Turbulence reduction and poloidal shear steepening in reversed shear plasmas investigated by light scattering on Tore Supra

Infrared light scattering is used to characterize turbulence during reverse d shear scenario obtained by a fast current ramp-up on Tore Supra [Hoang et al., Phys. Rev. Lett. 84, 4593 (2000)]. The preformed hollow profile is fr eezed using ion cyclotron resonance minority heating (ICRH). Turbulence int ensity is recorded to dramatically decrease during the ramp-up and to recov er smoothly as the discharge is ended. Simultaneously, a confinement improv ement of about 40% is achieved for about 2 s. Although the diagnostic perfo rms a line average and the measurement is dominated by turbulence at the ed ge, we show that the core behavior can be deduced. Core turbulence decrease is in agreement with the effective heat diffusivity behavior. With the sam e optical device, the radial electric shear is deduced from the frequency s pectra Doppler shift. Turbulence reduction is thus demonstrated to be corre lated with the E x B shear steepening. (C) 2001 American Institute of Physi cs. [DOI: 10.1063/1.1330202].