Sheared flow effects on ballooning instabilities in three-dimensional equilibria

Hegna, C. C.

doi:10.1063/1.2136870

Cited by 4 publications

(3 citation statements)

References 24 publications

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“…Poloidal and toroidal shear flows that naturally arise under quasi-steady-state conditions in the presence of diffusive transport are shown to have a stabilizing influence, as expected, but surprisingly over all ranges of toroidal mode number n, not just for intermediate to high-n, as is usually observed for purely toroidal shear flows. [17][18][19][20] This nearly uniform stabilizing influence of poloidal shear flows in all n space is shown to result from poloidal variations in the flow, in particular a nearly stagnant point at the outer midplane in both single-and double-null geometries that accompanies the radial shear. Another consequence of the shear flows is shown to be poloidal localization of the ballooning/peeling modes to the midplane, thus reducing the number of ELM filaments seen in nonlinear calculations without the shear flows.…”

Section: Introductionmentioning

confidence: 93%

“…A considerable amount of theoretical and computational work has been done to study these modes in linear and nonlinear regimes. [28][29][30][31][32][33][34][35] Since the edge and H-mode physics are closely affected by the presence of shear flows, a number of authors have also examined their role in high-n ballooning modes [17][18][19][20] and ballooning/peeling modes. 36 However, generally only toroidal shear flows have been considered.…”

Section: Effects Of Shear Flows On Edge-localized Modesmentioning

confidence: 99%

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Shear flows at the tokamak edge and their interaction with edge-localized modes

Aydemir

2007

Physics of Plasmas

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Shear flows in the scrape-off layer (SOL) and the edge pedestal region of tokamaks are shown to arise naturally out of transport processes in a magnetohydrodynamic model. In quasi-steady-state conditions, collisional resistivity coupled with a simple bootstrap current model necessarily leads to poloidal and toroidal flows, mainly localized to the edge and SOL. The role of these flows in the grad-B drift direction dependence of the power threshold for the L (low) to H (high) transition, and their effect on core rotation, are discussed. Theoretical predictions based on symmetries of the underlying equations, coupled with computational results, are found to be in agreement with observations in Alcator C-Mod [Phys. Plasmas 12, 056111 (2005)]. The effects of these self-consistent flows on linear peeling/ballooning modes and their nonlinear consequences are also examined.

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Section: Introductionmentioning

confidence: 93%

Section: Effects Of Shear Flows On Edge-localized Modesmentioning

confidence: 99%

Shear flows at the tokamak edge and their interaction with edge-localized modes

Aydemir

2007

Physics of Plasmas

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“…It is widely recognized that ballooning mode instability is very likely to be unavoidable in toroidal plasmas [1][2][3][4][5]. * Authors to whom any correspondence should be addressed.…”

Section: Introductionmentioning

confidence: 99%

Extended instability of kinetic ballooning modes induced by ion temperature gradient and impurity in tokamaks

Shen¹,

Dong²,

Peng³

et al. 2022

Nucl. Fusion

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Kinetic ballooning mode (KBM) instability in tokamak plasmas is studied with gyrokinetic theory. It is extended and improved of the theory of higher order kinetic ballooning mode which is not subject to second stabilization [Hirose A, et al. 1994 Phys. Rev. Lett. 72 3993]. It is verified that increasing ion temperature gradient ($\eta_i$) significantly enhances the instability of KBMs, and causes extended instability in the second MHD stable regime. Especially when $\eta_i\gtrsim 1$, the mode is unstable almost in the whole $\beta$ space from an experimental point of view, here $\beta$ is the ratio of kinetic pressure to magnetic pressure. It is found that for the case with relatively low safety factor ($1\lesssim q \lesssim 2$) and $\eta_i\approx 0$, which is generally in central regions of a tokamak plasma, the KBM instability can also be extended to the region of $\beta$ or $\alpha$ far beyond the second MHD stability boundary due only to impurity effect, here $\alpha=-Rq^2 (d\beta/dr)$. It has characteristics similar to the extended instability induced by $\eta_i$, while its instability window is narrower than the latter. From the observations, the unstable window of extended instability is finite. As the safety factor $q$ and magnetic shear $\hat {s}$ ($\equiv rdq/qdr$) increases, the “theoretical” critical $\beta$ of the second stable regime decreases and the $\alpha$ threshold increases. When the impurity effect is combined with the $\eta_i$ effect, the unstable window of the extended instability induced by $\eta_i$ can be greatly reduced by the former. In some cases, for instance as $\eta_i\gtrsim 1$ and $q$ is larger, and in the presence of impurity with mediate mass number, the impurity effect can even make the second stable regime of “experimental significance” re-appear in KBMs. The influence of $\eta_i$ and impurity effects on broadening mode structure of KBMs are smaller than $q$ and $\hat {s}$. The stability boundary based on $\hat {s}-\alpha$ plane conforms the above results.

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Impurity effects on kinetic ballooning instability in high q regions of tokamak plasmas

Li,

Dong,

Liu

et al. 2024

Nucl. Fusion

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The impurity effects on kinetic ballooning modes (KBMs) in high q regions of tokamak plasmas are numerically investigated, using a full gyrokinetic description for main and impurity ions. The findings indicate that the critical plasma pressure ratio β_c for KBM is significantly lower (higher) than the first (second) stable threshold β_MHD predicted by MHD theory. The KBM exhibits stronger extended instabilities for low q, while the instability region of KBM becomes significantly narrow for high q, resulting in a more concentrated instability window (localized in narrow medium-β region). In addition, the contributions from electromagnetic effect and Shafranov shift are identified in detail, i.e. the electromagnetic destabilizing effect is suppressed by the Shafranov shift, and strong stabilizing effect of Shafranov shift for high β_e or α on KBM is pronounced. The impurity ions with negative gradient ratio L_ez have stabilizing (destabilizing) effects on KBMs for high (low) q. Such distinct behaviors are demonstrated to be associated with the used parameters close to the second and first unstable α region, i.e. the instability reduces and enhances with the increase of α, respectively. Significantly, the presence of impurities, regardless of the peaking directions of their density profiles, serves to stabilize KBMs and reduce the window of extended instability for high q region, which is in significant contrast with the results for low q region. Furthermore, high concentration, steep negative gradient ratio L_ez, and high mass numbers of impurity ions as well as large temperature gradient η_i and η_e contribute to the stabilization of KBMs and improvement of confinement in high q regions of tokamaks. The region of stabilizing effect of magnetic shear on KBM, is found not belonging to the conventional magnetic shear region, but shows strong sensitivity to q value.

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Sheared flow effects on ballooning instabilities in three-dimensional equilibria

Cited by 4 publications

References 24 publications

Shear flows at the tokamak edge and their interaction with edge-localized modes

Shear flows at the tokamak edge and their interaction with edge-localized modes

Extended instability of kinetic ballooning modes induced by ion temperature gradient and impurity in tokamaks

Impurity effects on kinetic ballooning instability in high q regions of tokamak plasmas

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