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

In most helical systems, electron-internal transport barriers (e-ITBs) 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, quasi-coherent modes are frequently observed before and/or after the e-ITB phenomenon at the radial location of the transport barrier foot. Such modes vanish as the barrier is fully developed.