Active control of internal transport barrier formation due to off-axis electron-cyclotron heating in GAMMA 10 experiments

The controlled formation of an internal transport barrier (ITB) is observed in GAMMA 10 [T. Cho , Nucl. Fusion 45, 1650 (2005)]. The barrier is localized within a layer of a strongly sheared E(r)xB plasma rotation (5.5 < r(c)<= 10 cm). This high-vorticity layer is formed and maintained by off-axis electron-cyclotron heating, which generates a cylindrical layer (4 < r(c)< 7 cm) with a high-energy electron population that modifies the initial Gaussian radial potential profile into a nonmonotonic one with a hump structure. The local gradients of T-i and T-e are appreciably enhanced in the ITB layer, similarly to those of the ITB in tokamaks and stellarators. Reductions in the effective ion and electron thermal diffusivities are obtained in the barrier layer. A reduction of the observed low-frequency turbulence in the ITB layer and a partial decoupling of the turbulent structures localized on either side of the layer are demonstrated by two-dimensional x-ray diagnostics. (C) 2008 American Institute of Physics.