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TJ-II:Turbulence: Difference between revisions
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It is much more convenient to resort to the Rescaled-Range analysis technique and the determination of the Hurst exponent. We have shown that this type of analysis is far more robust with respect to random noise perturbations than the direct determination of the ACF or the Probability of Return. | It is much more convenient to resort to the Rescaled-Range analysis technique and the determination of the Hurst exponent. We have shown that this type of analysis is far more robust with respect to random noise perturbations than the direct determination of the ACF or the Probability of Return. | ||
The analysis of data from Langmuir probes taken at the plasma edge in a wide variety of fusion devices reveals the existence of self-similar behaviour or long-range correlations in all devices studied. The observed variation of the Hurst exponent in the plasma edge, 0.62 < H < 0.75, is small in spite of the variety of devices. On the other hand, the variation of H in the scrape-off layer is much larger. In Wendelstein VII-AS, a slight decrease in H at the sheared flow layer was observed, possibly corresponding to a local decorrelation effect. | The analysis of data from Langmuir probes taken at the plasma edge in a wide variety of fusion devices reveals the existence of self-similar behaviour or long-range correlations in all devices studied. The observed variation of the Hurst exponent in the plasma edge, 0.62 < H < 0.75, is small in spite of the variety of devices. | ||
<ref>[http://link.aip.org/link/?PHPAEN/5/3632/1 B.A. Carreras et al, Self-similarity of the plasma edge fluctuations, Phys. Plasmas '''5''', 10 (1998) 3632]</ref> | |||
On the other hand, the variation of H in the scrape-off layer is much larger. In Wendelstein VII-AS, a slight decrease in H at the sheared flow layer was observed, possibly corresponding to a local decorrelation effect. | |||
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). | 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). | ||
<ref>[http://link.aip.org/link/?PHPAEN/6/485/1 B.A. Carreras et al, Long-range time dependence in the cross-correlation function, Phys. Plasmas '''6''', 2 (1999) 485]</ref> | <ref>[http://link.aip.org/link/?PHPAEN/6/485/1 B.A. Carreras et al, Long-range time dependence in the cross-correlation function, Phys. Plasmas '''6''', 2 (1999) 485]</ref> | ||
<ref>[http://link.aip.org/link/?PHPAEN/6/1885/1 B.A. Carreras et al, Experimental evidence of long-range correlation and self-similarity in plasma fluctuations, Phys. Plasmas '''6''', 5 (1999) 1885]</ref> | <ref>[http://link.aip.org/link/?PHPAEN/6/1885/1 B.A. Carreras et al, Experimental evidence of long-range correlation and self-similarity in plasma fluctuations, Phys. Plasmas '''6''', 5 (1999) 1885]</ref> | ||