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  • 1
    Electronic Resource
    Electronic Resource
    Doping and residual impurities in GaAs layers grown by close-spaced vapor transport (1993)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 73 (1993), S. 1288-1296 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The close-spaced vapor transport (CSVT) technique is used to grow GaAs epitaxial layers from various n- or p-type doped GaAs sources. The transport agent is H2O with PH2O = 4.58 Torr. n-type layers can be grown with Te- or Ge-doped GaAs sources. The transport coefficients of both dopants (ratio of the electrically active dopant concentration in the layer to the electrically active dopant concentration in the source) is 100% for Te or Ge, in the substrate temperature range comprised between 750 and 850 °C. p-type layers are obtained with Zn-doped GaAs sources. The transport coefficient of Zn is about 1% and is also independent of the substrate temperature. The transport coefficients and their independence on temperature are in agreement with a mass-transport controlled model based on the hypothesis that the transport reactions of GaAs and the doping impurities are in equilibrium at the source and substrate temperatures. Si-doped GaAs cannot be used as a source to obtain conductive n-type layers. When undoped semi-insulating (SI)-GaAs wafers are used as sources in CSVT, n-type layers are obtained. They are characterized by ND−NA=9×1015–3×1016 cm−3 and μ300K=3000–4000 cm2 V−1 s−1, independent of the temperature, in the temperature range investigated. Glow discharge mass spectroscopy analyses performed on a source and on a layer indicate that C, O, Si, and S are the major residual impurities in the GaAs layer. All these impurities have their origin in the technique (reactor, transport agent). Ge is also present in the layers, as indicated by photoluminescence. It is a minor impurity. Its origin is probably the SI-GaAs source.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.353246
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  • 2
    Electronic Resource
    Electronic Resource
    Charge separation at a plasma edge in the presence of a density gradient (2000)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 7 (2000), S. 2517-2525 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A fully kinetic code for ions [one dimensional (1-D) in space, and using the three velocity dimensions in velocity space] is used to study the problem of the formation of a charge separation with the self-consistent electric field in a plasma in the presence of a density gradient. Electrons are treated using an adiabatic law. Graphical results are presented which follow the formation of a 1-D steady state showing the formation of an oscillating positive potential bump toward the edge of the plasma. These oscillations are closely associated with the gyration of the ions. It is also shown that the presence of a small fraction of impurity ions at the plasma edge can have a significant effect on the rapid buildup of the potential at the edge, and in increasing the charge separation and the associated electric field at the edge, in comparison to the case when no impurity ions are included. The present results show the importance of a kinetic solution to the problem of the equilibrium electric field and charge separation in the presence of a density gradient, and point to the important role played by the finite ions' gyroradius and the important contribution of impurity ions in this case. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.874091
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  • 3
    Electronic Resource
    Electronic Resource
    Vlasov gyrokinetic simulations of ion-temperature-gradient driven instabilities (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 202-217 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: An Eulerian code that solves the gyrokinetic Vlasov equation in slab geometry is presented. It takes into account the E×B and polarization drifts in the plane perpendicular to the magnetic field, and kinetic effects in the parallel direction. The finite Larmor radius is modelled by a convolution operator. The relation is established between this model and others proposed previously, and they are shown to be equivalent in the limit of long wavelengths and small Larmor radii. The code is applied to investigate ion-temperature-gradient modes in the quasi-neutral regime, with adiabatic electrons. Numerical results are reported for a wide range of parameters, including density and temperature profiles, magnetic field strength, and ion to electron temperature ratio. Normally the plasma evolves towards long wavelength structures, although in some cases (when Landau damping is very weak) more strongly turbulent regimes are observed. Test particles are used to compute diffusion coefficients both in real space and velocity space. For the most strongly turbulent regimes, particle diffusion coefficients are of order 20 m2 s−1. The saturation mechanism is also investigated. Many previous numerical results obtained with particle codes are confirmed, but the Vlasov Eulerian technique allows a much finer resolution of structures both in real space and velocity space.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871846
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  • 4
