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  • American Institute of Physics (AIP)  (6)
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  • 1
    Electronic Resource
    Electronic Resource
    Fast wave poloidal flow generation in a plasma (1994)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 1 (1994), S. 3890-3895 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Fast wave poloidal flow generation at the plasma edge of a tokamak is studied with a one-dimensional slab model. In the absence of mode conversion, the poloidal flow can be generated by a spatial change of plasma velocity and current density profiles due to strong minority ion power absorption at the minority ion cyclotron resonance. The electromagnetic force plays a more important role in the flow generation than does the plasma Reynolds stress. In the presence of mode conversion, the flow is mainly generated by interference between the long-wavelength transmitted fast wave and the short-wavelength ion Bernstein wave (IBW) from mode conversion. Flow shear generated in the presence of mode conversion varies spatially with a scale length similar to the IBW wavelength. With mode conversion, the plasma Reynolds stress becomes more important in the flow shear generation than the electromagnetic force. For both cases, the plasma Reynolds stress and the electromagnetic force are out of phase, so that the resultant flow shear is smaller than the larger of the two. The short scale length flow shear enhances the turbulence stabilization. © 1994 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.870860
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  • 2
    Electronic Resource
    Electronic Resource
    Theory of electric-field curvature effects on long-wavelength drift wave turbulence (1994)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 1 (1994), S. 1142-1153 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A simple model of long-wavelength drift waves is used to study the way in which a radial electric-field profile influences the growth and saturation of turbulence. For a fixed external field, the effect of curvature (∂2Er/∂r2) dominates that of shear (∂Er/∂r). In the linear regime, both affect the average k(parallel) at which ion damping occurs: shear by shifting the eigenmode off the resonant surface and curvature by changing the eigenmode width. Curvature damps more efficiently and also shifts the real frequency of the drift wave, changing the instability drive. In the nonlinear regime, radial trapping at large fluctuation levels limits the ability of an external electric-field profile to affect the spatial structure. Changes in damping are now less effective than the feedback between frequency shift and drive. The importance of the frequency shift caused by electric-field curvature in the presence of finite-amplitude fluctuations has been demonstrated by numerical calculations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.870765
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  • 3
    Electronic Resource
    Electronic Resource
    Bifurcations and modulational interaction in negative compressibility turbulence (1994)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 1 (1994), S. 2700-2710 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The results of detailed numerical studies of phenomena in negative compressibility turbulence with sheared perpendicular (i.e., poloidal) flow are presented. The turbulence model is based on the parallel ion flow gradient instability, a representative paradigm for ion drift waves. Studies of coupled turbulence and mean flow evolution indicate the existence of two distinct nonlinear states. In the first state, saturation occurs via nonlinear transfer to damped high-k modes and sheared flow is heavily damped. In the second state, the turbulence level is controlled by the self-consistently generated sheared flow. Transition between these states is determined by the competition between instability growth and damping of rotation. The dynamics of the observed transition is well described and consistent with a simple set of coupled envelope equations. Modulational interaction between small scale turbulence and large scale m≠0 shear flows is observed, as well.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.870597
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  • 4
    Electronic Resource
    Electronic Resource
    Effect of a gradient in parallel current on field error profiles in a reversed-field pinch (1991)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 3 (1991), S. 428-435 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Magnetic perturbations caused by external field errors are an important consideration in both the current operation and future design of reversed-field pinch (RFP) devices. Recently, it has been shown that perturbation profiles calculated in the presence of a relaxed RFP equilibrium (J0 =λB0, λ=const) are amplified when compared to those calculated in vacuum. This work extends the approach to include a partially relaxed equilibrium [λ=λ(r)], which is experimentally realistic, and the effects of finite resistivity, viscosity, and flow. The finite gradient in λ produces an amplification of the radial magnetic field perturbation much larger than that predicted for constant λ. Physically, this is due to the presence of an additional perturbed current associated with the gradient in λ which alters the curvature of the perturbation. Finite resistivity, viscosity, and flow act to reduce this amplification.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.859884
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  • 5
    Electronic Resource
    Electronic Resource
    Nonlinear coupling of tearing fluctuations in the Madison Symmetric Torusf| (1993)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 5 (1993), S. 2540-2545 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Three-wave, nonlinear, tearing mode coupling has been measured in the Madison Symmetric Torus (MST) reversed-field pinch (RFP) [Fusion Technol. 19, 131 (1991)] using bispectral analysis of edge magnetic fluctuations resolved in "k-space.'' The strength of nonlinear three-wave interactions satisfying the sum rules m1+m2=m3 and n1+n2=n3 is measured by the bicoherency. In the RFP, m=1, n∼2R/a (6 for MST) internally resonant modes are linearly unstable and grow to large amplitude. Large values of bicoherency occur for two m=1 modes coupled to an m=2 mode and the coupling of intermediate toroidal modes, e.g., n=6 and 7 coupled to n=13. These experimental bispectral features agree with predicted bispectral features derived from magnetohydrodynamic (MHD) computation. However, in the experiment, enhanced coupling occurs in the "crash'' phase of a sawtooth oscillation concomitant with a broadened mode spectrum suggesting the onset of a nonlinear cascade.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.860741
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  • 6
    Electronic Resource
    Electronic Resource
    Turbulent transport in the Madison Symmetric Torus reversed-field pinch (1992)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 4 (1992), S. 2136-2141 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Measurements of edge turbulence and the associated transport are ongoing in the Madison Symmetric Torus (MST) reversed-field pinch [Fusion Technol. 19, 131 (1991)] using magnetic and electrostatic probes. Magnetic fluctuations are dominated by m=1 and n ∼2R/a, tearing modes. Particle losses induced by magnetic field fluctuations have been found to be ambipolar (〈J˜(parallel) B˜r〉/B0=0). Electrostatic fluctuations are broadband and turbulent, with mode widths Δm∼3–7 and Δn∼70–150. Particle, parallel current, and energy transport arising from coherent motion with the fluctuating E˜×B drift have been measured. Particle transport via this channel is comparable to the total particle loss from MST. Energy transport (from 〈P˜E˜φ 〉/B0) due to electrostatic fluctuations is relatively small, and parallel current transport (from 〈J˜(parallel) E˜φ〉/B0) may be small as well.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.860020
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