H-mode and transport barriers in helical systems

This paper presents the physics of two bifurcations in confinement of helical devices—(1) to the H-mode and (2) to internal transport barrier (ITB)-like electron temperature profiles as they develop under neoclassical electron root conditions in 3-dimensional systems. With their characteristics—low or negative magnetic shear, strong toroidal flow damping, experimental variability of poloidal flow damping, radial electric field enforced by ambipolarity, diagnostic access to sophisticated spatial and temporal structures of turbulence thanks to low-power operation with external confinement—helical devices provide unique contributions to the physics of transport barriers. The bifurcation to confinement with external transport barrier seems to be soft and the leading role of the electric field gradient is confirmed; the one to ITB-like core profiles is a hard transition and it is the electric field which governs it. The paper summarizes the status of H-mode research in helical systems and discusses the impact of the electron root on core confinement.