot. The neutron emission anisotropy seems to be lower than that registered in other plasma-focus experiments. The neutron energy spectra, which were measured by means of a scintillating probe in the upstream direction, have been shifted toward lower energy (2.2-2.3 MeV). View full abstract»

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  • Nonideal MHD plasma regimes in the study of dynamic Z pinches

    Publication Year: 2002 , Page(s): 517 - 523
    Cited by:  Papers (4)
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    Detailed simulations of wire-array Z pinches with Radiation MHD codes require calculations which span a broad range of plasma parameters. An introductory study of the various regimes accessed by nonequilibrium Z pinches of the type fielded on the Z accelerator at Sandia National Laboratories is presented. In particular, the physics of wire initiation and breakdown and liner implosion are considered. The conditions for ideal and some nonideal MHD dynamics, including fluid viscosity, resistivity, and Hall current dynamics are reviewed. A discussion of the relevance of MHD simulations and modeling to the various plasma regimes is given. View full abstract»

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  • Prolonged plasma production at current-driven implosion of wire arrays on Angara-5-1 facility

    Publication Year: 2002 , Page(s): 559 - 566
    Cited by:  Papers (39)
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    Results of experimental investigation and modeling of prolonged plasma production during implosion of cylindrical wire arrays are presented. Results of the radiography of dense cores of imploding wire array and the measurements of internal azimuthal magnetic field in wire array give new experimental evidences of prolonged plasma production phenomenon. This phenomenon is an important property of current-driven implosion of the wire arrays at current rise rates ∼(0.5-1)1014 A/s. The prolonged plasma production can determine the current and the density profiles before final stage of a Z pinch compression, and also in the moment of Z pinch stagnation. From this point of view, the requirement that residual uncompressed plasma should not shunt the current at the discharge periphery becomes of the greatest importance. The conditions exist when the prolonged plasma production isn't an obstacle for the achievement of high-power X-ray emission from Z pinch. Presented experimental results on multiwire array implosion can be explained on the basis of prolonged plasma production without referring to multiwire array azimuthal structure. View full abstract»

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  • Experimental study of an argon-hydrogen Z pinch plasma radiation source

    Publication Year: 2002 , Page(s): 498 - 511
    Cited by:  Papers (3)
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    Experiments with Ar-H2 double gas puffs have been conducted on the GIT-12 generator at the current level of 2.1-2.4 MA and 250-350 ns implosion times. The argon-hydrogen mixture was used as a working medium in the outer shell of a double gas puff. The goal of the experiments was to verify whether the use of the argon-hydrogen mixture in the outer shell can improve the stability of a double gas puff implosion and provide increased argon K-shell radiation yields. The experiments showed that hydrogen admixture results in the change of implosion dynamics: decreased implosion velocities and compression ratios. The experimental data does not allow a conclusion that the use of an Ar-H2 mixture in the outer shell of a double gas puff significantly improves the implosion stability. An increase in the hydrogen percentage in the outer shell leads to a decrease in plasma density and temperature, and as a result, reduced K-shell radiation yields and powers. View full abstract»

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  • Time-resolved spectroscopy and energy-resolved imaging of coated wire arrays as the interwire spacing is decreased

    Publication Year: 2002 , Page(s): 547 - 551
    Cited by:  Papers (1)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (218 KB) |  | HTML iconHTML  

    We have spectroscopically observed, as suggested by Sanford et al. (1996), a transition from discrete wire implosions to the implosion of a more continuous plasma shell in aluminum-wire-array Z pinches driven by a 7-TW electrical generator. The time history of magnesium dopant K-shell X-ray emission depended on the interwire spacing, as well as whether the dopant was introduced as an alloy in or a coating on the aluminum wires. We diagnose the final assembly of the hot, dense K-shell emitting core, a region of the pinch not accessible to laser interferometry or optical/UV emission measurements. View full abstract»

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  • Azimuthal structure in wire-array Z pinch experiments

    Publication Year: 2002 , Page(s): 538 - 546
    Cited by:  Papers (3)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (530 KB) |  | HTML iconHTML  

    Nested wire-array loads on Z with a high degree of azimuthal symmetry in the outside current-return-shell (which surrounds a z pinch) exhibit a 6±2 azimuthal mode structure in the axial X-ray emission near stagnation when the pinch is viewed from above through a small radiation-exit-hole in the anode. MACH3 numerical simulations show that if a low-number mode like 6 does exist in the initial density distribution, it will remain throughout the implosion, amplifying near the time of stagnation. And if a random-density pattern initially exists with a significantly higher mode number, it will coalesce to a reduced mode number of about 4 to 8 by the time stagnation is neared, in agreement with experiment. Current-return-shells with a nine-fold periodic slot structure (which permit side-on viewing of the pinch) are used as examples of loads that develop azimuthal mode structures in their X-ray emission. View full abstract»

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IEEE Transactions on Plasma Sciences focuses on plasma science and engineering, including: magnetofluid dynamics and thermionics; plasma dynamics; gaseous electronics and arc technology.

 

 

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Editor-in-Chief
Steven J. Gitomer, Ph.D.
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