Abstract
The understanding of the nature, driving mechanisms, and dynamics of turbulence activity in tokamak plasmas is a major challenge in fusion research. In the ISTTOK tokamak, edge turbulence activity has been extensively studied using both limiter and electrode biasing to induce local sheared electric fields, thus affecting turbulence and consequently particle confinement. Due to the fast time scales involved plasmas (pulse length of 30ms and particle confinement of the order of 0.3ms) and the turbulent nature of ISTTOK plasmas, not only turbulence but also magnetohydrodynamic (MHD) activity occurs at rapid bursts. Therefore, resolving the modes both spatially and during time (in particular, regarding the energy/frequency spectrum) requires advanced time-frequency analysis tools, capable of tracking short lasting (0.1ms) MHD modes. In this article, we explore the capabilities of the empirical mode decomposition method as a tool for turbulence and MHD instability analysis. The time evolution of the energy content associated with fluctuating potential, edge plasma density (obtained from Langmuir probes), and perturbed magnetic fields (Mirnov coil signals) is obtained, and a comparison is made with other advanced time-frequency signal estimators.
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