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:<math>L = 2 \pi r \frac{B_\phi}{B_\theta} = 2 \pi R q</math> | :<math>L = 2 \pi r \frac{B_\phi}{B_\theta} = 2 \pi R q</math> | ||
where q is the [[ | where q is the [[Rotational transform|safety factor]], approximated by | ||
''q = r B<sub>φ</sub> / R B<sub>θ</sub>''. | ''q = r B<sub>φ</sub> / R B<sub>θ</sub>''. | ||