Overview of TJ-II Experiments

Full text: This paper presents an overview of experimental results on particle control using Li-coating, transport and plasma bifurcations in TJ-II. The results show a great improvement of plasma particle control after Li-coating, in comparison with the operation under Boron coated walls. The beneficial Li properties have a strong effect on this device allowing the access to improved confinement regimes. The effect of low order magnetic resonances have been also explored showing a local decreasing of heat diffusivity, while an increase of fast ion flux due to Alfven modes in NBI regime. Perturbative experiments of gas injection have shown a transition from peaked (central impurity accumulation) to a broader profiles (with lower-central Zeff profile). These profiles, tagged as 'bell' and dome', play a role for the density limit achieved under pure NBI heating. Laser blow-off has been used to inject trace boron impurities to study impurity transport. The parallel transport of boron ions has been measured with two filter-copes monitoring B-II line on opposite sides of the torus. The L-H transition appears during NBI injection and is characterised by the increase of diamagnetic energy, the decrease of Hα emission, the reduction of turbulence, and the development of steep density gradients. The turbulence reduction precedes the increase in the mean sheared flow, but is simultaneous with the increase in the low frequency oscillating sheared flow. This type of spontaneous transitions is added to the ones that happen at lower densities, which correspond to the shear flow development and can be also provoked by biasing. During both low and high plasma density bifurcations, the correlation length of the plasma potential becomes of the order of the machine size. The radial structure of plasma fluctuations has been investigated in the plasma boundary region of the TJ-II stellarator. Potential transport events are observed to be born in the proximity of the edge velocity shear layer and propagate outwards in the SOL, whereas they propagate inwards in the edge region, with radial velocities of 1 - 10 km/s. Electron Bernstein Waves heating experiments are carried using the OXB mode conversion. Those waves are also very efficient to drive non-inductive currents and hence can be used to tailor the rotational transform profile and to compensate bootstrap current. (author)