Optimization of the Internal Magnetic Configuration for High Bootstrap Current Fraction and High Beta for Steady-state

A systematic scan of the safety factor ($q$) profile has been used to study the optimum for steady-state operation, which requires the maximum possible beta and bootstrap current fraction ($f_{BS}$) and good alignment between the total current density and the bootstrap current density ($J_{BS}$). The $n_e$, $T_e$, and $T_i$ profiles at constant $\beta_N = 2.7$ were measured in a scan of the minimum $q$ ($1.12$, and 3.8 at $q_{min} = 1.1$. These opposite trends in $\beta_N$ and $f_{BS}$, and the improved current profile alignment for $q_{min}<2$, point to intermediate $q_{min}$ and $q_{95}$ as optimal for steady-state operation.