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  • 1985-1989  (5)
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  • 1
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: The Phaedrus diagnostic system makes optimum use of probes and miniature gridded energy analyzers to measure detailed parameters such as density, plasma potential, and electron and ion energy. This requires miniaturization and provision for flexible access. Other diagnostics are made as flexible as possible. A multiple-aperture, end-loss analyzer is rotatable, allowing complete mapping of the end wall loss contours. Self-emissive probes are used to measure plasma potential contours in the end plugs. A single-channel neutral energy analyzer is movable and can be swept in energy. Secondary emission detector arrays supplement the neutral energy analyzer and give a measure of the ion energy versus pitch angle. Calibrated diamagnetic loops are used to measure the total plasma energy and its axial extent in each cell. Both permanent and insertable fast ion gauges are available. Completing the diagnostic system is a computerized data-acquisition system that is flexible and receptive to the time varying demands of the many ongoing experiments.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Electrostatic end plugging is observed in a completely axisymmetric, three cell tandem mirror under conditions where the central-cell plasma density is always larger than the end-cell density. A factor of 4 increase in the central-cell density, to a maximum of 1.2×1013 cm−3 with simultaneous plasma beta of 13%, occurs upon application of the end plugging potential. Ion confining potentials of 25 V and 80 V at the two ends of the device, respectively, result in a factor of 2.5 increase in the axial confinement time for Tic =40 eV in agreement with the collisional flow model for ion confinement. The non-Boltzmann ion confining potential is caused by electron heating in the end cells by rf near the ion-cyclotron frequency. The initial central-cell density rise is caused by an increase in the ionization rate that occurs because of an increase in the electron temperature. The density remains high throughout the end-cell heating pulse as a result of increased particle confinement time. There is no nonambipolar radial ion loss in the core plasma (r≤16 cm) but inward radial transport of ions is observed at a rate consistent with ion–neutral collisions and a radial electric field in the negative radial direction. Steady-state thermal-barrier-like potential dips that are in agreement with the Boltzmann model for potentials are observed in the transitions between the central cell and the end cells.
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Stabilization of interchange instabilities in the Phaedrus tandem mirror [Phys. Rev. Lett. 51, 1955 (1983)] through the use of externally applied rf at a single frequency is demonstrated over a wide range of frequency (1.03Ωi 〈ω〈4.1Ωi). Here rf stabilized plasmas are reported with ion temperature up to 550 eV and beta up to 8%. Calculations of the rf electric fields show that the dominant mode is the m=+1 fast wave. When the rf frequency is near Ωi the calculated ponderomotive forces on ions and electrons are shown to be comparable. For ω〉1.3Ωi it is shown that the force acts primarily on electrons. Stable plasmas are only achieved when the net radial ponderomotive force is calculated to be stabilizing and comparable to the curvature force. Results are also reported for rf applied simultaneously at two frequencies, one which is stabilizing and one which is destabilizing. Abrupt changes in stability are observed as the rf power is increased whenever one of the rf frequencies is effective at plasma heating.
    Type of Medium: Electronic Resource
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  • 4
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Fueling and heating of tandem mirror end cells by rf at the ion-cyclotron frequency have been studied experimentally in the Phaedrus Tandem Mirror. The end cell density is found to increase linerly with rf voltage. The total plasma energy is observed to increase with rf power with no evidence of saturation at high power. The plasma axial length decreases with increasing rf power down to a length of approximately the distance between the two resonance locations in the end cell. The highest density and average ion energy are achieved with the resonance closest to the midplane. It is necessary to assume that the rf electric field at the resonance decreases with increasing density and with distance from the antenna in order to model the ion particle and power balance. The particle and power balance model predicts that dense, hot end cells may be maintained with E+∼1 V/cm and a fueling efficiency of a few percent when the resonance is close to the midplane because of better ion confinement and small plasma volume. Monte Carlo simulation of the trapping process shows that E2+ ∝T1.5ic is required to maintain a given rf trapping efficiency.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 62 (1987), S. 1054-1058 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We have studied the first stages of low-pressure oxidation of amorphous silicon using Auger electron spectroscopy. The application of the principal component analysis to the Si L2,3 core-valence-valence transition spectra, acquired during the oxidation, allows us for the interpretation of the oxidation kinetics of the a-Si. Using the target transformation method we have isolated the Auger spectra of the components present during the oxidation process. We observe an intermediate state in the Si-SiO2 interface formed during the oxidation. This state was attributed to a SiOx-type compound.
    Type of Medium: Electronic Resource
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