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  • 1
    Electronic Resource
    Electronic Resource
    Global gyrokinetic simulation of tokamak transport (1999)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 6 (1999), S. 1227-1245 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A kinetic simulation code based on the gyrokinetic ion dynamics in global general metric (including a tokamak with circular or noncircular cross-section) has been developed. This gyrokinetic simulation is capable of examining the global and semi-global driftwave structures and their associated transport in a tokamak plasma. We investigate the property of the ion temperature gradient (ITG) or ηi(ηi≡∂ ln Ti/∂ ln ni) driven drift waves in a tokamak plasma. The emergent semi-global drift wave modes give rise to thermal transport characterized by the Bohm scaling. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.873366
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  • 2
    Electronic Resource
    Electronic Resource
    Theory of self-organized critical transport in tokamak plasmas (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 1289-1307 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A theoretical and computational study of the ion temperature gradient (ITG) and ηi instabilities in tokamak plasmas has been carried out. In a toroidal geometry the modes have a radially extended structure and their eigenfrequencies are constant over many rational surfaces that are coupled through toroidicity. These nonlocal properties of the ITG modes impose a strong constraint on the drift mode fluctuations and the associated transport, showing self-organized criticality. As any significant deviation away from marginal stability causes rapid temperature relaxation and intermittent bursts, the modes hover near marginality and exhibit strong kinetic characteristics. As a result of this, the temperature relaxation is self-similar and nonlocal, leading to radially increasing heat diffusivity. The nonlocal transport leads to Bohm-like diffusion scaling. Heat input regulates the deviation of the temperature gradient away from marginality. We present a critical gradient transport model that describes such a self-organized relaxed state. Some of the important aspects in tokamak transport like Bohm diffusion, near marginal stability, radially increasing fluctuation energy and heat diffusivity, intermittency of the wave excitation, and resilient tendency of the plasma profile can be described by this model, and these prominent features are found to belong to one physical category that originates from the radially extended nonlocal drift modes. The obtained transport properties and scalings are globally consistent with experimental observations of low confinement mode (L-mode) discharges. The nonlocal modes can be disintegrated into smaller radial islands by a poloidal shear flow, suggesting that the transport changes from Bohm-like to near gyro-Bohm. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871754
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  • 3
    Electronic Resource
    Electronic Resource
    Beam orbit analysis in a wiggler by using a noncanonical perturbation method (1995)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 2 (1995), S. 1316-1325 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: In a free-electron laser (FEL) with a long wiggler, an additional focusing field is required to keep beam particles from diverging. An additional long scale transverse motion induced by the focusing field modulates the longitudinal motion and sometimes destroys the axial phase relation between the beam particles and radiation field, even if the beam is successfully transmitted. In order to investigate such a delicate phase relation under a complex wiggler configuration, a systematic perturbation method that employs noncanonical variables and a Lie transformation developed by Cary and Littlejohn [Ann. Phys. 151, 1 (1983)] is presented. The method is applied to a focusing wiggler, which has a parabolically curved pole face. Radial space charge force is taken into account in the analysis and the maximum current that can propagate the wiggler is estimated for an electron beam whose energy is around 1 MeV. The effect of the beam current on the axial phase relation is also investigated and found to be small. The present noncanonical approach is found to be transparent and comprehensive for this kind of problem, compared with the standard canonical perturbation method and applicable to arbitrarily complex wigglers. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871343
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  • 4
    Electronic Resource
    Electronic Resource
    Plasma jet formation and magnetic-field generation in the intense laser plasma under oblique incidence (1999)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 6 (1999), S. 2855-2861 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Long-scale jet-like x-ray emission was observed in the experiments on the interactions of 100 TW laser light with plasmas. The jet formation is investigated by simulations with a two-dimensional particle code. When an S-polarized intense laser is irradiated obliquely on an overdense plasma, collimated MeV electrons are observed from the critical surface in the specular reflection direction. These electrons are found to be accelerated through the coronal plasma by the reflected laser light, which was modulated at the reflection point. The quasisteady magnetic channel occurs simultaneously and collimates the energetic electrons along the specular direction. In the case of P-polarized laser, it is found that an outgoing electron stream is induced at the critical surface due to Brunel mechanism. Megagauss quasistatic magnetic fields are generated and pinch the electron stream. The angle of ejected electron depends on the electron's energy. The emission direction of the jet generated by the P-polarized light is determined by the canonical momentum conservation along the target surface. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.873243
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  • 5
    Electronic Resource
    Electronic Resource
