Application of ECRH/ECCD on FTU: an Overview of Recent Results

Abstract. The ECRH system (1.6 MW, 0.5s, 140 GHz) has been widely exploited on FTU (4-8 T; 0.3-1.6 MA; 0.3- 4x10 20 m-3) in the last 10 years. In this paper we report the main recent results obtained applying EC waves for suprathermal current drive, MHD activity control and disruption mitigation. The mostly used suprathermal EC absorption scheme in FTU is at down-shifted frequencies, with central BTo (7 T) higher than the resonant one (5 T). Experiments carried out with Ip=500kA, ne=6-8 1019m-3 and N//EC ranging from 0 to ±0.5 have shown a measured overall current drive (CD) efficiency well above that due to the sum of those expected for the two waves. This fact is ascribed to the existence of a synergy that apparently increases the EC efficiency close to 0.1 1020AW-1m-3. The consequent modification of the current profile is well supported by FEB data. EC waves are the most promising candidate for NTM stabilization/control on ITER. Experiments on real time m=2, n=1 mode stabilization with EC waves are reported, and a description of the control algorithm, based on ECE signals for the detection of the island position and the EC deposition radius, is given. Experiments to study the influence of ECRH power on the disruption evolution have been carried out. The disruptions have been induced by injection of impurities or by increasing the electron density above the Greenwald limit using gas puffing. Magnetic coil measurements and soft-x ray tomography reconstruction results indicate that ECRH is acting on the magnetic island evolution, leading to disruption avoidance and/or to current decay softening, depending on the deposition position. ECRH is found to be effective in disruption avoidance when the deposition location is r/a=0.6 in Mo injection cases and r/a=0 in density limits cases.