J. Bleuel

Self-similarity of the plasma edge fluctuations

The rescaled range analysis techniques are used to investigate long-range dependence in plasma edge fluctuations [Mandelbrot and Wallis, Water Resources Res. 4, 909 (1969)]. This technology has been applied to data from several confinement devices such as tokamaks, stellarators, and reversed-field pinch. The results reveal the self-similar character of the electrostatic fluctuations at the plasma edge with self-similarity parameters ranging from 0.62 to 0.72. These results show that the tail of the autocorrelation function decays as a power law for time lags longer than the decorrelation time and as long as times of the order of the confinement time. In cold plasma devices (Te<1 eV at the core), there is no evidence of algebraic tails in the autocorrelation function. Some other characteristic features of the autocorrelation function and power spectrum have been investigated. All of these features are consistent with plasma transport as characterized by self-organized criticality.

Statistical characterization of fluctuation wave forms in the boundary region of fusion and nonfusion plasmas

The statistical properties of plasma fluctuations have been investigated in the plasma boundary region of fusion (tokamaks and stellarators) and nonfusion plasmas. Fluctuations in ion saturation current and floating potential have a near-Gaussian character in the proximity of the velocity shear layer (rsh). However, fluctuations deviate from a Gaussian distribution when moving inside of the plasma edge (r rsh). Furthermore, fluctuations show sporadic pulses that are asymmetric in time. The present analysis shows a coupling of those pulses and the averaged flow in the shear layer region.

Experimental Evidence of Long-Range Correlations and Self-Similarity in Plasma Fluctuations

To better understand long time transport dynamics, techniques to investigate long-range dependences in plasma fluctuations have been applied to data from several confinement devices including tokamaks, stellarators, and reversed field pinch. The results reveal the self-similar character of the edge plasma fluctuations. This implies that the tail of the autocorrelation function decays as a power law and suggests that there is a superdiffusive component of the anomalous transport. Rescaled fluctuation and turbulent flux spectra from different devices also show a strong similarity. For a range of parameters corresponding to the tokamak ohmic regime and equivalent power for other devices, the spectral decay index may show a universal character.

Intermittency of plasma edge fluctuation data: Multifractal analysis

Plasma edge fluctuations show a degree of intermittency similar to fluid turbulence. Using fluctuation measurements obtained with Langmuir probe data from two confinement devices, it is shown that plasma fluctuations have a multifractal character over the fluctuation range of scales with intermittency levels comparable to the levels measured in neutral fluid turbulence. In the mesoscale range, that is, for time scales between 10 times the turbulence decorrelation time and plasma confinement time, plasma fluctuations have a structure closer to a monofractal with very low intermittency.

Parallel correlation measurements in the scrape-off layer of the Joint European Torus

In the scrape-off layer of the Joint European Torus [P.-H. Rebut, R. J. Bickerton, and B. E. Keen, Nucl. Fusion 25, 1011 (1985)], Langmuir probes were used to measure the correlation of ion saturation current fluctuations parallel to the magnetic-field B. It was possible to realize three different probe tip separations (connection lengths) ∥B under almost identical plasma conditions and magnetic configuration. Among these, a probe tip separation of 66 m ∥B represents the largest value ever realized for this type of experiment. It was found that the correlation is reduced from ⩾80% at 0.75 m probe tip separation ∥B in the divertor to ≈40% at 23 m, but remains at ≈40% even for the 66 m separation. An upper limit for the ratio between the average wave numbers parallel and perpendicular to the magnetic field |k∥/k⊥|<0.0022 is found, but the results are in good agreement with a parallel wave number k∥=0.

Characterization of the frequency ranges of the plasma edge fluctuation spectra

Frequency spectra of fluctuations for the ion saturation current, floating potential, and turbulent transport measured in the plasma edge of plasma confinement experiments (tokamaks and stellarators) have been analyzed to identify the frequency ranges characterized by a power dependence. Three main regions can be identified. For the intermediate frequency region, the decay of the spectra is close to 1/f, as is expected in self-organized criticality systems. This region is particularly important for the role that it plays in plasma transport and the self-similarity of the fluctuations and fluxes. The effect of plasma rotation on the decay indices has also been studied.

Turbulent transport studies in the JET edge plasmas in limiter configuration

Electrostatic turbulence has been investigated in the plasma boundary region of the JET tokamak. The normalized fluctuation levels are in the range of 10-20% at the plasma radius where the poloidal velocity of the fluctuation reverses sign. In limiter plasmas, the E × B turbulent transport can account for a significant part of the total particle and energy losses. Three regions in the power spectra of the fluctuations with different frequency dependences have been found. The spectral region with power decay close to 1/f has been identified in the frequency range where the turbulent flux, which comes in bursts, is dominant. This result could be an indication of transport close to instability thresholds.