EVIDENCE OF INHOMOGENEOUS THERMAL TRANSPORT IN RTP

There are strong indications that the radial electron thermal diffusiv ity is not a smooth function of the radius, but shows strong local var iations. To investigate this irregularity systematically, a number of dedicated experiments have been carried out in the RTP tokamak: intens e central heating, pellet ablation experiments, perturbative transport studies and steady state off-axis heating. The crucial diagnostic in these experiments is the Thomson scattering system, with a radial reso lution of 1% of the minor radius. In sawtoothing plasmas a transport b arrier has been identified near the sawtooth inversion radius. With st eady state off-axis heating, hollow current density profiles have been sustained, and the region of reversed magnetic shear is characterized by very low thermal transport.