Core transport reduction in tokamak plasmas with modified magnetic shear

Bell, M.G.; Bell, R.E.; Efthimion, P. C.; Ernst, D. R.; Fredrickson, E. D.; Levinton, F. M.; Manickam, J.; Mazzucato, E.; Schmidt, G. L.; Synakowski, E. J.; Zarnstorff, M. C.

doi:10.1088/0741-3335/41/3a/065

Cited by 38 publications

(50 citation statements)

References 35 publications

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“…In general the electron transport reduction is transient while the ion transport can be transient or an enduring core barrier can form depending on the total input power. These results are similar to those from other tokamaks, [3][4][5][6][7][8] and integer q surfaces are widely recognized as zones of improved confinement in plasma devices.…”

Section: Discussionsupporting

confidence: 88%

“…1 and 2͒ as well as JT-60U, 3 RTP, 4 TFTR, 5 JET, 6 ASDEX-U, 7 and T10, 8 it has been observed that the formation and performance of ITB discharges is linked to surfaces where the safety factor q has low-order rational values, particularly integer surfaces. In experiments in these devices, transport improvement either was initiated at and/or transport barriers were tied to locations with q = integer.…”

Section: Introductionmentioning

confidence: 99%

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Core barrier formation near integer q surfaces in DIII-D

Austin¹,

Burrell

Waltz

et al. 2006

Physics of Plasmas

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Recent DIII-D experiments have significantly improved the understanding of internal transport barriers (ITBs) that are triggered close to the time when an integer value of the minimum in q is crossed. While this phenomenon has been observed on many tokamaks, the extensive transport and fluctuation diagnostics on DIII-D have permitted a detailed study of the generation mechanisms of q-triggered ITBs as pertaining to turbulence suppression dynamics, shear flows, and energetic particle modes. In these discharges, the evolution of the q profile is measured using motional Stark effect polarimetry and the integer qmin crossings are further pinpointed in time by the observation of Alfvén cascades. High time resolution measurements of the ion and electron temperatures and the toroidal rotation show that the start of improved confinement is simultaneous in all three channels, and that this event precedes the traversal of integer qmin by 5–20ms. There is no significant low-frequency magnetohydrodynamic activity prior to or just after the crossing of the integer qmin and hence magnetic reconnection is determined not to be the precipitant of the confinement change. Instead, results from the GYRO code point to the effects of zonal flows near low order rational q values as playing a role in ITB triggering. A reduction in local turbulent fluctuations is observed at the start of the temperature rise and, concurrently, an increase in turbulence poloidal flow velocity and flow shear is measured with the beam emission spectroscopy diagnostic. For the case of a transition to an enduring internal barrier the fluctuation level remains at a reduced amplitude. The timing and nature of the temperature, rotation, and fluctuation changes leading to internal barriers suggests transport improvement due to increased shear flow arising from the zonal flow structures.

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Section: Discussionsupporting

confidence: 88%

Section: Introductionmentioning

confidence: 99%

Core barrier formation near integer q surfaces in DIII-D

Austin¹,

Burrell

Waltz

et al. 2006

Physics of Plasmas

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“…Such flows are thought to be implicated in the formation and evolution of transport barriers in tokamaks and stellarators. 2,4 On the theoretical side, a number of researchers [5][6][7][8][9][10] have examined the possibility of suppressing turbulence by radial decorrelation due to strongly sheared EÃB flow shear and have contributed a variety of models which exhibit this effect ͑some of this work is reviewed in Refs. 4 -11͒.…”

Section: Introductionmentioning

confidence: 99%

The nonlinear dynamics of the modulational instability of drift waves and the associated zonal flows

2001

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The linear and nonlinear dynamics of zonal flows and their interactions with drift wave turbulence is considered in the simple but illuminating generalized Charney–Hasegawa–Mima model due to Smolyakov et al. [Phys. Plasmas 7, 1349 (2000)]. Two positive definite, exact, integral invariants associated with the full generalized Charney–Hasegawa–Mima system are derived. For an initial monochromatic drift wave pump with small but finite amplitude, a modulational instability can occur, characterized by growing zonal flow and sideband perturbations (i.e., a four-wave interaction). The pump threshold for instability is readily satisfied, depending on the zonal flow wave number. The fully nonlinear Charney–Hasegawa–Mima equations are solved with a numerical scheme which is validated by demonstrating the conservation of the two exact invariants. The simulations show that the validity of the four-wave model is limited to approximately three instability growth times. The radial structure of the zonal flow can be “jet-like” or highly oscillatory depending upon the ratio of the system size to the density scale length and initial conditions. It is found that zonal flows can be dramatically reduced if the most unstable zonal flow wave number does not fit into the system.

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“…Analysis of negative central shear (NCS) tokamak discharges has shown that an ion internal transport barrier (ITB) may develop near the position where the safety factor q reaches its minimum [1][2][3][4][5][6]. There is also evidence that, in addition to the NCS q profile, E × B flow shear in the core plays an important role in stabilizing turbulence [7][8][9].…”

Section: Introductionmentioning

confidence: 99%

First principles model of internal transport barriers in negative central shear discharges

Rogister¹

2001

Nucl. Fusion

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Abstract. At and around minima or maxima of the safety factor q, overlapping, and therefore toroidicity induced coupling of drift eigenmodes centred on neighbouring rational surfaces r l,m defined by q(r l,m ) = m/l and r l,m±1 defined by q(r l,m±1 ) = (m ± 1)/l, is generally negligible; l and m(m ± 1) are the toroidal and (main) poloidal mode numbers, respectively. Under these conditions, a careful analysis shows that the growth rate of the ion temperature gradient (ITG) mode is proportional to the absolute value of the magnetic shear parameter; its radial width and frequency (in the E × B rotating frame) are larger in the axisymmetric torus than in the plasma slab or cylinder, especially at small values of k θ ai (k θ is the poloidal mode number and ai the characteristic ion Larmor radius). These results provide a straightforward theoretical interpretation of the origin of internal ion transport barriers (ITBs) in discharges with negative central shear if the ITG instability is indeed the main channel for ion energy transport. In agreement with recent experiments, the mechanism does not degrade for Te ∼ Ti, an important result for extrapolation to reactors. Once initiated, an ITB will locally increase the curvature of the ion temperature profile and, consequently, the gradients of the radial electric field and E × B rotation profiles (until neoclassical (or sub-neoclassical) transport comes into play).The damping associated with velocity shear yields an upper bound on the unstable mode number range, whereas Landau damping is expected to introduce a lower bound. In cases where toroidal effects are subdominant, the ratio of the damping rate associated with velocity shear to the growth rate associated with magnetic shear leads naturally to the Hamaguchi-Horton parameter (divided by the factor ηi − 2/3 which characterizes the departure from the instability threshold (ηi is the ratio of the density and temperature length scales)) when introducing the most unfavourable mode number. Toroidal effects are always dominant for sufficiently weak magnetic shear or sufficiently large values of |LN |/R (where LN is the density gradient length), and therefore before steepening of the profiles has occurred near the zero magnetic shear layer; the counterpart of the Hamaguchi-Horton characteristic parameter appropriate to that case is also obtained, considering again the most unfavourable mode number.

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Core transport reduction in tokamak plasmas with modified magnetic shear

Cited by 38 publications

References 35 publications

Core barrier formation near integer q surfaces in DIII-D

Core barrier formation near integer q surfaces in DIII-D

The nonlinear dynamics of the modulational instability of drift waves and the associated zonal flows

First principles model of internal transport barriers in negative central shear discharges

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