    Zonal flows in gyrofluid simulations of slab electron temperature gradient turbulence (2002)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 9 (2002), S. 1241-1254 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Zonal flows can be generated by drift wave turbulence through the nonlinear interactions. Three-dimensional gyrofluid simulations show that in a turbulent system driven by electrostatic sheared slab electron temperature gradient (ETG) modes, the excitation of zonal flows is a slower process, which indicates the amplitudes grow up approximately proportionally to time. The zonal flows are very weak compared with the background turbulence and hardly work for suppressing the turbulent electron heat transport. The dynamics of zonal flows is numerically explored by starting the zonal flow component in the quasi-steady state. Meanwhile, the basic modulation processes between zonal flows and the pump waves as well as the sidebands in an ETG turbulence–zonal flows system are theoretically analyzed using a four-wave or five-wave (including two pump waves) coupling model. Through comparing the spectral evolution of both ETG turbulence and zonal flows in the simulations with the modulational instability analysis based on a five-wave model, it is identified that the zonal flows may originate from the beating of any two pump waves in the ETG turbulence, the evolution of zonal flows is determined by a modulational instability that strongly depends on the spectra of the turbulence, usually a marginal instability observed in ETG turbulence simulations. The saturation of zonal flows is testified to be subject to the same modulation process that becomes stable. Afterwards, the zonal flows enter a periodic oscillation phase. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1455628
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  • 6
    Electronic Resource
    Electronic Resource
    Kinetic resonance damping rate of the toroidal ion temperature gradient mode (1994)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 1 (1994), S. 927-936 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The linear damping rates of the toroidal ion temperature gradient (ηi) mode due to the toroidal resonance are calculated in the local kinetic limit. The well-known Landau contour method is generalized to treat the analytic continuation problem of the guiding center dispersion function in the toroidal resonance system where the resonance occurs from both the magnetic ∇B-curvature drift and the parallel ion transit drift. A detailed numerical analysis is presented for the dependence of the damping rate of the toroidal ηi mode on various parameters such as εn, ky, and the trapped electron fraction. In addition, a consideration is presented on the decay problem of the ballistic response by phase mixing in the toroidal system, which is directly related to the kinetic damping problem of the normal modes by the toroidal resonance.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.870751
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  • 7
    Electronic Resource
    Electronic Resource
    High energy ions and nuclear fusion in laser–cluster interaction (2002)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 9 (2002), S. 589-601 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Highly efficient energy conversion of laser to high-energy components of ions and resulting rapid expansion take place upon the laser irradiation of clusters. Depending on the interrelationship between cluster size a and the electron excursion length ξe (which is related to the laser field amplitude), the expansion characteristics is categorized into the Coulomb explosion (a(very-much-less-than)ξe) and alternative hydrodynamic ambipolar expansion (a(very-much-greater-than)ξe), revealing different features in the ion energy distribution. In the Coulomb explosion regime, a flat and/or slightly inverted ion energy distribution effective for enhanced fusion reactivity is generated. The critical cluster size and laser field amplitude which demarcate the expansion characteristics is investigated and the optimal fusion cross section is found in the domain of Coulomb explosion. A high fusion neutron yield is expected in the Coulomb explosion regime with greater cluster size and higher packing fraction. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1418433
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  • 8
    Electronic Resource
    Electronic Resource
    Poloidal shear flow effect on toroidal ion temperature gradient mode: A theory and simulation (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 3689-3695 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The effect of poloidal shear flow on the global structure and stability of toroidal ion temperature gradient mode is studied theoretically and also numerically using a toroidal particle simulation code. Reasonable agreements are found between the theoretical and simulation results; both indicating that with increasing shear flow the toroidal ion temperature gradient mode is stabilized in the increase of poloidal asymmetry and in the reduction of radial mode width. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871939
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  • 9
    Electronic Resource
    Electronic Resource
    Efficient carrier blocking by an attractive potential in strained Si1−xGex/Si single quantum well (1996)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 69 (1996), S. 635-637 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Efficient carrier blocking by a single "attractive'' potential is demonstrated using a strained Si0.85Ge0.15/Si quantum well (QW). The carrier blocking becomes pronounced at increased temperatures as most of the carriers at the QW bound state are thermalized with the barrier band edge. The carrier blocking efficiency at 100 K is almost of the order of unity as compared to insufficient carrier blocking due to a large "repulsive'' potential associated with SiO2. The carrier blocking arises from efficient hole reemission from the QW that reflects a highly efficient carrier capture to the strained Si1−xGex/Si QWs from Si barriers. It is further shown that a single QW is the optimized geometry for efficient carrier blocking and that multiple well potentials lead only to a reduced blocking efficiency. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.117931
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  • 10
    Electronic Resource
    Electronic Resource
    Photoluminescence study of Si1−xGex/Si surface quantum wells (1997)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 70 (1997), S. 2837-2839 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We report a photoluminescence (PL) study of Si1−xGex/Si surface quantum wells (SFQWs). The PL peak energies are found to be affected by strain relaxation in Ge-rich SFQWs while a PL energy lowering was observed for x≤0.47 as compared to buried quantum wells capped with Si. Exciton localization in the lateral direction is suggested to be the dominant PL mechanism in SFQWs rather than perpendicular confinement effects that are expected for SFQWs. PL degradation and a spectral dominance switch over to newly developing lower energy peaks were clearly observed after prolonged air exposure. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.119019
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