Rotation profile flattening and toroidal flow shear reversal due to the coupling of magnetic islands in tokamaks

Tobias, Benjamin; Chen, M.; Classen, I. G. J.; Domier, Calvin; Fitzpatrick, Richard; Grierson, B. A.; Luhmann, Neville C.; Muscatello, C. M.; Okabayashi, M.; Olofsson, Karl; Paz-Soldan, C.

doi:10.1063/1.4946026

Cited by 20 publications

(14 citation statements)

References 41 publications

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“…On the one hand, this approximation neglects the toroidal mode coupling effect, which possibly affects the solution of the mode/magnetic island during the non-linear stage before the occurrence of EF penetration. However, this nonlinear toroidal mode coupling will be suppressed by the plasma rotation shear (and hence frequency difference between different rational surfaces) with the absence of any modes/magnetic island [51]. On the other hand, the modifications of plasma response expected from a toroidal plasma ideal MHD modes is also neglected, which will modify tearing parity resonant fields at the corresponding rational surface [52].…”

Section: Discussion and Summarymentioning

confidence: 99%

Nonlinear modeling of the scaling law for the $m/n = 3/2$ error field penetration threshold

Logan

Park

et al. 2020

Nucl. Fusion

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The scaling law for the m/n = 3/2 error field (EF) penetration threshold is predicted numerically based on nonlinear single-fluid and two-fluid modeling using the TM1 code. The simulated penetration threshold of radial magnetic field b r at the plasma edge is scaled to the electron density n e , temperature T e , viscous time τ µ , toroidal field B t and the natural frequency ω in the form of b r /B t ∝ n αn e T α T e τ αµ µ B α B t ω αω by scanning these parameters separately. Here, α n , α T , α µ , α B and α ω are the scaling coefficients on n e , T e , τ µ , B t and ω, respectively. Singlefluid modeling shows that the 3/2 EF threshold scales as b r /B t ∝ n 0.56 e T 0.6 e τ −0.59 µ B −1.15 t ω, which is similar with the analytical scaling law in both the Rutherford and visco-resistive regimes. However, two-fluid modeling shows that the scaling law differs significantly in particular regarding the dependence on plasma rotation. In detail, the scaling coefficient α n on density decreases from 0.67 to 0.56 and α T on temperature decreases from 0.67 to 0.32, while α µ on viscous time is around -0.45 and α B on toroidal field decreases slightly from -1.15 to -1, when the ratio |ω E /ω * e | between plasma rotation frequency ω E and diamagnetic drift frequency ω * e varies from 0 to 10. Scans of the plasma rotation reveals that the penetration threshold linearly depends on the perpendicular rotation frequency (or natural frequency) ω ⊥e = ω E + ω * e , and there is a minimum in the required field amplitude when ω ⊥e ∼ 0. In addition, the enduring mystery of non-zero penetration threshold at zero plasma natural frequency in EF experiments is resolved by two-fluid simulations. We find that the very small island and smooth bifurcation in EF penetration near zero frequency is hard to detect in the experiment, leading to a finite penetration threshold within the capability of the experimental measurements.

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Section: Discussion and Summarymentioning

confidence: 99%

Nonlinear modeling of the scaling law for the $m/n = 3/2$ error field penetration threshold

Logan

Park

et al. 2020

Nucl. Fusion

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“…This effect will generate magnetic perturbations with broad spectrum [23,24], which in turn further advances the heat transport under multiple LMs. In the modeling, the central rotation profile is not near zero, however the global rotation is almost zero after mode locking or field penetration in DIII-D experiments [24], indicating there should be additional LMs, momentum transport or changes in the momentum sources inside the q = 2 rational surface and their effect should be taken into account. Future work to use a full toroidal geometry would allow one to further take into account these effects and to give a more precise consistence with experiment.…”

Section: Discussion and Summarymentioning

confidence: 99%

“…Multiple LMs usually happen after the occurence of mode locking or field penetration [18,19], due to the braking and destabilizing effects caused by the wide bandwidth of EF [11,20,21] and toroidal coupling effects [22][23][24]. These multiple LMs challenge the study of the transition process from LMs to TQ in both experiment and theory.…”

