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In [[Tokamak|tokamak]] research, the quantity ''q = 2π/ι'' is preferred (called the "safety factor"). | In [[Tokamak|tokamak]] research, the quantity ''q = 2π/ι'' is preferred (called the "safety factor"). | ||
In a circular [[Tokamak|tokamak]], ''q'' can be approximated by | In a circular [[Tokamak|tokamak]], | ||
the equations of a field line on the flux surface are, approximately: | |||
<ref>K. Miyamoto, ''Plasma Physics and Controlled Nuclear Fusion'', Springer-Verlag (2005) ISBN 3540242171</ref> | |||
:<math>\frac{r d\theta}{B_\theta} = \frac{Rd\phi}{B_\phi}</math> | |||
where ''φ'' and ''θ'' are the [[Toroidal coordinates|toroidal and poloidal angles]], respectively. | |||
Thus ''q = dφ/dθ'' can be approximated by | |||
:<math>q = \frac{r B_\phi}{R B_\theta}</math> | |||
== References == | |||
<references /> | |||