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  • 1
    Electronic Resource
    Electronic Resource
    Global simulations of ion turbulence with magnetic shear reversal (2001)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 8 (2001), S. 2793-2803 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: This paper presents the results of three-dimensional fluid global simulations of electrostatic ion turbulence in tokamaks with reversed magnetic shear. It is found that a transport barrier appears at the location of magnetic shear reversal. This is due to a rarefaction of resonant surfaces in this region. For the same reason, the barrier is more pronounced when the minimum of the safety factor is a simple rational number. The barrier is broadened by velocity shear effects. It is also found that large-scale transport events hardly cross a transport barrier. Finally, a significant amount of toroidal rotation is generated by the turbulence. This rotation changes its sign at the position of magnetic shear reversal, as expected from a quasi-linear estimate of the Reynolds stress tensor. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1367320
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  • 2
    Electronic Resource
    Electronic Resource
    Tokamak turbulence with stochastic field lines (1998)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 5 (1998), S. 4271-4279 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Three-dimensional numerical simulations of ballooning turbulence in a tokamak plasma with stochastic magnetic field lines are presented. Three main features are observed. First, the level of pressure fluctuations decreases in the ergodic layer. Second, this is essentially due to a suppression of large scale structures. Finally, the turbulent heat diffusivity does not decrease in the stochastic layer due to an increase of electric field fluctuations. These observations are in agreement with turbulence measurements on Tore Supra [Equipe Tore Supra, Proceedings, 13th International Conference on Plasma Physics and Controlled Nuclear Fusion, Washington, 1990 (International Atomic Energy Agency, Vienna, 1991), Vol. 1, p. 549]. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.873163
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  • 3
    Electronic Resource
    Electronic Resource
    Action at distance and Bohm scaling of turbulence in tokamaks (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 1898-1907 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Recent transport experiments in tokamaks have suggested the concept of "action at distance'' in which the local turbulence depends on gradients at a distance larger than the correlation length. Furthermore, the scaling of the ion thermal diffusivity is not always consistent with local gyro-Bohm-like transport but rather scales worse than Bohm-like. This work is an attempt to reconcile these observations with simplified numerical simulations of toroidal ion temperature gradient (ITG) mode turbulence using a fast two-dimensional (2-D) inhomogeneous full radius turbulence code. It is found that action at a distance is possible, but only at weak damping rates, since the propagation range is given simply by the curvature drift group velocity divided by the average damping rate. The correlation lengths always scale linearly with the gyroradius. It is found that Bohm scaling or worse is possible when the gradients are close to the ITG threshold and the radial modes keep the turbulence level small enough to avoid destroying the slow to form global eigenmodes. In contrast to local ITG ballooning modes, the global eigenmode growth rates decrease with increasing gyroradius from the effect of larger diamagnetic rotational shear. This behavior results in an increase of the correlation time with increasing gyroradius, when the gradients are close to the thresholds. Thus, at sufficiently large relative gyroradius, the breaking of gyro-Bohm scaling can result from increased stability not mixing length. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871985
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  • 4
    Electronic Resource
    Electronic Resource
    Self-sustained magnetic islands (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 4628-4636 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Numerical simulations of a single magnetic island evolution are presented in the regime where the island width is smaller than an ion Larmor radius. It is shown that the island rotation is controlled by particle diffusion due to collisions or a background of microturbulence. More precisely, for vanishing density gradient, the sign of rotation frequency is determined by the off-diagonal diffusion term, i.e., by the coefficient of the temperature gradient in the particle flux. As expected from the theory of a stationary island, there exist cases where linearly stable magnetic perturbations are nonlinearly self-sustained. This situation corresponds to large poloidal beta and temperature gradient. The drive is due to diamagnetic frequency effects. However, this situation is not generic, and islands can also decay. It is found that a magnetic island is self-sustained for a negative off-diagonal diffusion coefficient. This case occurs in a tokamak if the inward particle pinch is due to the temperature gradient. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871586
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  • 5
    Electronic Resource
    Electronic Resource
    Anomalous particle pinch in tokamaks (2000)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 7 (2000), S. 4197-4207 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The diffusion coefficient in phase space usually varies with the particle energy. A consequence is the dependence of the fluid particle flux on the temperature gradient. If the diffusion coefficient in phase space decreases with the energy in the bulk of the thermal distribution function, the particle thermodiffusion coefficient which links the particle flux to the temperature gradient is negative. This is a possible explanation for the inward particle pinch that is observed in tokamaks. A quasilinear theory shows that such a thermodiffusion is generic for a tokamak electrostatic turbulence at low frequency. This effect adds to the particle flux associated with the radial gradient of magnetic field. This behavior is illustrated with a perturbed electric potential, for which the trajectories of charged particle guiding centers are calculated. The diffusion coefficient of particles is computed and compared to the quasilinear theory, which predicts a divergence at low velocity. It is shown that at low velocity, the actual diffusion coefficient increases, but remains lower than the quasilinear value. Nevertheless, this differential diffusion between cold and fast particles leads to an inward flux of particles. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1308082
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  • 6
