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The drift-kinetic equation is solved for low density TJ-II plasmas employing slowly varying, time-dependent profiles. This allows to simulate density ramp-up experiments and to describe from first principles the formation and physics of the radial electric field shear, which is associated to the transition from electron to ion root. We show that the range of frequencies of plasma potential fluctuations in which zonal flows are experimentally observed is neoclassically undamped in a neighbourhood of the transition. This makes the electron root regime of stellarators, close to the transition to ion root, a propitious regime for the study of zonal-flow evolution. We present simulations of collisionless relaxation of zonal flows, in the sense of the Rosenbluth and Hinton test, that show an oscillatory behaviour in qualitative agreement with the experiment close to the transition. [ABSTRACT FROM AUTHOR]

Poloidal and toroidal velocities of fully ionized carbon are measured by means of charge exchange recombination spectroscopy in the TJ-II stellarator. We present a detailed treatment of the 3D geometry and show that flow measurements, performed at different locations of the same flux surface, are compatible with flow incompressibility for the low density plasmas under study (line averaged electron densities ne ≤ 1019 m-3). Furthermore, comparison with neoclassical calculations shows quantitative agreement with the measured radial electric field and ion parallel mass flow in the absence of an external momentum input. [ABSTRACT FROM AUTHOR]

A description and transport analysis (energy and particles) of neutral beam heated plasmas is shown for two almost identical magnetic configurations except for a ≈1% difference in their rotational transform, .... Due to the low magnetic shear of TJ-II magnetic fields, such difference takes the low order rational ... = 8/5 to the plasma edge in one of them, while in the other the 8/5 value is near the edge but inside the plasma. A transport transition happens at lower density and plasma energy for the configuration that includes the ... = 8/5, which exhibits better particle confinement already before the transition. [ABSTRACT FROM AUTHOR]

The dynamics of fluctuating electric field structures in the edge of the TJ-II stellarator, which display zonal-flow-like traits, is studied. These structures have been shown to be global and affect particle transport dynamically (Alonso J et al 2012 Nucl. Fusion 52 063010). In this paper we discuss the possible drive (Reynolds stress) and damping (neoclassical viscosity, geodesic transfer) mechanisms for the associated E × B velocity. We show that (a) while the observed turbulence-driven forces can provide the necessary perpendicular acceleration, a causal relation could not be firmly established, possibly because of the locality of the Reynolds stress measurements, (b) the calculated neoclassical viscosity and damping times are comparable to the observed zonal-flow relaxation times and (c) although an accompanying density modulation is observed to be associated with the zonal flow, it is not consistent with the excitation of pressure sidebands, as those present in geodesic acoustic oscillations, caused by the compression of the E × B flow field. [ABSTRACT FROM AUTHOR]

The results of global linear gyrokinetic simulations of residual flows carried out with the code EUTERPE in the TJ-II three-dimensional geometry are reported. The linear response of the plasma to potential perturbations homogeneous in a magnetic surface shows several oscillation frequencies: a Geodesic-acoustic-mode-like frequency, in qualitative agreement with the formula given by Sugama and Watanabe (2006 Plasma Phys. 72 825), and a much lower frequency oscillation in agreement with the predictions of Mishchenko et al (2008 Phys. Plasmas 15 072309) and Helander et al (2011 Plasma Phys. Control. Fusion 53 054006) for stellarators. The dependence of both oscillations on ion and electron temperatures and the magnetic configuration is studied. The low-frequency oscillations are in the frequency range supporting the long-range correlations between potential signals experimentally observed in TJ-II. [ABSTRACT FROM AUTHOR]

A new algorithm based on metaheuristics has been developed to perform stellarator optimization. This algorithm, which is inspired by the behaviour of bees and is called distributed asynchronous bees, has been used for the optimization under three criteria: minimization of B × grad(B) drift, Mercier and ballooning stability. This algorithm is tested by partially optimizing TJ-II and, afterwards, a three-period optimized configuration is found by performing a full optimization that starts from a three-period heliac. [ABSTRACT FROM AUTHOR]

The drift kinetic equation is solved for low density TJ-II plasmas employing slowly varying, time-dependent profiles. This allows us to simulate density ramp-up experiments and describe from first principles the formation and physics of the radial electric field shear layer. The main features of the transition are perfectly captured by the calculation, and good quantitative agreement is also found. The results presented here, that should be valid for other nonquasisymmetric stellarators, provide a funda-mental explanation for a wealth of experimental observations connected to the shear layer emergence in TJ-II. The key quantity is the neoclassical viscosity, which is shown to go smoothly to zero when the critical density is approached from below. This makes it possible for turbulence-related phenomena, and particularly zonal flows, to arise in the neighborhood of the transition. [ABSTRACT FROM AUTHOR]

The validity of the traditional local diffusive approach and of the use of monoenergetic calculations has been studied for the stellarator TJ-II [Alejaldre et al., Fusion Technol. 17, 131 (1990)]: it is shown to be doubtful, under some circumstances, even in a purely collisional description of transport. The diffusion in physical space starting from Dirac-delta-like initial conditions has been studied using the code Integrator of Stochastic Differential Equations for Plasmas by Castejón et al. [Plasma Phys. Controlled Fusion 49, 753 (2007)]. Particles may experience large radial excursions from their original magnetic surfaces in a single collisional time. The contribution of these particles to the flux may make it nondiffusive; non-Gaussian density distributions, characterized by long tails, are observed. In the velocity space, there are important variations in the average particle kinetic energy after one collision time. We discuss the effect of this fact over the calculation of monoenergetic transport coefficients and their convolution. A simple analysis based on Hurst exponents has shown nevertheless that the description of transport by means of a pinch term and an effective transport coefficient is more correct than expected. [ABSTRACT FROM AUTHOR]

The response of transport in low density (line average ) plasmas to electron cyclotron resonance heating (ECH) power, 100 kW ≲ QECH ≲ 400 kW, is documented for the TJ-II Heliac-type stellarator. Radially resolved electron heat balance shows no significant differences between boron or lithium coating of the vacuum chamber walls. The main trends in electron heat transport are found to be similar to other stellarator/heliotron devices and are compatible with neoclassical calculations in the bulk of the plasma. According to our calculations the heat fluxes are anomalous near the edge, ρ ≳ 0.8 where ρ is the normalized minor radius, but the uncertainties there are large. Particle transport in the density gradient region, ρ ≈ 0.8, has little sensitivity to the variation of heating power and is compatible with neoclassical predictions. Neoclassical transport of particles and electron heat is found to be dominant in the gradient regions of typical ECH plasmas of the TJ-II Heliac. [ABSTRACT FROM AUTHOR]