Additional heating and reversed magnetic shear in the FTU Tokamak

After one year of shutdown devoted to the installation of the toroidal molybdenum limiter, the Frascati Tokamak Upgrade (FTU) has recently resumed operation. The experimental programme is focused on the three radiofrequency (RF) heating and non-inductive current drive systems: electron cyclotron resonant heating (ECRH), lower hybrid current drive (LHCD) and ion Bernstein wave heating (IBWH). The main aim of the experiments reported here is to demonstrate LHCD at high density and to sustain and heat reversed magnetic shear configurations, with a large reversal radius (r(s)/a approximate to 0.5). A transient heating of the shear reversed discharges achieved in ohmic conditions has been performed through ECRH. The results show that good confinement properties are maintained within the shear reversal surface. The FTU 8 GHz LH system was upgraded so that up to 1.4 MW of LH could be injected into the plasma. A value of 0.14 x 10(20) A . W-1 . m(-2) was obtained for the figure of merit of the CD efficiency. Furthermore, in ohmic and LH heated discharges a confinement time of up to 1.6 times the ITER89-P value was observed. The first shear reversed discharge driven by 0.8 MW of LHCD was produced at I-p = 350 kA, (n) over bar(e) = 7 x 10(19) m(-3); it shows a regime where regular m = 2, n = 1 relaxations with extremely large amplitude ((B) over tilde(theta)/B-theta = 2.5% at the plasma edge) coexist with a high temperature plasma (T-e0 = 1.6 keV).