Electron Internal Transport Barriers and Magnetic Topology in the Stellarator TJ-II

In most helical systems electron Internal Transport Barriers (e-ITB) are observed in Electron Cyclotron Heated (ECH) plasmas with high heating power density. In the stellarator TJ-II, e-ITBs are easily achievable by positioning a low order rational surface close to the plasma core, because this increases the density range in which the e-ITB can form. Experiments with different low order rationals show a dependence of the threshold density and barrier quality on the order of the rational (3/2, 4/2, 5/3,...). In addition, during the formation of e-ITB quasi-coherent modes are frequently observed in the plasma core region. The mode can exist before or after the e-ITB phenomenon at the radial location of the transport barrier foot but vanishes as the barrier is fully developed. I.