    Electronic Resource
    Electronic Resource
    Kinetic theory of magnetic island stability in tokamaks (1994)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 1 (1994), S. 1890-1904 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The nonlinear behavior of low- and large-wave number tearing modes is studied. The emphasis is laid on diamagnetic effects. A kinetic equation, including transport processes associated with a background of microturbulence, is used to describe the electron component. Such transport processes are shown to play a significant rôle in the adjustment of density and temperature profile, and also in the calculation of the island rotation frequency. The fluctuating electric potential is calculated self-consistently, using the differential response of electrons and ions. Four regimes are considered, related to island width (smaller or larger than an ion Larmor radius) and transport regime (electron–ion collisions or electroviscosity dominated). It is shown that diamagnetism does not influence the island stability for small island width in the viscous regime, as long as the constant Ψ constraint is maintained. It turns out that the release of this constraint may strongly modify the previously calculated stability thresholds. Finally, it is found that diamagnetism is destabilizing (stabilizing) for island width smaller (larger) than an ion Larmor radius, in both resistive and viscous regimes. A typical island evolution scenario is studied which shows that even large-scale tearing modes with positive Δ' could saturate to island width of order of a few ion Larmor radii. Illustrative Δ' threshold and island saturation size are calculated.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.870645
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  • 7
    Electronic Resource
    Electronic Resource
    Turbulence reduction and poloidal shear steepening in reversed shear plasmas investigated by light scattering on Tore Supra (2001)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 8 (2001), S. 186-192 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Infrared light scattering is used to characterize turbulence during reversed shear scenario obtained by a fast current ramp-up on Tore Supra [Hoang et al., Phys. Rev. Lett. 84, 4593 (2000)]. The preformed hollow profile is freezed using ion cyclotron resonance minority heating (ICRH). Turbulence intensity is recorded to dramatically decrease during the ramp-up and to recover smoothly as the discharge is ended. Simultaneously, a confinement improvement of about 40% is achieved for about 2 s. Although the diagnostic performs a line average and the measurement is dominated by turbulence at the edge, we show that the core behavior can be deduced. Core turbulence decrease is in agreement with the effective heat diffusivity behavior. With the same optical device, the radial electric shear is deduced from the frequency spectra Doppler shift. Turbulence reduction is thus demonstrated to be correlated with the E×B shear steepening. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1330202
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  • 8
    Electronic Resource
    Electronic Resource
    Theory and simulation of rotational shear stabilization of turbulence : 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. 1784-1792 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Numerical simulations of ion temperature gradient (ITG) mode transport with gyrofluid flux tube codes first lead to the rule that the turbulence is quenched when the critical E×B rotational shear rate γE−crit exceeds the maximum of ballooning mode growth rates γ0 without E×B shear [Waltz, Kerbel, and Milovich, Phys. Plasmas 1, 2229 (1994)]. The present work revisits the flux tube simulations reformulated in terms of Floquet ballooning modes which convect in the ballooning mode angle. This new formulation avoids linearly unstable "box modes" from discretizing in the ballooning angle and illustrates the true nonlinear nature of the stabilization in toroidal geometry. The linear eigenmodes can be linearly stable at small E×B shear rates, yet Floquet mode convective amplification allows turbulence to persist unless the critical shear rate is exceeded. The flux tube simulations and the γE−crit(approximate)γ0 quench rule are valid only at vanishing relative gyroradius. Modifications and limits of validity on the quench rule are suggested from analyzing the finite relative gyroradius "ballooning-Schrödinger equation" [R. L. Dewar, Plasma Phys. Controlled Fusion 39, 437 (1997)], which treats general "profile shear" (x variation in γ0) and "profile curvature" (x2 profile variation). © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872847
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  • 9
    Electronic Resource
    Electronic Resource
    Kelvin–Helmholtz instabilities in tokamak edge plasmas (1999)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 6 (1999), S. 3955-3965 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The parallel Kelvin–Helmholtz instability is investigated as a possible explanation for poloidal asymmetries of density fluctuations which reverse with the plasma current direction. It is shown that these modes are localized around the position where the radial gradient of parallel velocity is maximum. Two mechanisms lead to unstable Kelvin–Helmholtz modes; the acceleration of ions in a presheath and the anomalous Stringer spin-up due to asymmetries of the particle flux. Up–down asymmetries are explained by combining these two effects. Depending on the limiter configuration, the Stringer effect amplifies or weakens the flow due to presheath acceleration. This type of asymmetry reverses with the plasma current direction. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.873659
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  • 10
    Electronic Resource
    Electronic Resource
    Heat flux driven ion turbulence (1998)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 5 (1998), S. 2836-2845 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: This work is an analysis of an ion turbulence in a tokamak in the case where the thermal flux is fixed and the temperature profile is allowed to fluctuate. The system exhibits some features of self-organized critical systems. In particular, avalanches are observed. Also the frequency spectrum of the thermal flux exhibits a structure similar to the one of a sand pile automaton, including a 1/f behavior. However, the time average temperature profile is found to be supercritical, i.e., the temperature gradient stays above the critical value. Moreover, the heat diffusivity is not the same for a turbulence calculated at fixed flux than at fixed temperature gradient, with the same time averaged temperature. More precisely the diffusivity at fixed temperature is found to be larger in the edge and smaller close to the heat source. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.873003
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