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Magnetic shear: Difference between revisions
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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. | ||
== Global magnetic shear == | |||
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]]. | |||
The global magnetic shear is defined as | |||
:<math>s = \frac{r}{q} \frac{dq}{dr} = -\frac{r}{\iota} \frac{d\iota}{dr}</math> | |||
High values of magnetic shear provide stability, since the radial extension of helically resonant modes is reduced. | |||
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. | |||
<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> | |||
== Local magnetic shear == | == Local magnetic shear == | ||
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== References == | == References == | ||
<references /> | |||
<references/> | |||