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  • 21
    Electronic Resource
    Electronic Resource
    Intermittency of plasma edge fluctuation data: Multifractal analysis (2000)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 7 (2000), S. 3278-3287 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Plasma edge fluctuations show a degree of intermittency similar to fluid turbulence. Using fluctuation measurements obtained with Langmuir probe data from two confinement devices, it is shown that plasma fluctuations have a multifractal character over the fluctuation range of scales with intermittency levels comparable to the levels measured in neutral fluid turbulence. In the mesoscale range, that is, for time scales between 10 times the turbulence decorrelation time and plasma confinement time, plasma fluctuations have a structure closer to a monofractal with very low intermittency. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.874193
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  • 22
    Electronic Resource
    Electronic Resource
    On the applicability of local asymptotic stability criteria to stellarator stability (2001)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 8 (2001), S. 990-996 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: An intrinsic consequence of the three-dimensional nature of the stellarator equilibrium may be the existence of local flattening of the pressure profile at the resonant surfaces. This local flattening of the pressure profile significantly changes the stability properties. The localized interchange modes are stabilized, and a new instability branch controls the critical beta. This instability is strongly stabilized by shear at high poloidal mode numbers. As a consequence, the plasma stability properties change, and the asymptotically derived local stability criteria often used in stellarator design are no longer applicable. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1349877
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  • 23
    Electronic Resource
    Electronic Resource
    Characterization of the frequency ranges of the plasma edge fluctuation spectra (1999)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 6 (1999), S. 4615-4621 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Frequency spectra of fluctuations for the ion saturation current, floating potential, and turbulent transport measured in the plasma edge of plasma confinement experiments (tokamaks and stellarators) have been analyzed to identify the frequency ranges characterized by a power dependence. Three main regions can be identified. For the intermediate frequency region, the decay of the spectra is close to 1/f, as is expected in self-organized criticality systems. This region is particularly important for the role that it plays in plasma transport and the self-similarity of the fluctuations and fluxes. The effect of plasma rotation on the decay indices has also been studied. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.873748
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  • 24
    Electronic Resource
    Electronic Resource
    On the stiffness of the sand pile profile (1998)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 5 (1998), S. 1206-1208 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The existence of a critical gradient is an important feature of many transport models. [M. Kotschenreuther et al., Phys. Plasmas 2, 2381 (1995)]. However, fundamental differences exist in the dynamics near marginal stability, depending on whether the transport phenomena are controlled by strict (linear) marginal stability or by a self-organized criticality. One of the most striking differences is in the stiffness of the profiles. In this paper, a sand pile model is used to gain some basic understanding of the stiffness of the profile under different dynamics. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872628
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  • 25
    Electronic Resource
    Electronic Resource
    J* optimization of small aspect ratio stellarator/tokamak hybrid devices : 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. 1752-1758 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A new class of low aspect ratio toroidal hybrid stellarators is found using a more general plasma confinement optimization criterion than quasisymmetrization. The plasma current profile and shape of the outer magnetic flux surface are used as control variables to achieve near constancy of the longitudinal invariant J* on internal flux surfaces (quasiomnigeneity), in addition to a number of other desirable physics target properties. A range of compact (small aspect ratio A), low plasma current devices have been found with significantly improved confinement, both for thermal as well as energetic (collisionless) particle components. With reasonable increases in magnetic field and geometric size, such devices can also be scaled to confine 3.5 MeV alpha particle orbits.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872844
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  • 26
    Electronic Resource
    Electronic Resource
    Internal disruptions in Heliotron E* (1998)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 5 (1998), S. 3700-3707 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: In Heliotron E [K. Uo, Nucl. Fusion 25, 1243 (1985)] shifted-in vacuum magnetic field configuration, the q profile varies from just above 2 at the magnetic axis to 0.4 at the plasma edge. For low-β plasmas, resistive interchange modes are the dominant low-n instabilities at the plasma core. They saturate at low fluctuation levels. Above a threshold value, the ideal m/n=2/1 modes become unstable. They can be resonant or nonresonant modes depending on the value of q(0). In either case, their nonlinear evolution leads to a sudden crash of the pressure within the r/a=0.3 radius (sawtooth oscillation). When beta is increased further, the q=2 surface moves out of the plasma, and the ideal m/n=2/1 modes are effectively stabilized when q(0)〈1.85. As a result, the sawtooth oscillations are suppressed. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872980
