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  • Oxford University Press  (253)
  • American Institute of Physics (AIP)
  • American Chemical Society (ACS)
  • 21
    Electronic Resource
    Electronic Resource
    Properties of a vacuum ultraviolet laser created plasma sheet for a microwave reflector (1995)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 78 (1995), S. 6974-6979 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A 193 nm excimer laser and a custom fabricated cylindrical lens system is used to produce a plasma sheet of 8 cm×30 cm×0.4 cm in tetrakis(dimethylamino)ethylene (TMAE), a low ionization energy organic gas. Plasma density variation due to photon absorption is studied by scanning the filling pressure of TMAE between 12 and 150 mTorr. A high density (n≥2.0×1013 cm−3), low temperature (Te≈0.8 eV) plasma sheet of 4 mm thickness is obtained with less than 50% spatial density variation over the 30 cm axial length. Charge recombination is found to be the dominant process for t≤1.2 μs with the plasma diffusion playing a perturbational role. A one-dimensional plasma model is utilized to model the experimental plasma data by treating the diffusion as a perturbation. This study shows that the recombination coefficient is 1.8±0.1×10−7 cm3 s−1 and the diffusion coefficient is 2.8±0.4×104 cm2 s−1. The plasma sheet has attractive properties for a microwave agile mirror. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.360773
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  • 22
    Electronic Resource
    Electronic Resource
    High-purity ion beam production at high current densities with a liquid-helium-cooled series-field-coil extraction ion diode (1991)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 70 (1991), S. 2926-2938 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Experiments are described in which a high-purity, high-power (0.15 TW, 1 MeV) proton beam is generated from an ion source consisting of H2 gas frozen onto a liquid-helium-cooled copper anode at 4.2 K in a series-field-coil extraction diode on the 0.7 TW HydraMITE-II accelerator. Peak anode proton current densities of 2 kA/cm2 were measured. This current density is a factor of 100 higher than those obtained in previous liquid-helium-cooled cryogenic diode experiments on small accelerators and is in the range required for high-power ion beam applications. Thomson parabola, Faraday cup, and carbon activation measurements indicate an ion beam proton fraction close to 100% for the cryogenic source, compared to 50–70% for the standard hydrocarbon anode tested. The cryogenic proton source is believed to consist of no more than a few monolayers of molecular hydrogen. The hydrogen-coated cryogenic anode shows a faster initial anode turn-on than other materials. However, source-limited emission from the thin hydrogen layer results in a somewhat longer current risetime, reduced ion diode efficiency, lower proton current enhancement over the Child–Langmuir limit, and a proton spectrum of lower average energy than for the hydrocarbon anode. Techniques to overcome these limitations are discussed. Cryogenic ion sources consisting of frozen N2, CH4, and Ne have also been studied. In each case, high intensity beams consisting predominantly of components of the refrigerated gas were produced.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.349317
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  • 23
    Electronic Resource
    Electronic Resource
    Lithium niobate stress gauge current diagnostic for noninductive measurement of fast-rise-time multimegampere currents (1990)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 68 (1990), S. 4917-4928 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Accurate modeling of load behavior in Z-pinch plasma radiation sources driven by high-current generators requires the measurement of fast-rise-time multimegampere currents close to the load. Conventional current diagnostics mounted in inductive cavities (such as B-dot loops and Rogowski coils) fail at small radius because of electrical breakdown produced by high dI/dt. In this paper, we describe the use of large-signal, nanosecond-time-resolution lithium niobate piezoelectric stress gauges to directly measure the magnetic pressure B2/2μ0=μ0I2/8π2r2 generated at radius r by a current I flowing in a radial transmission line. Current measurements have been performed at radius r=2.54×10−2 m on Sandia National Laboratories' Proto-II (10 TW) and SATURN (30 TW) gas puff Z-pinch experiments with maximum currents of 10.1 MA and dI/dt to 2.1×1014 A/s. Comparisons with Faraday rotation and B-dot current diagnostic measurements at large radius are presented. Bremsstrahlung noise problems unique to the SATURN gas puff source are discussed. For a Y-cut lithium niobate stress gauge on a pure tungsten electrode, current densities up to I/2πr=78 MA/m can be measured before the electrode yield strength and the piezoelectric operating stress limit are exceeded. Above the Hugoniot elastic limit of the electrode material, the dynamic range and accuracy of the diagnostic are greatly reduced, but it appears that the technique can be extended to higher current densities using an X-cut quartz piezoelectric element and a tungsten-sapphire electrode impedance stack.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.347077
