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  • 1
    Electronic Resource
    Electronic Resource
    Experiments on space-charge waves in electron beams propagating through a resistive-wall channel (1999)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 86 (1999), S. 1699-1709 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Localized longitudinal space-charge waves in a transport channel with a resistive wall are investigated experimentally. The space-charge waves with various pulse widths are generated in electron beams with energies of 3.0–8.5 keV and currents of 30–135 mA, and they are transported through a resistive channel with a resistance of 5–10 kΩ and a length of 1 m. The resistive channel, made of a glass tube with a resistive material coating, is surrounded by a long solenoid which provides the focusing for the beam. The localized space-charge waves are measured with current monitors and energy analyzers at the entrance and exit of the resistive channel, and the wave-amplitude growth and damping rates are determined. The measured results are compared with the theory in the long-wavelength range. For localized waves with short widths, where the long-wavelength approximation is not valid any more, dispersion and distributed capacitance effects on the growth/damping rates are discussed. In addition, preliminary results on the energy width measurements of space-charge waves are presented. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.370950
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  • 2
    Electronic Resource
    Electronic Resource
    Thickness dependence of giant magnetoresistance effect in granular Cu–Co thin films (1999)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 85 (1999), S. 4471-4473 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: This work investigates the magnetic and transport properties of Cu–Co thin films with a special reference to their dependence on the film thickness. Such dependencies of the giant magnetoresistance (GMR) effect in silver-based magnetic alloys, such as Ag–Fe, Ag–Co, and Ag–FeNi films, have recently been found, and they were interpreted within the framework of surface spin-flipping scattering. This article reports on similar thickness dependence in the Cu-based alloy, although the spin-orbit interaction in Cu films is much weaker than in Ag films. A reduction of the GMR in the thinnest samples by a factor of 6, compared to the value of as-deposited bulk samples (8.6% in 50 kOe and at 5 K), was accompanied by an increase in resistivity by no more than 50%. A novel vapor-mixing technique of simultaneous sputtering from two sources was used to deposit Cu80Co20 granular thin films of the 20% nominal Co volume fraction. The thickness of the films, ranging from 8 to 400 nm, was measured by the small-angle x-ray reflectivity method. The average size of the Co particles ranging from 2.5 to 3 nm was determined from the superparamagnetic behavior of the Cu–Co granular films. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.370378
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  • 3
    Electronic Resource
    Electronic Resource
    Effects of orientation on the time decay of magnetization for cobalt–alloy thin film media (1999)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 85 (1999), S. 4997-4999 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The dependence of the time decay of magnetization on orientation ratio was investigated for longitudinal Co–alloy thin film media. The coercivity orientation ratio was controlled by the degree of mechanical texture. For oriented samples, it was found that the remanent magnetization along the circumferential direction decayed faster with time than that along the radial direction when the applied reverse magnetic field was near the remanent coercivity. However, the remanent magnetization along the circumferential direction decayed more slowly with time than that along the radial direction when the applied reverse magnetic field was less than roughly half the remanent coercivity. Anisotropic interactions and magnetic anisotropy distributions appear to be the cause for the different time decay of magnetization along the circumferential and radial directions for oriented media. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.370070
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  • 4
    Electronic Resource
    Electronic Resource
    Comparison of Zakharov simulation and open boundary Vlasov simulation of strong Langmuir turbulence (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 111-121 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Both Zakharov equations and Vlasov equations are solved numerically to study the strong Langmuir turbulence developed in a plasma driven by an external pump field oscillating at the plasma frequency. A steady state turbulence in Vlasov simulations is reached by using open boundary conditions under which hot particles generated by strong caviton fields are replaced by initial cold Maxwellian particles when they cross the boundaries of simulation domain. A similar steady turbulent state in Zakharov simulations is easily achieved by implementing a phenomenological damping model and using periodic boundary conditions. Simulation results of these two different models are compared and investigated. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871837
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  • 5
    Electronic Resource
    Electronic Resource
    Longitudinal space-charge waves and instabilities in intense beams : 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. 2064-2070 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Space-charge effects and collective behavior play an important role in the dynamics of intense beams for accelerator applications. One of the research issues is the longitudinal space-charge waves and instabilities in high current accelerators. At the University of Maryland, an experimental program has been conducted to systematically study space-charge waves and longitudinal instabilities in space-charge dominated electron beams. This includes the generation of localized space-charge waves in an active device; the propagation and end reflection of space-charge waves in a conducting transport channel; and the longitudinal instabilities in a resistive-wall channel. This paper reports and discuss our experimental results. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872878
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  • 6
    Electronic Resource
    Electronic Resource
