Parameter Space for Self-Consistent High $\beta_N$, High $\ell_i$ Discharges in Steady-State

A tokamak scenario with a peaked current profile ($\ell_i$>0.9), is a promising candidate for a high $\beta_N$ and Q power plant because of the increased stability limits and confinement. Model equilibria have been used to show that the present DIII-D experiments in this range of $\ell_i$, with $\beta_N$=5 and evolving current density (J) profile, can be extended to 100$\%$ noninductively driven current with stationary, self-consistent J and pressure (P) profiles. $\beta_N$=4, $q_{95}$<6.5, bootstrap current fraction $f_{BS}$<0.46 is predicted stable to ideal low-n modes without requirement for a conducting wall, while $\beta_N$=5, $f_{BS}$<0.6 is predicted stable when the effect of the vacuum vessel is included. These results reflect the trade-off between high $f_{BS}$ and high $\beta_N$ that is required because, as $\beta_N$ is increased, bootstrap current in the plasma outer half, from the H-mode pedestal and the broad pressure profile, reduces $\ell_i$ and the ideal stability limit. Full simulations using the TGLF transport model and the DIII-D current drive/heating sources yield similar parameters.