This paper presents the latest results on confinement studies in the TJ-II stellarator. The inherently strong plasma–wall interaction of TJ-II has been successfully reduced after lithium coating by vacuum evaporation. Besides H retention and low Z, Li was chosen because there exists a reactor-oriented interest in this element, thus giving special relevance to the investigation of its properties. The Li-coating has led to important changes in plasma performance. Particularly, the effective density limit in NBI plasmas has been extended reaching central values of 8 × 1019 m−3 and T e ≈ 250–300 eV, with peaked density, rather flat T e profiles and higher ion temperatures. Due to the achieved density control, a second type of transition has been added to the low density ones previously observed in ECRH plasmas: higher density transitions characterized by the fall in Hα emission, the onset of steep density gradient and the reduction in the turbulence; which are characteristics of transition to the H mode. Confinement studies in ECH plasmas indicate that lowest order magnetic resonances, even in a low shear environment, locally reduce the effective electron heat diffusivities, while Alfven eigenmodes destabilized in NBI plasmas can influence fast ion confinement.

Interpretative transport has been used to revisit the global scalings of TJ-II plasmas with electron cyclotron resonance heating (ECH), from a local perspective. Density, rotational transform and ECH power scans were analysed based mainly upon Thomson scattering data (electron density and temperature) in steady state discharges. The thermal diffusivity χe is estimated assuming pure diffusion and negligible convective heat fluxes in a set of 201 discharges. The density scan indicates that for normalized minor radii ρ ≲ 0.4 there is no significant change of χe with density in the range studied , while in the density gradient region, χe decreases with line density. In the rotational transform scan, there is an overall tendency of χe to decrease with increasing rotational transform, but we also find that the presence of a low order rational value of the rotational transform is accompanied by a local lowering of χe. Finally, in the ECH power scan, χe is found to have an overall increment in 0.2 < ρ < 0.6 when Q ECH increases from 200 to 400 kW, although it is less significant in the density gradient region.

Low order rational numbers of the rotational transform (magnetic resonances, in short) have been considered deleterious for magnetic confinement in toroidal devices. The design and operation of stellarators has been based on the notion that either the resonances should be relegated to small volumes with high magnetic shear, or they had to be simply avoided. Low density, low collisionality discharges of the low magnetic shear TJ-II device show that these resonances can benefit electron heat confinement. Only in conditions of very low shear over an extended radial portion of the plasma, do the resonances have a detrimental effect on confinement. The experiments consist of Electron Cyclotron Heated (ECH) plasmas where a moderated Ohmic current is induced. A distinctive feature of the TJ-II Heliac is its high rotational transform.

This work surveys the main results concerning the effects of the rotational transform, its low order rational values and its shear on the confining properties of low shear devices. It is meant to promote further studies aimed at clarifying their role in future, reactor grade, devices. 1-D transport studies are encouraged as the effects of rotational transform on confinement appear to be of local nature. Low order rational values of the rotational transform are associated with both degraded and improved confinement, being the magnetic shear a plausible cause for the difference. Very small shear values are enough to avoid deletereous effects of the low order rationals in high rotational transform discharges, but further experiments are needed to elucidate whether there is a threshold shear that depends on the rotational transform itself.