ECRH at high heating power density in FTU tokamak

Plasma absorption at the fundamental EC resonance of e.m. waves at 140 GHz, 0.8 MW, Ordinary mode, provides in the FTU tokamak a localized (minimum deltar similar or equal to2.5 cm) and intense (up to similar or equal to 100 MW/m(3)) heat source for the electrons. It is shown that at high density (similar or equal to 10(20) m gamma - (-3)) ions are heated (DeltaT(i)/T-i = 0.25) by collisional e - i energy transfer and assuming ion thermal diffusivity about three times larger (at most) than predictions in order to account ;for the observed T-i. Off-center localization (r(abs)/a similar or equal to 0.2) of the ECRH heat source changes the electron temperature (and current density) profile in a way to fully stabilize sawteeth. On-axis absorption accelerates the sawtooth crash repetition rate if P-ecrh less than or equal to P-oh, but with P-ecrh > P-oh the sawtooth period is increased. A critical shear model for the sawtooth crash repetition rate is shown to be in agreement with its experimental dependence on resonance position and heating power. In discharges characterized by a flat temperature profile in the ohmic phase, ECRH may de-stabilize tearing modes of different order, with a strong degradation of core energy confinement. (C) 2001 Published by Elsevier Science B.V.