    Global energy confinement under coupling of the transport process with magnetohydrodynamic modes (1995)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 2 (1995), S. 2753-2759 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Macroscopic instabilities are governed by transport-determined plasma profiles and transport, in turn, is affected by instabilities. It is clear that neither transport process nor the behavior of instabilities can be treated in isolation. A model is developed for investigating the effects of the instabilities on global energy confinement in tokamaks. The results clearly demonstrate that magnetohydrodynamic (MHD) instabilities coupled with the transport process change not only the value of global energy confinement time, but also, perhaps more importantly, the energy confinement scaling law. While considering MHD effects, global energy confinement becomes sensitively dependent on the boundary transport conditions, and the cases with and without MHD stability can result in quite different parameter behavior of the global energy confinement. Such coupling should be considered when attempts are made to analyze the global energy confinement in tokamaks. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871240
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  • 7
    Electronic Resource
    Electronic Resource
    Vlasov simulation of modulational instability and Langmuir collapse (1995)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 2 (1995), S. 1129-1139 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Vlasov simulations are developed to study the one-dimensional modulational instability and Langmuir collapse in a plasma driven by an external field. Both electrons and ions in the plasma are described by the Vlasov equations. The external driving field energy density is ∼10−3 times the initial electron thermal energy density. The plasma has a mass ratio of mi/me=1836, and a temperature ratio of Ti/Te=0.1 or 1.0, where mi, me, Ti, and Te are the ion mass, electron mass, ion temperature, and electron temperature. The modulational instability is saturated by the formation of Langmuir cavitons which later collapse and burn out due to the strong wave–particle interactions. The particle heating during a single Langmuir collapse is small under this weak driving field. The accelerated electron and ion jet-like streams in phase space as well as the nucleation of cavitons are observed during the evolution. Zakharov equations with the same parameters and similar initial conditions are also simulated, and the results are compared with those of the Vlasov simulations. The two models agree with each other quantitatively up to the first collapse following the saturation of exponential growth; dynamic differences appear after the first collapse. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871391
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  • 8
    Electronic Resource
    Electronic Resource
    Space-charge solitary waves in charged particle beams in resistive-wall transport channels (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 669-672 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The properties of solitary waves in space-charge dominated charged particle beams are investigated theoretically. Similarly to linear space-charge waves, both slow and fast solitary waves can exist. In a dissipative environment, e.g. a resistive-wall channel, it is shown that the slow space-charge solitary waves grow in amplitude, while the fast space-charge solitary waves decay. However, the growth and decay rates differ from those of the linear waves. A possible experiment with an electron beam is proposed to demonstrate these phenomena. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871893
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  • 9
    Electronic Resource
    Electronic Resource
    Dielectric studies of Zn1−xMnxSe epilayers (1999)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 86 (1999), S. 1630-1633 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We report on the dielectric properties of Zn1−xMnxSe(0≤x≤0.78) epilayers, by capacitance and dissipation factor measurements at temperature 5 K〈T〈475 K and frequency 20 Hz〈f〈100 kHz. A Debye-like relaxational contribution to the dielectric response is observed, which requires the presence of charge redistribution. The relaxation is found to be a thermally activated process, and the activation energies obtained from the dissipation factor and capacitance are in good agreement. The capacitance is found to increase with a decrease in test frequency. From our results it is established that the dielectric response is caused by carrier hopping among structural defects. A monotonic variation is found in the relationship between the activation energy and the Mn concentration. This monotonic variation is interpreted in terms of the four-center model, in which the number of Mn atoms appearing in the nearest-neighbor sites of the defect can have four possible configurations. The measured defect behavior reflects the overall average of all energy levels involved in the center, and the result is weighted by the relative concentrations. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.370938
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  • 10
    Electronic Resource
    Electronic Resource
    Magnetic properties of RFe11.3Nb0.7 compounds (R=rare earth) (1999)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 85 (1999), S. 4684-4686 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Structural and magnetic properties of RFe11.3Nb0.7 compounds with rare-earth elements (R=Y, Sm, Gd, Tb, Dy, Ho, and Er) have been investigated. The Curie temperature increases for R=Sm–Gd, then decreases from R=Gd to Er. In the compound with R=Tb, with increasing temperature, a spin reorientation from an easy magnetization direction in the plane to an easy cone occurs at 365 K. In the compound with R=Er, a spin reorientation from easy plane to easy axis is found at about 40 K. Two spin reorientations take place in the compound with R=Dy, from easy plane to a complex structure at 125 K and from complex structure to easy axis at 210 K. At room temperature, the easy magnetization direction is along the c axis for R=Y, Sm, Gd, Dy, Ho, and Er, and in the plane for R=Tb. Studies of the magnetic anisotropy in YFe11.3Nb0.7 and GdFe11.3Nb0.7 point out that the Fe sublattice anisotropy is of easy axis type. In HoFe11.3Nb0.7, a first-order magnetization process takes place below 150 K when an external magnetic field is applied along the hard magnetization direction. The exchange-interaction constants JR–Fe decreases with increasing atomic number of R, as is generally found in rare-earth transition-metal compounds. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.370447
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