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Description
In this work, we use the new Spectral Gas Puff Imaging diagnostic of the TJ-II stellarator to study edge turbulence. This diagnostic allows measuring electron density and temperature fluctuations simultaneously in a two-dimensional region in the plasma edge.
We compare the properties of turbulence in two heating phases, namely the low-density Electron Cyclotron Resonance Heating phase and the high-density Neutral Beam Injection phase. We find that the plasma edge region, 0.7 < ρ < 1, can be divided into at least two zones. In the interior region, ρ < 0.8, turbulent structures are small (∼0.5 cm), while the electron density, ne, and the electron temperature, Te, are weakly correlated and in phase opposition. In the edge region, 0.8 < ρ < 1, turbulent structures are larger (∼1 cm in ECRH and ∼3 cm in NBI), while ne and Te are strongly correlated and in phase. The structures are elongated and not aligned with flux surfaces.
Rescaled range analysis shows that electron density and temperature fluctuations are persistent, indicating the presence of some turbulence regulation mechanism. Notably, electron temperature fluctuations are more persistent than electron density fluctuations. Simultaneously, causal analysis, based on the transfer entropy, reveals a net causal information flow from electron temperature to electron density fluctuations. These two results suggest that a turbulent mechanism is regulating edge electron temperature fluctuations, while electron density fluctuations follow. Also, Entropy-Complexity analysis shows that edge fluctuations exhibit nontrivial chaotic behaviour, especially in the high-density phase.