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  • 1
    Electronic Resource
    Electronic Resource
    Response of internal transport barriers to central electron heating and current drive on ASDEX Upgrade : The 41st annual meeting of the division of plasma physics of the american physical society (2000)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 7 (2000), S. 1839-1844 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Internal transport barriers with the central electron temperature as large as the central ion temperature both in excess of 10 keV have been achieved in the Axi-symmetric divertor experiment (ASDEX Upgrade) [H. Vernickel et al., J. Nucl. Mater. 128, 71 (1984)]. By applying central electron cyclotron heating and current drive to negative central shear discharges, established by neutral beam heating in the current ramp, the core electron temperatures could be raised by more than a factor of 2. Despite the fivefold increase of the central electron heat flux, the ion and electron energy and also angular momentum transport did not deteriorate. For neutral beam injection alone and also with additional central electron cyclotron counter-current drive, a double tearing mode and the associated detrimental effect on the plasma confinement is destabilized only transiently, when the minimum of the safety factor (qmin) passes through 2. For co-current drive, however, the confinement does not recover after qmin has dropped below 2, as the (2,1) mode persists due to the influence of the current profile modification in the very plasma center. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.874006
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  • 2
    Electronic Resource
    Electronic Resource
    Paraxial WKB description of short wavelength eigenmodes in a tokamak (2001)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 8 (2001), S. 3664-3672 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A novel asymptotic approach for solving the eigenvalue problem in the short-wavelength limit is developed. This approach, called the paraxial WKB method, is presented and applied to the analysis of gradient-driven instabilities in a tokamak. In some respects, the paraxial WKB method is more general than the ballooning representations usually used for this problem. For instance, the paraxial WKB method can describe drift-ballooning instabilities in the presence of sheared plasma rotation when the ballooning representation fails. Being different from other techniques, the paraxial WKB method not only sheds new light on the physics of drift-ballooning instabilities, but also provides a description for short-wave instabilities of other types. Other possible extensions of the paraxial WKB technique, such as the application to three-dimensional systems like stellarators, are briefly discussed. Finally, advantages and disadvantages of the paraxial WKB method in comparison with the ballooning representation are considered. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1383797
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  • 3
    Electronic Resource
    Electronic Resource
    Boundary conditions for a Gaussian wave beam (2001)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 8 (2001), S. 4325-4330 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The beam tracing technique allows the description of short-wavelength wave beams by means of a set of ordinary differential equations, in which the effects of diffraction (neglected by the standard geometrical-optics procedure) are taken into account. The propagation of the beam is expressed in terms of a central ray and a set of parameters connected with the curvature of the wave front and the amplitude profile. Two crucial issues for the applicability of this technique to situations of experimental interest are the beam reflection from a mirror and the possibility of following the beam in its vacuum–plasma crossing. These problems are investigated in the paper. The parameters of the incident beam are regarded as known quantities and their values after reflection or inside the plasma are obtained. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1401116
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  • 4
    Electronic Resource
    Electronic Resource
    Beam tracing in inhomogeneous anisotropic plasmas (1998)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 5 (1998), S. 3529-3541 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: An asymptotic method for solving the wave equation in the short-wavelength limit is presented. This method, called beam tracing, takes into account the wave properties, i.e., diffraction and interference. It reduces the full wave equation to a set of ordinary differential equations. In this respect, it differs from all other asymptotic techniques describing diffraction which end up with much more complicated partial differential equations. The resulting system of beam tracing equations is expressed in terms of the same Hamiltonian function as in geometric optics (ray tracing) and, similar to the ray tracing, allows powerful numerical solving algorithms. Thus the beam tracing combines the simplicity of ray tracing with a description of the wave phenomena, which are not included in the ray tracing. The beam tracing technique provides an efficient tool for calculation of wave fields in all problems where the short-wave approximation is applicable such as rf heating, current drive and plasma diagnostics with microwave beams. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.873070
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  • 5
    Electronic Resource
    Electronic Resource
    Paraxial Gaussian wave beam propagation in an anisotropic inhomogeneous plasma (1999)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 6 (1999), S. 5-11 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The beam tracing technique describes electromagnetic wave beam propagation in the short wavelength limit, including diffraction effects, not described by the usual geometric optics. It involves a set of ordinary differential equations: a central or reference ray is determined and around this the beam profile and the shape of the phase front are calculated. Here, these equations are obtained for a Gaussian wave beam. Analytic solutions in an inhomogeneous magnetized plasma are given for perpendicular propagation in a slab geometry. The beam is found to show a strong deformation of its cross section due to the anisotropy of the medium. The rotation of the cross section and that of the curvature of the phase front are discussed. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.873254
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