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  • 24
    Electronic Resource
    Electronic Resource
    Recommended Bethe–Bloch stopping power parameters for Kapton and Havar (1990)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 67 (1990), S. 1639-1642 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Previous measurements of the stopping power of Kapton and Havar for 1.1–4.4-MeV protons have been analyzed with modified Bethe–Bloch theory in order to extract some of the parameters required in the formalism. Results suggest the possibility of systematic errors, in that the Havar data appear to have been slightly high and the Kapton data quite low when compared with other reported measurements. Currently recommended values of the mean excitation energy and Barkas-effect parameter are, respectively, 79 eV and 1.34 (Kapton) and 296 eV and 1.36 (Havar). The need for further accurate measurements for Kapton in particular is emphasized.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.345630
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  • 25
    Electronic Resource
    Electronic Resource
    Thermal and magnetic properties of Bi2CuO4 (abstract) (1990)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 67 (1990), S. 5761-5761 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Since the discovery of the high-Tc superconductors, there is a great interest in the magnetic properties of copper oxides due to the possibility of a magnetic origin of the pairing mechanism. With this perspective, we decide to reinvestigate the structure and the magnetic properties of Bi2CuO4, a compound we use successfully as precursor of the Bi-Sr-Ca-Cu high-Tc superconductors. A neutron powder diffraction experiment allowed us to resolve the ambiguity in the structure, and revealed the existence of a magnetic phase transition to a 3D antiferromagnetic ordered state below 50 K. The nuclear structure can be described as formed by stacks of CuO4 units in the c-axis direction, linked in the stacks and with units in other stacks by BiO2 chains, so each stack is connected with four stacks. The space group was confirmed to be P4/ncc. The magnetic space group is P4/n'cc. There is a ferromagnetic ordering of the magnetic moments on Cu atoms along the stacks, and an antiferromagnetic ordering between the stacks. We present an interpretation of the magnetic properties of this compound, at the light of our previous results, and a discussion of a calorimetric experiment.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.345956
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  • 26
    Electronic Resource
    Electronic Resource
    Methods of calculating the Barkas-effect correction to Bethe–Bloch stopping power (1990)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 67 (1990), S. 6613-6620 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The low-velocity Barkas-effect correction term for the Bethe–Bloch stopping-power formula has been calculated with each of the three extant formalisms for targets with atomic numbers from 1 to 18 and for projectile velocities from 0.025c to 0.3c. Comparison of each with measurements on aluminum revealed which formalism provided the best fit over three different projectile velocity intervals. The method yielding the generally superior agreement was used to calculate the stopping power of each element (except Li, F, Na, Mg, P, and Cl) for protons and alpha particles with energies between 0.5 and 5.0 MeV. Results corresponded very closely to those of an existing fit to most known experimental data.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.345094
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  • 27
    Electronic Resource
    Electronic Resource
    Transport by intermittent convection in the boundary of the DIII-D tokamak (2001)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 8 (2001), S. 4826-4833 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Intermittent plasma objects (IPOs) featuring higher pressure than the surrounding plasma, and responsible for ∼50% of the E×BT radial transport, are observed in the scrape off layer (SOL) and edge of the DIII-D tokamak [J. Watkins et al., Rev. Sci. Instrum. 63, 4728 (1992)]. Conditional averaging reveals that the IPOs, produced at a rate of ∼3×103 s−1, are positively charged and also polarized, featuring poloidal electric fields of up to 4000 V/m. The IPOs move poloidally at speeds of up to 5000 m/s and radially with E×BT/B2 velocities of ∼2600 m/s near the last closed flux surface (LCFS), and ∼330 m/s near the wall. The IPOs slow down as they shrink in radial size from 4 cm at the LCFS to 0.5 cm near the wall. The IPOs appear in the SOL of both L and H mode discharges and are responsible for nearly 50% of the SOL radial E×B transport at all radii; however, they are highly reduced in absolute amplitude in H-mode conditions. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1406940
