TJ-II:Turbulence: Difference between revisions

Non-linear interactions can be detected by means of higher-order spectra (e.g. quadratic interactions can be detected through the bi-spectrum). With Fourier analysis, however, in order to achieve statistically significant values for the bi-spectrum, very long time series are necessary. This fact has mostly precluded its use in fields like plasma turbulence, since long steady-state data series are not generally available. In our work, for the first time, the bicoherence was calculated using wavelet transforms, thus making the detection of non-linear interactions with time resolution possible.

<ref>[http://link.aps.org/doi/10.1103/PhysRevLett.74.395 B.Ph. van Milligen et al, Nonlinear phenomena and intermittency in plasma turbulence, Phys. Rev. Lett. '''74''', 3 (1995) 395]</ref>

<ref>B.Ph. van Milligen et al, Statistically robust linear and non-linear wavelet analysis applied to plasma edge turbulence, Rev. Sci. Instrum. '''68''' (1997) 967</ref>

<ref>[http://link.aps.org/doi/10.1103/PhysRevLett.84.4842 P.H. Diamond et al, In search of the elusive zonal flow using cross-bicoherence analysis, Phys. Rev. Lett. '''84''', 12 (2000) 4842]</ref>

<ref>[http://link.aps.org/doi/10.1103/PhysRevLett.80.4438 B.A. Carreras et al, Long-range time correlations in plasma edge turbulence, Phys. Rev. Lett. '''80''', (1998) 4438]</ref>

<ref>[http://link.aps.org/doi/10.1103/PhysRevLett.83.3653 B.A. Carreras et al, Self-similarity properties of the probability distribution function of turbulence-induced particle fluxes at the plasma edge, Phys. Rev. Lett. '''83''' (1999) 3653]</ref>

<ref>C. Hidalgo et al, Empirical similarity in the probability density function of turbulent transport in the edge plasma region in fusion plasmas, Plasma Phys. Control. Fusion '''44''' (2002) 1557</ref>

<ref>B.Ph. Van Milligen et al, Additional evidence for the universality of turbulent fluctuations and fluxes in the scrape-off layer region of fusion plasmas, Physics of Plasmas '''12''' (2005) 052507</ref>

The repeated occurrence of values of H differing significantly from the value corresponding to random noise (H = 0.5) in all machines points to a universal aspect of the underlying turbulence. Further, the degree of self-similarity detected implies the existence of long-range correlations (with respect to the correlation time).

In this framework, an important technique is the quiet-time analysis.

<ref>[http://link.aps.org/doi/10.1103/PhysRevE.66.036124 R. Sánchez et al, Quiet-time statistics: A tool to probe the dynamics of self-organized-criticality systems from within the strong overlapping regime, Phys. Rev. E, '''66''' (2002) 036124]</ref>

<ref>[http://link.aps.org/doi/10.1103/PhysRevLett.90.185005 R. Sánchez et al, Quiet-time statistics of electrostatic turbulent fluxes from the JET tokamak and the W7-AS and TJ-II stellarators, Phys. Rev. Lett. '''90''', 185005 (2003)]</ref>

An important effort has also been made to identify and classify turbulence,

<ref>I. García-Cortés et al, Turbulent transport studies in the JET edge plasmas in limiter configuration, Plasma Phys. Control. Fusion '''42''' (2000) 389</ref>

<ref>C. Hidalgo et al, Intermittency and structures in edge plasma turbulence, Comptes Rendus Physique '''7''', 6 (2006) 679</ref>  

to analyse its spectra,

<ref>[http://link.aps.org/doi/10.1103/PhysRevLett.82.3621 M. A. Pedrosa et al, Empirical similarity of frequency spectra of the edge plasma fluctuations in toroidal magnetic confinement systems, Phys. Rev. Lett. '''82''' (1999) 3621]</ref>

<ref>M. A. Pedrosa et al, Studies of spectra of the edge plasma fluctuations in toroidal magnetic confinement systems, J. Plasma Fusion. Res. SERIES, '''2''' (1999) 77</ref>

to determine its relation with local plasma parameters (rational surfaces, gradients, electric fields),

<ref>M.A. Pedrosa et al, Role of rational surfaces on fluctuations and transport in the plasma edge of the TJ-II stellarator, Czechoslovak Journal of Physics, '''50''', 12 (2000) 1463</ref>

<ref>M.A. Pedrosa et al, Edge turbulence during limiter biasing experiments in the TJ-II stellarator, Czechoslovak Journal of Physics, '''53''' (2003) 877</ref>

and to visualise turbulent structures.