Effect of tearing modes on temperature and density profiles and on the perpendicular transport in the W VII-A stellarator

In the ohmically heated W VII-A stellarator, the behaviour of which is similar to that of a medium sized tokamak, the additional shearless external rotational transform t0 (Δι0/ι0 < 1%, ι = 1/q) improves the stability properties and allows stationary and reproducible helium discharges with large amplitude tearing modes to be produced for 0.10 ≤ ι0 ≤ 0.15. Discharges of this type are used to study, by means of Thomson scattering, the modification of electron temperature and density profiles by large amplitude tearing modes. The two-dimensional shapes of the profiles found experimentally can be attributed to the magnetic islands predicted by tearing mode theory. These shapes are further used to define an effective, locally enhanced radial transport coefficient κ⊥,e in a one-dimensional heat transport code. In this way, the measured temperature profiles can be reproduced quite well and the energy confinement time of discharges with tearing mode activity can be predicted quantitatively. The transport model is used to investigate the explicit dependence of κ⊥,e on the plasma current and to study the importance of plasma current driven instabilities for the energy confinement in the W VII-A stellarator as well as in tokamaks.