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  • 1
    Electronic Resource
    Electronic Resource
    Anomalous diffusion and exit time distribution of particle tracers in plasma turbulence model (2001)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 8 (2001), S. 5096-5103 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: To explore the character of transport in a plasma turbulence model with avalanche transport, the motion of tracer particles has been followed. Both the time evolution of the moments of the distribution function of the tracer particle radial positions, 〈|r(t)−r(0)|n〉, and their finite scale Lyapunov number are used to determine the anomalous diffusion exponent, ν. The numerical results show that the transport mechanism is superdiffusive with an exponent ν close to 0.88±0.07. The distribution of the exit times of particles trapped into stochastic jets is also determined. These particles have the lowest separation rate at the low resonant surfaces. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1416180
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  • 2
    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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  • 3
    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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  • 4
    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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  • 5
    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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  • 6
    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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  • 7
    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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  • 8
    Electronic Resource
    Electronic Resource
    A model realization of self-organized criticality for plasma confinement (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 2903-2911 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A model for plasma transport near marginal stability is presented. The model is based on subcri- tical resistive pressure-gradient-driven turbulence. Three-dimensional nonlinear calculations based on this model show effective transport for subcritical mean profiles. This model exhibits some of the characteristic properties of self-organized criticality. Perturbative transport techniques are used to elucidate the transport properties. Propagation of positive and negative pulses is studied. The observed results suggest a possible explanation of the apparent nonlocal effects ob- served with perturbative experiments in tokamaks. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871650
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  • 9
    Electronic Resource
    Electronic Resource
    Radiation-driven turbulence at the plasma edge in toroidal devices (1994)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 1 (1994), S. 3871-3882 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The radiation drives for plasma edge parameters are reexamined. By including the electron density gradient, the radiation drive through density fluctuations can be strongly destabilizing even in a parameter regime away from force balance. This results in the dominance of the magnitude of the radiation power (γn drive) over its derivative (γT drive) for plasma edge parameters of Ohmic tokamak discharges. This leads to instability over a range of poloidal mode numbers much broader than in the case of the γT drive and to temperature fluctuations of opposite phase to the density fluctuations. Nonlinear numerical calculations show that this opposite phase relationship persists nonlinearly. As a consequence, the fluctuations induce a heat flux that is observed to be inward, while the particle and energy fluxes that they generate are calculated to be outward. © 1994 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.870859
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  • 10
    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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