Impact on MHD stability and energy confinement of localized ECRH on FTU tokamak

Gaussian beam steering (2 beams, *400 kW each) and toroidal field tuning are used to control ECRH localization for MHD stabilization and energy transport studies on FTU tokamak. It is shown that by locating the absorption of one beam inside the m=2 island, and of a second one close to the s.t. inversion radius, both instabilities can be suppressed, with a significant impact on energy confinement. MHD suppression allows the analysis of the underlying energy transport. It is shown that the electron temperature and pressure profiles are stiff when ECRH is localized off-axis, and that a finite amount of heat pumping against the temperature gradient must be accounted for to explain steady-state profiles at high density (^//>JC,«1020m"3). The resistance of the thermal response to change with the heating source term can be analyzed in terms of two models implying profile stiffness: the effect of a critical temperature gradient, possibly related to ETG modes, and the tendency of a tokamak plasma to stick to preferred profiles. In particular, it is shown that the assumption of constant magnetic entropy, implying relaxation of the toroidal current density, allows good reproduction of the experimental profiles and of the main observation of a step-wise radial profile of the effective electron thermal diffusivity. Profile stiffness is observed both at high density, with significant e-i coupling, and at low density when much less power is conducted through ions.