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  • American Institute of Physics (AIP)  (7)
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  • 1
    Electronic Resource
    Electronic Resource
    Ion divergence in magnetically insulated diodes (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 2175-2182 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Magnetically insulated ion diodes are being developed to drive inertial confinement fusion. Ion beam microdivergence must be reduced to achieve the very high beam intensities required to achieve this goal. Three-dimensional particle-in-cell simulations [Phys. Rev. Lett. 67, 3094 (1991)] indicate that instability-induced fluctuations can produce significant ion divergence during acceleration. These simulations exhibit a fast growing mode early in time, which has been identified as the diocotron instability. The divergence generated by this mode is modest, due to the relatively high-frequency ((approximately-greater-than)1 GHz). Later, a low-frequency low-phase-velocity instability develops with a frequency that is approximately the reciprocal of the ion transit time. This instability couples effectively to the ions, and can generate unacceptably large ion divergences ((approximately-greater-than)30 mrad). Linear stability theory reveals that this mode has structure parallel to the applied magnetic field and is related to the modified two-stream instability. Measurements of ion density fluctuations and energy-momentum correlations have confirmed that instabilities develop in ion diodes and contribute to the ion divergence. In addition, spectroscopic measurements indicate that lithium ions have a significant transverse temperature very close to the emission surface. Passive thin-film lithium fluoride (LiF) anodes have larger transverse beam temperatures than laser-irradiated active sources. Calculations of the ion beam source divergence for the LiF film due to surface roughness and the possible loss of adhesion and fragmentation of this film are presented. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871672
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  • 2
    Electronic Resource
    Electronic Resource
    Statistics of surface flow structures on decaying grid turbulence (1986)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 29 (1986), S. 2388-2397 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The surface flow pattern of decaying turbulence generated by a vertical bar grid in a towing tank has been studied. The velocities and positions of about 2×104 Al aluminum filing tracers following the surface motion have been measured and statistically analyzed. From ten photo series, each of 10 mesh width span and 40 mesh widths long, 2000 surface eddies were identified and studied. The flow field is decomposed into "closed'' rotating flow structures (the surface eddies) and "open'' flow structures (rivers) of predominantly translational motion. There are also relatively stagnant areas that grow in number and size with time. Immediately behind the grid the average energy densities of the surface eddies ee and the rivers er are about equal but ee decays at a greater rate than er. The eddy size distribution gradually shifts to larger eddies and an increasing fraction of the fluctuating kinetic energy is transferred into the rivers. In the collision-dominated equilibrium region, the eddy and the river energy distributions can be described by Boltzmann-type functions. These distributions may then be characterized by "temperature'' θe and θr for eddies and rivers. These temperatures decay steadily throughout the observation period, but remain approximately equal to each other. The relationship between the surface flow dynamics and the subsurface and bulk turbulent grid-flow is examined.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.865532
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  • 3
    Electronic Resource
    Electronic Resource
    Formation and decay of a spheromak plasma (1993)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 5 (1993), S. 1816-1827 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The magnetic properties of the spheromak configuration produced by a combination of slow theta and Z discharges in the University of Maryland Spheromak experiment (MS) are reported. The magnetic structure of the plasma in MS has been mapped out by arrays of passive magnetic pickup coils. The Taylor relaxation process is observed during the formation phase. The magnetic profile evolves in such a way that the ratio of poloidal current Ip to poloidal flux ψ in the plasma approaches a constant value, where μ0Ip=kelψ. When the spheromak is formed, the magnetic field configuration is close to Taylor's minimum energy state, μ0j=kB. This constant k is related to the size of the spheromak produced. A spheromak with 1.0 T maximum field, corresponding to 650 kA poloidal current, has been produced in MS. However, due to the high plasma density (6–8×1020 m−3) and the presence of low-Z impurities (mainly carbon and oxygen), the plasma is radiation dominated with electron temperature ≤15 eV. The magnetic field decays exponentially during the decay phase. Axisymmetric equilibrium states that could exist in the configuration are calculated with a Grad–Shafranov equilibrium code. Comparison of the numerical calculation with the experimental measurements indicates that the magnetic-field structure stays close to the equilibrium state as the plasma decays.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.860818
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  • 4
    Electronic Resource
    Electronic Resource