    Electronic Resource
    Electronic Resource
    A hybrid Eulerian Vlasov code. I. Study of high-frequency beatwave experiment and Manley–Rowe action evolution in a finite causal system (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 650-668 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: For the first time, beatwave simulations relevant to the UCLA experiment (University of California at Los Angeles) [see Clayton et al., Phys. Rev. Lett. 70, 37 (1993), and also Phys. Plasmas 1, 1753 (1994)] have been made with a relativistic Eulerian Hilbert–Vlasov code for a realistically high ratio of driver frequency to plasma wave frequency (ω0/ωp≈30). Some of the more striking features that have emerged from the Hilbert–Vlasov simulations are discussed in this paper, with particular emphasis on particle dynamics in phase space with beam injection, and action transfer results obtained from the derivation of the integrated Manley–Rowe relations derived for a finite causal system. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871892
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  • 5
    Electronic Resource
    Electronic Resource
    A hybrid semi-Lagrangian Vlasov code. II. Numerical simulation of chirped beatwave experiment (1998)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 5 (1998), S. 4041-4054 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: In this paper the results are reported of a set of numerical simulations applicable to the problem of particle acceleration by the beating of very high-frequency laser waves (the ratio of driver frequency to plasma frequency close to ω0/ωp=30), including for the first time beat frequency chirping (i.e., pump frequency linearly decreasing with time) at various rates in order to counteract relativistic electron detuning of the plasma oscillation frequency and the resulting Rosenbluth–Liu amplitude limit. This amplitude limit can indeed be evaded, giving extremely high acceleration gradients of more than 25 GeV/m, for the case of numerical parameters relevant to the UCLA experiment (University of California at Los Angeles). A chirp rate that is too fast leads to a sudden loss of frequency lock and a subsequent decay of the driven electric field, in agreement with the behavior of a similarly driven relativistic Lagrangian Rosenbluth–Liu oscillator. Even at chirp rates somewhat below this limiting value a remarkably localized and extraordinarily intense (but as yet ill-understood) electric field structure is formed, one that is unlikely to be compatible with the general requirement for smooth and coherent acceleration. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.873126
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  • 6
    Electronic Resource
    Electronic Resource
    Effect of impurity ions on the charge separation and velocity shear at a plasma edge (1999)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 6 (1999), S. 1401-1404 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The problem of the formation of charge separation in a plasma in the presence of a steep density gradient, the self-consistent electric field and the associated E×B velocity, are studied using a two-dimensional (2D) gyro-kinetic Vlasov code for the ions, with electrons following an adiabatic law. The code shows the formation of a one-dimensional (1D) equilibrium charge at the plasma edge. It is also shown that the presence of a small fraction of impurity ions at the plasma edge can have a significant effect in increasing the effective charge separation and the associated electric field. The present results show that only a kinetic code can solve the problem of the equilibrium electric field in the presence of a density gradient, and point to the important role played by the ions' gyro-radius in establishing a charge separation at a plasma edge. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.873486
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  • 7
    Electronic Resource
    Electronic Resource
    Two-stage electron acceleration by simultaneous stimulated Raman backward and forward scattering (1995)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 2 (1995), S. 3115-3129 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The coexistence of stimulated Raman forward and backward scattering of intense electromagnetic radiation, which can occur, for instance, in laser fusion plasmas, is investigated. The simultaneous Raman forward and backward scattering is shown to create an electrostatic field structure which is exceptionally efficient in producing highly relativistic electrons. The mechanism of the electron acceleration is analyzed both by Vlasov–Maxwell simulations with self-consistent fields and by test particle calculations with prescribed electrostatic fields. The Vlasov–Maxwell simulations reveal that the two plasma waves generated by the backward and forward scattering are spatially separated, and thus form a two-stage electron "accelerator.'' © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871144
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  • 8
    Electronic Resource
    Electronic Resource