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  • 27
    Electronic Resource
    Electronic Resource
    The dynamics of marginality and self-organized criticality as a paradigm for turbulent transport (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 1858-1866 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A general paradigm, based on the concept of self-organized criticality (SOC), for turbulent transport in magnetically confined plasmas, has been recently suggested as an explanation for some of the apparent discrepancies between most theoretical models of turbulent transport and experimental observations of the transport in magnetically confined plasmas. This model describes the dynamics of the transport without relying on the underlying local fluctuation mechanisms. Computations based on a cellular automata realization of such a model have found that noise-driven SOC systems can maintain average profiles that are linearly stable (submarginal) and yet are able to sustain active transport dynamics. It is also found that the dominant scales in the transport dynamics in the absence of sheared flow are system scales rather than the underlying local fluctuation scales. The addition of sheared flow into the dynamics leads to a large reduction of the system-scale transport events and a commensurate increase in the fluctuation-scale transport events needed to maintain the constant flux. The dynamics of these models and the potential ramifications for transport studies are discussed. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871681
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  • 28
    Electronic Resource
    Electronic Resource
    Dynamics of second-order phase transitions in resistive pressure-gradient-driven turbulence (1995)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 2 (1995), S. 2744-2752 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The phase transition model for the transition from a low-confinement to a high-confinement regime is based on general properties of the coupling between turbulence and averaged sheared flows. To extract these generic properties several turbulence models have been investigated. In particular, the structure of the turbulent Reynolds stress tensor is crucial to the phase transition model. For a broad range in parameters of the resistive pressure-gradient-driven turbulence, we find a strong competition in the Reynolds stress tensor between the dynamo instability term and the turbulent viscosity. This result is contrary to expectations based on simplified local models. This competition allows the transition to take place only near marginal stability. The particle flux is calculated, and its reduction at the transition is caused not only by fluctuation suppression but also by the change in the relative phase of the pressure and potential fluctuations. The results obtained with this turbulence model are presented here, with special emphasis on the structure of the Reynolds stress and the existence of limit cycle solutions, which may be related to the observed phenomenon of dithering. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871239
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  • 29
    Electronic Resource
    Electronic Resource
    On the statistical mechanics of self-organized profiles (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 3745-3753 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The radial structure of tokamak profiles determined by anomalous transport is elucidated by studying the statistical mechanics of a sand pile automaton for which the toppling conditions depend on local gradient, alone. In this representation, the sand pile dynamics is Markovian, and spatial profiles may be obtained from calculated expectation values of the local gradient. The Markovian structure of the dynamics is exploited to analytically derive a local gradient probability distribution function from a generalized kinetic equation. For homogeneous, weak noise, the calculated expectation value of the gradient is well below the marginally stable state. In the over-driven limit (i.e., strong homogeneous noise), a region of super-critical gradient is shown to form near the bottom of the pile. For the case of localized noise, the mean self-organized profile is always sub-critical. These results are consistent with numerical studies of simple automata. Their relevance to and implications for tokamak confinement are discussed. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871508
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  • 30
    Electronic Resource
    Electronic Resource
    Modification of tokamak edge turbulence using feedbackf| (1994)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 1 (1994), S. 1606-1611 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Using active feedback, the turbulent fluctuation levels have been reduced by as much as a factor of 2 in the edge of the Texas Experimental Tokamak (TEXT) [K. W. Gentle, Nucl. Fusion Technol. 1, 479 (1981)]. A probe system was used to drive a suppressor wave in the TEXT limiter shadow. A decrease in the local turbulence-induced particle flux has been seen, but a global change in the particle transport at the present time has not been observed. By changing the phase shift and gain of the feedback network, the amplitude of the turbulence was increased by a factor of 10.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.870661
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