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| Thus, in 3 dimensions, the shear is a 3 x 3 tensor. | | Thus, in 3 dimensions, the shear is a 3 x 3 tensor. |
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| == Global magnetic shear ==
| | That's way the besstet answer so far! |
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| In the context of magnetic confinement, and assuming the existence of toroidally nested magnetic [[Flux surface|flux surfaces]], the only relevant variation of the direction of the magnetic field is the radial gradient of the [[Rotational transform|rotational transform]].
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| The global magnetic shear is defined as
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| :<math>s = \frac{r}{q} \frac{dq}{dr} = -\frac{r}{\iota} \frac{d\iota}{dr}</math>
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| High values of magnetic shear provide stability, since the radial extension of helically resonant modes is reduced.
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| Negative shear also provides stability, possibly because convective cells, generated by curvature-driven instabilities, are sheared apart as the field lines twist around the torus.
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| <ref>[http://link.aip.org/link/?PHPAEN/3/2221/1 T.M. Antonsen, Jr., et al, ''Physical mechanism of enhanced stability from negative shear in tokamaks: Implications for edge transport and the L-H transition'', Phys. Plasmas '''3''', 2221 (1996)]</ref>
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| == Local magnetic shear == | | == Local magnetic shear == |