= Uniform-twist central axial field bound
{title2=$B_z(0)^2\leq(1+\gamma^2)B_p^2$}
For a <flux tube> with $B_\phi=\gamma(s/s_0)B_z$, regularity at the axis and <cylindrical magnetostatic pressure balance> give $B_z(0)^2=B_p^2-2\mu_0p(0)+\gamma^2\langle B_z^2\rangle$. The <flux-tube axial field virial identity> and $p\geq0$ give the bound. A twisted <toroidal magnetic field> can support a central axial field above the untwisted <magnetic pressure> limit even though its cross-sectional mean obeys that limit.
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