    Structure of benzene and phenol chemisorbed on Ni{110} (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 98 (1993), S. 5855-5862 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The chemisorption of benzene and phenol on a clean Ni{110}–(1×1) surface and an oxygen predosed Ni{110}–(3×1)–O surface near room temperature has been investigated by time-of-flight scattering and recoiling spectrometry accompanied by shadow cone calculations. The Ne scattering and H, C, and O recoiling fluxes exhibited strong angular anisotropies as a function of beam incident (α) and crystal azimuthal (δ) angles. These anisotropies are due to C and O atoms shadowing their neighboring atoms within the benzene molecules and resulting phenoxide species, demonstrating that scattering and recoiling spectrometry is capable of providing structural information on polyatomic molecular systems. The results show that both benzene and phenoxide are chemisorbed as molecules which have very good short-range order despite the absence of long-range order observable by low energy electron diffraction. Both benzene and phenoxide are oriented nearly parallel to the surface, with a maximum inclination angle of 15°. The C atoms in the para positions of benzene and the C–O bond in phenoxide are oriented along the 〈001(approximately-greater-than) azimuth. The C–H bond is bent out of the plane of the hexagonal ring so that the H atoms are above the C atom plane. Chemisorption on the oxygen predosed surface results in a reaction in which a H atom is abstracted from both benzene and phenol with the formation of surface hydroxide groups; the molecules remain well ordered on this surface also.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.464877
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  • 9
    Electronic Resource
    Electronic Resource
    Extraction studies on electron cyclotron resonance ion sources : Proceedings of the 6th international conference on ion sources (1996)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 67 (1996), S. 1350-1352 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: The production of radioactive ion beams in the SPIRAL project requires very efficient ion sources delivering beams with good optical properties. To improve these properties, the extraction conditions are under study. This paper will present the last results obtained with a new extraction system and will compare them to a numerical simulation. This comparison shows that the axial magnetic field influences the multicharged ion beam if the space charge during the acceleration is not compensated. In order to decrease the axial magnetic field effect on the extraction zone, a parallel beam can be formed with a multielectrode system. The first tests of this system will be presented. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1146662
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  • 10
    Electronic Resource
    Electronic Resource
    P–T–X relationships in the Precambrian Al–Mg-rich granulites from In Ouzzal, Hoggar, Algeria (1992)
    Oxford, UK : Blackwell Publishing Ltd
    Journal of metamorphic geology 10 (1992), S. 0 
    Details
    ISSN: 1525-1314
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: The Al–Mg-rich granulites from the In Ouzzal craton, Algeria, show a great diversity of mineral reactions which correspond to continuous equilibria as predicted by phase relationships in the FeO–MgO–Al2O3–SiO2 system. The sequence of mineral reactions can be subdivided into three distinct stages: (1) a high-P stage characterized by the growth of coarse mineral assemblages involving sapphirine and the disappearance of early corundum and spinel-bearing assemblages; (2) a high-T stage characterized by the development of Sa–Qz-bearing assemblages; and (3) a later stage, in which garnet-bearing assemblages are replaced by more or less fine symplectites involving cordierite.During the course of early mineral reactions, the distribution coefficient, Kd, between the various ferromagnesian phases decreased significantly whereas Al2O3 in pyroxene increased concomitantly. These observations, when combined with topological constraints, clearly indicate that the high-P stage 1 was accompanied by a significant rise in temperature (estimated at 150 ± 50° C) under near isobaric conditions, in agreement with the reaction textures. By stage 2, pressure and temperature were extreme as evidenced by the low Kd value between orthopyroxene and garnet (Kd= 2.06–1.99), the high alumina content in pyroxene (up to 11.8%) and the high magnesium content in garnet [100 Mg/(Mg + Fe) = 60.6]. Mineral thermometry based on Fe–Mg exchange between garnet and pyroxene and on Al-solubility in pyroxene gives temperatures close to 970 ± 70° C at 10 ± 1.5 kbar. These results are in agreement with the development of Sa–Qz assemblages on a local scale.Late mineral reactions have been produced during a decompression stage from about 9 to 6 kbar. Except for local re-equilibration of Mg and Fe at grain boundaries, there is no evidence for further reactions below 700° C.We interpreted the whole set of mineral reactions as due to changes in pressure and temperature during a tectonic episode located at c. 2 Ga. Because of the lack of evidence for further uplift after the thermal relaxation which occurred at c. 6 kbar, it is possible however that the exhumation of this granulitic terrane occurred in a later tectonic event unrelated to its formation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1525-1314.1992.tb00069.x
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