Improved Confinement Regimes for the Ignitor Experiment$^1$

The Ignitor experiment$^2$ is designed to achieve ignition in well confined deuterium-tritium plasmas, with Ohmic heating only or small amounts of additional RF heating. Thanks to its unique features ($B_T \leq 13$ T, $I_p \leq 11$ MA, $\left\langle n\right\rangle \leq 5\times 10^{20} \rm m^{-3} $), Ignitor is the only device capable of accessing plasma parameters that are relevant to a net power producing D-T reactor. In fact, plasma regimes suitable to operate a reactor have yet to be produced. The Poloidal Field System in Ignitor makes it possible to operate with different configurations, from the reference ``extended limiter'' configuration to the configuration with double X-points just inside or just outside the first wall. X-point configurations are interesting as they may lead to a high confinement regime (H- mode) when the heating power is larger than a threshold value. H-mode operation in Ignitor has been assessed both with 0D calculations as well as 1 1/2D transport simulations. Quasi-stationary conditions can be attained when a process producing re-distribution of pressure and current profiles is active. The accessibility to improved confinement regimes with or without edge pedestal that have been observed on existing experiments is assessed.\\ $^1$Sponsored in part by ENEA of Italy and by the U.S. D.O.E.\\ $^2$B.Coppi, A.Airoldi, F.Bombarda, et al.,\textit{Nucl. Fusion} \textbf{41}, 1253 (2001)