First plasmas have been successfully achieved in the TJ-II stellarator using electron cyclotron resonance heating (f = 53.2 GHz, P ECRH = 250 kW). Initial experiments have explored the TJ-II flexibility in a wide range of plasma volumes, different rotational transform and magnetic well values. In this paper, the main results of this campaign are presented and, in particular, the influence of plasma wall interaction phenomena on TJ-II operation is discussed briefly.

This paper presents an overview of experimental results and progress made in investigating the link between magnetic topology, electric fields and transport in the TJ-II stellarator. The smooth change from positive to negative electric field observed in the core region as the density is raised is correlated with global and local transport data. A statistical description of transport is emerging as a new way to describe the coupling between profiles, plasma flows and turbulence. TJ-II experiments show that the location of rational surfaces inside the plasma can, in some circumstances, provide a trigger for the development of core transitions, providing a critical test for the various models that have been proposed to explain the appearance of transport barriers in relation to magnetic topology. In the plasma core, perpendicular rotation is strongly coupled to plasma density, showing a reversal consistent with neoclassical expectations. In contrast, spontaneous sheared flows in the plasma edge appear to be coupled strongly to plasma turbulence, consistent with the expectation for turbulent driven flows. The local injection of hydrocarbons through a mobile limiter and the erosion produced by plasmas with well-known edge parameters opens the possibility of performing carbon transport studies, relevant for understanding co-deposit formation in fusion devices.

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TJ-II is a four period, low magnetic shear stellarator (R = 1.5 m, a < 0.22 m, B 0 ≤ 1.2 T) which was designed to have a high degree of magnetic configuration flexibility. In the last experimental campaign, coupling of the full ECH power (P ECRH ≤ 600 kW) to the plasma has been possible using two ECRH transmission lines which have different power densities. Both helium and hydrogen fuelled plasmas have been investigated. This paper reviews the latest physics results in particle control, configuration effects, and transport and fluctuation studies.