An integrated data analysis system based on Bayesian inference has been developed for the TJ-II stellarator. It reconstructs the electron density profile at a single time point, using data from interferometry, reflectometry, Thomson Scattering, and the Helium beam, while providing a detailed error analysis. In this work, we present a novel analysis of the ambiguity inherent in profile reconstruction from reflectometry, and show how the Integrated Data Analysis approach elegantly resolves it. Several examples of the application of the technique are provided, in both low-density discharges with and without electrode biasing, and in high-density discharges with an (L-H) confinement transition.

The probability density function (PDF) of turbulent transport has been investigated in the plasma edge region of tokamak (JET) and stellarator (TJ-II) fusion devices. PDFs can be re-scaled using a functional form, PDF( E×B ) = L −1 g( E×B /L), where L is directly related with the level of fluctuations in the turbulent flux. This kind of re-scaling holds at different timescales in which the functional form of the PDF changes. The empirical similarity in the PDF of turbulent transport in the edge region in both the JET tokamak and the TJ-II stellarator supports the view that turbulent transport displays universality in fusion plasmas. These results emphasize the importance of the statistical description of transport processes in fusion plasmas as an alternative approach to the traditional way to characterize transport based on the computation of effective transport coefficients (i.e. diffusion coefficients) and on average quantities (i.e. average correlation lengths).

In this work we present the experimental setup, combining edge, and x-ray diagnostics, used to localize the currents generated by fast electrons confined near the plasma periphery in TJ-II plasmas. The method is similar to that used for the experimental determination of the magnetic field mapping in stellarators, just intercepting the trajectories of test electrons. The fact that stellarators can keep magnetic structures “frozen” in vacuum during the field plateau enables the detection of local currents near the plasma periphery in the TJ-II stellarator.

Experimental evidence for the controlled onset of fluctuations by varying the magnetic well in the plasma edge region of the TJ-II stellarator (major radius R=1.5 m, minor radius a<0.22 m, magnetic field B=1 T) has been obtained. The level of fluctuation, the degree of intermittency and the radial correlation of relevant quantities increase as the magnetic well is reduced. TJ-II possesses a high degree of flexibility, making it an ideal device for investigating the characteristics of transport close to instability thresholds.

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