A model for electron transport barriers in tokamaks, tested against experimental data from RTP

Recent experiments in the RTP tokamak have shown that the electron temperature, T-e, profile in EC heated L mode discharges can have only a small number of distinct profile shapes, depending on the location of the additional heating. The sharp transitions between the profile shapes are associated with the loss or gain of a low rational q surface. These observations suggest that the electron thermal diffusivity chi(e) is a direct function of the safety factor q, with alternating layers of low and high chi(e). It is shown that such a model not only excellently describes the distinct profile shapes but also predicts other, observed but so far unexplained, phenomena, such as sharp off-axis maxima of T-e observed with off-axis heating at specific radii.