On the radial scale of fluctuations in the TJ-II stellarator

The TJ-II stellarator was designed to have a high degree of magnetic configuration flexibility. The rotational transform can be varied between 0.9 and 2.5, and the magnetic well may be changed from -1% to 6%. This flexibility is a powerful tool for physics studies in fusion plasmas. The magnetic-well scan experiments in TJ-II support the view that plasma turbulence displays universality. These results emphasize the importance of the statistical description of transport processes in fusion plasmas as an alternative approach to the study of transport based on the computation of effective transport coefficients. Comparative studies in TJ-II stellarator and other devices show that fluctuations and E × B sheared flows organize themselves to be close to marginal stability. This property should be considered as a critical test for L-H transition models. The magnetic-configuration scan experiments in TJ-II show the complex interplay between transport and radial electric fields in the proximity of rational surfaces.