    Nanosecond time-resolved vacuum ultraviolet spectrometer for ion diode spectroscopy (1993)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 64 (1993), S. 2493-2495 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A 1-m normal incidence spectrometer has been modified for use as a diagnostic of ion diode plasmas. To improve instrumental sensitivity, an elliptical mirror images an anode surface plasma onto the entrance slit of an f/10 normal incidence spectrometer. The detector is a time-resolving copper iodide coated microchannel plate stripline framing camera with 60-μm resolution, limiting instrumental resolution to 1 A(ring) with a 600 l/mm grating in first order. Reflectivity of optics and photoelectron efficiency limit the spectral range from 400 to 2000 A(ring). With a 600-l/mm grating the detector spans a 600-A(ring) range. Applications of the instrument may include ion source divergence measurements from Doppler broadening, electric field measurements from Stark splittings or shifts, electron temperature from mean ionization state, and magnetic field measurements on high-power Z pinches from Zeeman splitting.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1143909
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  • 5
    Electronic Resource
    Electronic Resource
    Analyzing noisy spectral line shapes : Proceedings of the 9th topical conference on high temperature plasma diagnotics (1992)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 63 (1992), S. 4783-4785 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: In many situations it is necessary to analyze spectral line shapes where the data contain significant amounts of noise or statistical fluctuations. Our visible spectroscopy measurements exploring ion diode physics on the PBFA II accelerator typically result in noisy spectra because the harsh environment limits the number of photons collected. The spectral line profiles include contributions from Doppler (Ti∼1–3 keV), Stark (ne∼1017 cm−3), and instrument broadening, as well as from Stark shifting (E∼3–10 MV/cm) and Zeeman splitting (B∼2–10 T). We extract a range of parameters (e.g., ion temperature from Doppler broadening) that fit the data by determining a range of fits that are consistent with the uncertainty due to the noise in the data. The range of fits is generated by a Monte Carlo technique. This method effectively distinguishes between actual spectral features and artifacts due to noise. It provides not only estimates of physical parameters, but also their uncertainties. We evaluated the technique over a range of signal-to-noise ratios and found that it works well for our application.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1143562
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  • 6
    Electronic Resource
    Electronic Resource
    High-accuracy time- and space-resolved Stark shift measurements (invited) : Proceedings of the eleventh topical conference on high temperature plasma diagnostics (1997)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 68 (1997), S. 1009-1014 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: Stark-shift measurements using emission spectroscopy are a powerful tool for advancing understanding in many plasma physics experiments. We use simultaneous two-dimensional space- and time-resolved spectra to study the electric field evolution in the 20 TW Particle Beam Fusion Accelerator II ion diode acceleration gap. Fiber optic arrays transport light from the gap to remote streaked spectrographs operated in a multiplexed mode that enables recording time-resolved spectra from eight spatial locations on a single instrument. Design optimization and characterization measurements of the multiplexed spectrograph properties include the astigmatism, resolution, dispersion, and sensitivity. A semiautomated line-fitting procedure determines the Stark shift and the related uncertainties. Fields up to 10 MV/cm are measured with an accuracy ±2%–4%. Detailed tests of the procedure confirm that the uncertainty in the wavelength-shift error bars is less than ±20%. Development of an active spectroscopy probe technique that uses laser-induced fluorescence from an injected atomic beam to obtain three-dimensional space- and time-resolved measurements of the electric and magnetic fields is in progress. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1147826
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  • 7
    Electronic Resource
    Electronic Resource
    Absorption spectroscopy characterization measurements of a laser-produced Na atomic beam : Proceedings of the eleventh topical conference on high temperature plasma diagnostics (1997)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 68 (1997), S. 354-357 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A pulsed Na atomic beam source developed for spectroscopic diagnosis of a high-power ion diode is described. The goal is to produce a ∼1012-cm−3-density Na atomic beam that can be injected into the diode acceleration gap to measure electric and magnetic fields from the Stark and Zeeman effects through laser-induced fluorescence or absorption spectroscopy. A ∼10 ns full width at half-maximum (FWHM), 1.06 μm, 0.6 J/cm2 laser incident through a glass slide heats a Na-bearing thin film, creating a plasma that generates a sodium vapor plume. A ∼1 μs FWHM dye laser beam tuned to 5890 Å is used for absorption measurement of the Na I resonant doublet by viewing parallel to the film surface. The dye laser light is coupled through a fiber to a spectrograph with a time-integrated charge-coupled-device camera. A two-dimensional mapping of the Na vapor density is obtained through absorption measurements at different spatial locations. Time-of-flight and Doppler broadening of the absorption with ∼0.1 Å spectral resolution indicate that the Na neutral vapor temperature is about 0.5–2 eV. Laser-induced fluorescence from ∼1×1012 cm−3 Na I 3s-3p lines observed with a streaked spectrograph provides a signal level sufficient for ∼±0.06 Å wavelength shift measurements in a mock-up of an ion diode experiment. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1147828
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