High-bootstrap, noninductively sustained electron internal transport barriers in the Tokamak a Configuration Variable

Important ingredients of the advanced-tokamak route to fusion have been explored in depth in the Tokamak a Configuration Variable [F. Hofmann, J. B. Lister, M. Anton et al., Plasma Phys. Controlled Fusion 36, B277 (1994)] over the past two years. Using a uniquely powerful and flexible electron-cyclotron resonance heating (ECRH) system as the primary actuator, fully noninductive, steady-state electron internal transport barrier discharges have been generated with an electron-energy confinement time up to five times longer than in L mode, poloidal β up to 2.4, and bootstrap fraction up to 75%. Interpretative transport modeling confirms that the safety factor profile is nonmonotonic in these discharges. The formation of the barrier is a discrete event resulting in rapid and localized confinement improvement consistent with the time and location of magnetic-shear reversal. In steady state, however, the confinement quality appears to depend on the current gradient in a broader negative-shear region enclosed by the barrier, improving with increasing shear: in particular, the width and depth of the barrier can be controlled and finely tuned, along a magnetohydrodynamic-stable path, by manipulating the current profile with ECRH (six independently steerable 0.45 MW launchers). The crucial role of the current profile has been clearly demonstrated by applying small Ohmic current perturbations which dramatically alter the properties of the barrier, enhancing or reducing the confinement with negative and positive current, respectively, with negligible Ohmic heating. These results are in agreement with theoretical estimates: first-principle-based numerical simulations of microinstability dynamics and turbulence-driven transport predict a substantial suppression of turbulence and anomalous energy diffusivity near the location of the minimum in the safety factor