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and its magnitude is equal to the inverse radius of curvature. | and its magnitude is equal to the inverse radius of curvature. | ||
A plasma is stable against curvature-driven instabilities (e.g., ballooning modes) when | A plasma is stable against curvature-driven [[Plasma instability|instabilities]] (e.g., ballooning modes) when | ||
:<math>\vec \kappa \cdot \vec \nabla p < 0</math> | :<math>\vec \kappa \cdot \vec \nabla p < 0</math> | ||
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The tangent plane to any flux surface is spanned up by two tangent vectors: one is the normalized magnetic field vector (discussed above), and the other is | The tangent plane to any flux surface is spanned up by two tangent vectors: one is the normalized magnetic field vector (discussed above), and the other is | ||
:<math>\vec b_\perp = \frac{\vec \nabla \psi | :<math>\vec b_\perp = \frac{\vec \nabla \psi}{|\vec \nabla \psi|} \times \frac{\vec B}{|\vec B|}</math> | ||
The corresponding perpendicular curvature is | The corresponding perpendicular curvature (the curvature of the flux surface in the direction perpendicular to the magnetic field) is | ||
:<math>\vec \kappa_\perp = \vec b_\perp \cdot \vec \nabla \vec b_\perp</math> | :<math>\vec \kappa_\perp = \vec b_\perp \cdot \vec \nabla \vec b_\perp</math> |