Ion temperatures have been measured in the TJ-II stellarator using passive emission spectroscopy in parallel and perpendicular directions. A high-energy component was observed from the tails of the Hα line emission in both directions, suggesting a non-negligible population of suprathermal ions. The role of this fast ion energy component, which has been observed in both stellarator and tokamak plasmas when heated by electron cyclotron resonance heating in the additional heating of the plasma bulk is investigated by means of a simple model. The origin of such a fast component is discussed in terms of different mechanisms.

The role of a fast ion component on the heating of the plasma bulk

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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Overview of TJ-II experiments

Transitions to improved core electron heat confinement are triggered by low order rational magnetic surfaces in TJ-II electron cyclotron heated (ECH) plasmas. Experiments are performed changing the magnetic shear around the rational surface n = 3/m = 2 to study its influence on the transition; ECH power modulation is used to look at transport properties. The improvement in the electron heat confinement shows no obvious dependence on the magnetic shear. Transitions triggered by the rational surface n = 4/m = 2 show, in addition, an increase in the ion temperature synchronized with the increase in the electron temperature. Ion temperature changes had not been previously observed either in TJ-II or in any other helical device. SXR measurements demonstrate that, under certain circumstances, the rational surface positioned inside the plasma core region precedes and provides a trigger for the transition.

/pdf/transitions-to-improved-core-electron-heat-confinement-5ckgoenutz.pdf

Transitions to improved core electron heat confinement triggered by low order rational magnetic surfaces in the stellarator TJ-II

The ion collisional transport is estimated for the TJ-II device without the assumption of radially narrow particle trajectories, which is usually made in neoclassical transport studies. One million particles are followed in a realistic TJ-II magnetic configuration, taking into account collisions and the electric field. Global features of transport, not present in the customary neoclassical models, appear. The main results reached by considering the actual particle geometries are the monotonic increasing of heat and particle fluxes with minor radius, the non-diffusive character of transport, the appearance of asymmetries on the magnetic surfaces and the non-Maxwellian character of the distribution function.

Ion kinetic transport in the presence of collisions and electric field in TJ-II ECRH plasmas

Calculations of the monoenergetic radial diffusion coefficients are presented for several configurations of the TJ-II stellarator usually explored in operation. The neoclassical radial fluxes and the ambipolar electric field for the standard configuration are then studied for three different collisionality regimes, obtaining precise results in all cases.

/pdf/study-of-the-neoclassical-radial-electric-field-of-the-tj-ii-vqyhgoqwz1.pdf

Study of the neoclassical radial electric field of the TJ-II flexible heliac

Energy-resolved neutral particle fluxes are studied in the TJ-II stellarator by using measurements from a neutral particle analyzer. The average ion energy can be deduced up to positions outside th...

Sergei Yakovlevich Petrov

Papers

Energy-Resolved Neutral Particle Fluxes in TJ-II ECRH Plasmas