Description
The poloidal magnetic flux produced by the net plasma current changes by approximately 10% over the range of credible current profiles. The time derivative of the poloidal flux outside a surface of fixed toroidal flux $\psi_t$, is given by the loop voltage $V_\ell$, which implies the actual current profile is given by the spatial constancy of $V_\ell$. Using Boozer coordinates, the internal inductance $\ell_i$, and $q_{edge}/q_{axis}$ can be given using $I(\psi_t)/I_{plasma}$ and a surface-shape function $\sigma(\psi_t)$,which can be expanded in $\psi_t$. The ratio of the second derivative relative to the first of $\sigma$ is $\delta_t.$ A large $\ell_i$ is associated with tearing and disruptions. A large edge current density, given by $(dI/d\psi_t)_{edge}$, and a large triangularity $\delta_t$ are shown to reduce $\ell_i$ for a given $I(\psi_t)/I_{plasma}$. It is important to determine if there are any safe regions against disruptions in the space of $\ell_i$, $q_{edge}/q_{axis}$, edge current, and $\delta_t.$ The scatter diagram using $\ell_i$, $q_{edge}/q_{axis}$ for disruptions in JET-ILW of Figure 13 in Nucl. Fusion \textbf{60}, 066028 (2020) implies more quantities are needed to describe a disruption safe region.