Section: Nuclear Fusionmentioning

confidence: 99%

Fast and pervasive heat transport induced by multiple locked modes in DIII-D

et al. 2018

Nucl. Fusion

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This work presents the impact that multiple island chains co-existent across a tokamak plasma profile have on the heat transport and final temperature of that plasma. Numerical studies using the TM1 code show that error fields (EFs) with multiple poloidal components accelerate the core field penetration compared to pure m/n = 2/1 EF penetration (here m and n are the poloidal and toroidal mode numbers respectively). After field penetration, locked magnetic islands of m/n = 2/1, 3/1 and 4/1 flatten the temperature at the corresponding rational surfaces. The coexistence of these islands significantly enhances the plasma heat transport throughout a wide swath of plasma from the core 2/1 rational surface to plasma edge. The electron temperature T e profile from 2/1 to 4/1 rational surfaces can be nearly flattened even if there is no island overlap, and the temperature inside each island is determined by the boundary temperature at the outboard separatrix of the island. The resulting central T e decreases by more than 50%, in good agreement with experimental observations and much lower than modeling with only a single 2/1 locked island. Further comparisons of the T e profile between numerical modeling and DIII-D experiment indicates that the observed reduction in the edge temperature requires edge island overlap and stochasticity. Numerical scans reveal the T e profile decreases further when large EF amplitudes create larger islands, wider edge stochastic regions and secondary island structures. Scans of the relative phase between EF harmonics reveal that the 3/1 island width is most sensitive to the island phase and the central T e changes with the 3/1 island width. These results indicate that the coexistence of multiple LMs in tokamak plasmas deteriorate thermal confinement more than the sum of their isolated impacts would and that this may be responsible for the fast thermal quench observed prior to major disruptions.

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“…When the magnetic island width, w reaches to the critical value (Cw 4 = 21), the irreversible jump to the end branch takes place and the MHD mode frequency is significantly reduced to almost zero, which indicates the plasma toroidal rotation stops (flow damping) by the growth of the magnetic island. The toroidal flow damping due to growth of the magnetic island was found to be restricted inside the magnetic island in the plasma with MHD mode locking in JT-60 U [199] and DIII-D [212]. Highly localized flow damping inside the magnetic island results in the large flow shear at the boundary of the magnetic island because of the finite plasma toroidal flow driven by NBI torque outside the magnetic island.…”

Section: Toroidal Flow Damping By Magnetic Island and Stochastizationmentioning

confidence: 95%

Bifurcation phenomena in magnetically confined toroidal plasmas

Ida

2020

Advances in Physics: X

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Bifurcation phenomena observed in turbulence and transport, in topology of magnetic field and MHD, and the interplay between turbulence and MHD bifurcation in magnetically confined toroidal plasmas are reviewed. Two types of bifurcation phenomena in turbulent transport are discussed. One is significant change in the magnitude of the diffusion term and the other is a sign flip of non-diffusive term of transport. The bifurcation of diffusion term magnitude causes the transition to a so-called improved mode such as ion-electron root transition, L-mode to H-mode transition, and ITB formation. In contrast, the bifurcation of non-diffusion term sign causes the intrinsic flow reversal, convection reversal of bulk ion, and impurity ions. Two types of bifurcation phenomena in MHD are discussed. One is the bifurcation of static magnetic topology and the other is the bifurcation of MHD instability. The bifurcation of static magnetic topology is observed in between three magnetic topology (nested magnetic flux surface, magnetic island and stochastic magnetic field) in toroidal plasma. The bifurcation of MHD instability is observed as a parity change of MHD oscillation. The bifurcation of the magnetic island states caused by the interplay between turbulence and MHD is also discussed.

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Rotation profile flattening and toroidal flow shear reversal due to the coupling of magnetic islands in tokamaks

Cited by 20 publications

References 41 publications

Nonlinear modeling of the scaling law for the $m/n = 3/2$ error field penetration threshold

Nonlinear modeling of the scaling law for the $m/n = 3/2$ error field penetration threshold

Fast and pervasive heat transport induced by multiple locked modes in DIII-D

Bifurcation phenomena in magnetically confined toroidal plasmas

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