Investigation of nonlinear behaviour of low frequency drift modes in ECR-produced plasma of a linear device using wavelet and Hilbert-Huang transform

Abstract Investigation of the nonlinear behaviour of low-frequency drift modes in electron cyclotron resonance (ECR) plasma of the Ma gnetized P lasma L inear E xperimental (MaPLE) device has been conducted using continuous wavelet transform (CWT), Hilbert-Huang transformation (HHT), and largest Lyapunov exponent (LLE) techniques. The resulting CWT spectra indicated the energy transfer from the lower- to higher-frequency drift modes. The undulating nature of the contours in the CW spectra confirmed the shifting of the dominant frequency and the presence of nonlinearity in the system. Further, the CWT-based ridge plot showed that the degree of chaos and nonlinear mode coupling first decrease, then increase, and again decrease radially outward. The nonlinear interaction between various time scales has been further investigated extensively using the HHT. Wave decomposition during HHT involves empirical mode decomposition (EMD), which uses an adaptive posteriori -defined basis system and independent of linear approximations. The resulting HH spectra indicated the nonlinear interaction between different modes increases in the edge region. The LLE has been estimated to study the chaos present in the system globally. LLE verifies the order-chaos-order transition along with radial positions. The presence of E → × B → -shear flow has been identified as the primary driver of drift wave suppression and the route of nonlinear mode coupling and chaos generation in the signal.