Structures, wavelet and intermittency in tokamak edge turbulence

Abstract

The wavelet bicoherence technique is used to examine floating potential fluctuations in ADITYA tokamak edge plasma, which have shown vortex-like coherent structures on statistical basis [Proceedings of the 15th International Conference on Plasma Physics and Controlled Nuclear Fusion, Seville (International Atomic Energy Agency, Vienna, 1995), Vol. 1, p. 583]. On a single discharge basis, the data exhibit intermittent episodes of quadratic interaction in which low and high frequency components are coupled. The broadband nonlinear phase coupling is used as a diagnostic signature of localized coherent structure. It is observed that the wavelet bicoherence is high in the presence of sparse structures and small when the structures are closely packed. The turbulent edge plasma has regions of packed Gaussian structures and sparse non-Gaussian structures.

Observation of vortex-like coherent structures in the edge plasma of the ADITYA tokamak

“…In addition, we can observe non-Gaussian characteristics of the fluctuations in individual discharges. In a recent analysis of discharges included in the present paper, 4 we have shown that the turbulent fluctuations exhibit dominantly non-Gaussian statistics for frequency components higher than 20 kHz which corresponds to scale lengths smaller than 15 cm.…”

“…Such episodes of phase coupling are punctuated by quiescent periods of random fluctuation. 4 It should be mentioned that a recent attempt to observe conditional structures in the TEXT-Upgrade ͑Texas Experimental Tokamak͒ tokamak 25 has given a negative result. They use a pair of probes instead of a poloidal array and the sampling condition is only on the amplitude of the potential fluctuations.…”

“…These structures have to be obtained by using other complementary techniques. [3][4][5][6][7][8][9][10][11]14 From the wavelet bicoherence analysis of the data presented in this paper, we have shown that the fluctuation time series has ''episodes'' of packed Gaussian structures and isolated non-Gaussian structures. 4 The localized structures embedded in random fluctuations can also be recovered by using conditional averaging technique.…”

“…[1][2][3][4] Our measurements indicate that ͑a͒ the edge/ SOL turbulence often exhibits non-Gaussian statistics which is indicative of intermittency; 1 ͑b͒ the instantaneous particle flux ⌫ϰñ e Ẽ exhibits an episodic or, ''bursty'' character 2 and ͑c͒ the turbulent fluctuations ͑as examined by the wavelet bicoherence analysis͒ exhibit intermittent episodes of quadratic interaction in which large and small scales are coupled. 3,4 All these measurements point towards the existence of intermittent coherent structures in the tokamak edge/ SOL turbulence. It is therefore of considerable interest to make some direct measurements of coherent structures in the edge/SOL turbulence of the tokamak ADITYA.…”

Convective transport by intermittent blob-filaments: Comparison of theory and experiment

“…Wavelet analysis has been used to evaluate the blob size, lifetime, local intermittency measure, packing fraction, linear density of structures, and bicoherence in various devices. 22,126,144,150,152,164,171,177,178,180,189,191,218 Statistical analyses have searched for self-organized criticality or 'avalanche'-like behavior using techniques such as the waiting time distribution, self-similarity, and Hurst exponents. 125,151,173,187,226,237 The fractal structure was investigated in HYBTOK-II, T-10, LHD, and NAGDIS-II, 178,238,239 and the singular value (or proper or biorthogonal) decomposition technique was used to identify blob structures.…”