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  • 28
    Electronic Resource
    Electronic Resource
    Characterization of the separatrix plasma parameters in DIII-D (1998)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 5 (1998), S. 1410-1422 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Characterization of the plasma density and temperature at the last closed flux surface (the separatrix) of a tokamak requires accurate knowledge of the location of the separatrix. In this paper we discuss the effect of inaccuracy in the separatrix location on the measured parameters in DIII-D [Luxon et al., International Conference on Plasma Physics and Controlled Nuclear Fusion (International Atomic Energy Agency, Vienna, 1986), p. 159] An uncertainty in the separatrix position of ±0.5 cm, as expected in this device using magnetic reconstruction to determine the location of the separatrix, leads to unacceptably large uncertainty in the plasma parameters. Several techniques to improve the accuracy obtained from magnetic reconstruction are discussed. A new technique that is based on a characterization of the electron temperature profile is proposed. A comparison of the separatrix location defined in this manner with that obtained using magnetic reconstruction techniques suggests a systematic error in the reconstruction when the plasma is far from the walls and magnetic diagnostics. Determination of the perpendicular transport coefficients is given as an example of the improved statistics obtained using the new technique of defining the separatrix position.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872830
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  • 29
    Electronic Resource
    Electronic Resource
    Radiative divertor plasmas with convection in DIII-D : The 39th annual meeting of division of plasma physics of APS (1998)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 5 (1998), S. 1736-1743 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The radiation of divertor heat flux on DIII-D [J. Luxon et al., in Proceedings of the 11th International Conference on Plasma Physics and Controlled Nuclear Fusion (International Atomic Energy Agency, Vienna, 1987), p. 159] is shown to greatly exceed the limits imposed by assumptions of energy transport dominated by electron thermal conduction parallel to the magnetic field. Approximately 90% of the power flowing into the divertor is dissipated through low-Z radiation and plasma recombination. The dissipation is made possible by an extended region of low electron temperature in the divertor. A one-dimensional analysis of the parallel heat flux finds that the electron temperature profile is incompatible with conduction-dominated parallel transport. Plasma flow at up to the ion acoustic speed, produced by upstream ionization, can account for the parallel heat flux. Modeling with the two-dimensional fluid code UEDGE [T. Rognlien, J. L. Milovich, M. E. Rensink, and G. D. Porter, J. Nucl. Mater. 196–198, 347 (1992)] has reproduced many of the observed experimental features. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872842
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  • 30
    Electronic Resource
    Electronic Resource
    Tungsten wire-array Z-pinch experiments at 200 TW and 2 MJ : The 39th annual meeting of division of plasma physics of APS (1998)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 5 (1998), S. 2105-2111 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Here Z, a 60 TW/5 MJ electrical accelerator located at Sandia National Laboratories, has been used to implode tungsten wire-array Z pinches. These arrays consisted of large numbers of tungsten wires (120–300) with wire diameters of 7.5 to 15 μm placed in a symmetric cylindrical array. The experiments used array diameters ranging from 1.75 to 4 cm and lengths from 1 to 2 cm. A 2 cm long, 4 cm diam tungsten array consisting of 240, 7.5 μm diam wires (4.1 mg mass) achieved an x-ray power of ∼200 TW and an x-ray energy of nearly 2 MJ. Spectral data suggest an optically thick, Planckian-like radiator below 1000 eV. One surprising experimental result was the observation that the total radiated x-ray energies and x-ray powers were nearly independent of pinch length. These data are compared with two-dimensional radiation magnetohydrodynamic code calculations. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872881
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