Status of TJ-II Project

The neoclassical transport properties of TJ-II stellarator [C. Alejaldre et al., Fusion Technol. 13, 521 (1988)] are studied with the monoenergetic Monte Carlo technique. A compromise between the number of modes and particles and the required computing time to obtain reliable estimates, from the computational point of view, of the monoenergetic diffusion coefficients is shown to be of one thousand particles and one hundred harmonics, because of the rich magnetic-field structure of TJ-II. Although, these requirements are probably too demanding in making the transport estimations. The data base containing the normalized monoenergetic diffusion coefficient for several radial positions, radial electric fields and collisionalities have been fitted using a neural network. This fit reduces the number of points necessary in the data base and allows a smooth interpolation and extrapolation to perform the convolutions of the monoenergetic coefficients with the Maxwellian. For two different typical TJ-II discharges the ambipolar radial electric field, and the neoclassical particle and heat fluxes are presented both showing rather large positive radial electric fields at the plasma core and small negative fields at the edge. The neoclassical particle and energy confinement time are in surprisingly good agreement with the experimental energy balance analysis and the international stellarator scaling. Although no satisfactory explanation is available yet the large neoclassical diffusion caused by the complex ripple structure of TJ-II magnetic field may be an important ingredient.

Section: Tj-ii Stellaratormentioning

confidence: 99%

Section: Monoenergetic Coefficients and Monte Carlomentioning

confidence: 99%

Monte Carlo estimation of neoclassical transport for the TJ-II stellarator

Tribaldos¹

2001

“…Experiments in the TJ-II stellarator 2 have shown that the development of the plasma edge sheared flow layer requires a minimal value of the plasma density or density gradient. 3,4 In a series of experiments in this device, it has been shown that the density level required is reproducible and its value may depend on the edge rotational transform.…”

Section: Introductionmentioning

confidence: 99%

Critical transition for the edge shear layer formation: Comparison of model and experiment

Carreras

Garcia

Pedrosa³

et al. 2006

The experimental results for the emergence of the plasma edge shear flow layer in TJ-II [C. Alehaldre et al.Fusion Technol. 17, 131 (1990)] can be explained using a simple model for a second-order transition based on the sheared flow amplification by Reynolds stress and turbulence suppression by shearing. In the dynamics of the model, the resistive interchange instability is used. This model gives power dependence on density gradients before and after the transition, consistent with experiment.

“…The emergence of the plasma edge shear flow layer as the density increases in the TJ-II stellarator [8] is shown [9,10] to have the characteristic properties of a second order phase transition. It is consistent [11] with a simple transition model that couples shear flow amplification by turbulence [12,13] with turbulence suppression by sheared flows [14].…”

Section: Introductionmentioning

confidence: 99%

Dynamics of a one-dimensional model for the emergence of the plasma edge shear flow layer with momentum-conserving Reynolds stress

Calvo¹,

Carreras²

2007

A one-dimensional version of the second-order transition model based on the sheared flow amplification by Reynolds stress and turbulence supression by shearing is presented. The model discussed in this paper includes a form of the Reynolds stress which explicitly conserves momentum. A linear stability analysis of the critical point is performed. Then, it is shown that the dynamics of weakly unstable states is determined by a reduced equation for the shear flow. In the case in which the flow damping term is diffusive, the stationary solutions are those of the real Ginzburg-Landau equation.