Poloidal current density

Use Eq. (36) and the definition $g \equiv R B_{\phi}$, the poloidal components of the current density, $J_Z$ and $J_R$, can be written as

$$\mu_0 J_R = - \frac{1}{R} \frac{\partial g}{\partial Z},$$ (37)

and

$$\mu_0 J_Z = \frac{1}{R} \frac{\partial g}{\partial R},$$ (38)

respectively. The poloidal plasma current density is usually small and can be safely neglected in most cases of interest.