Nonlinear interactions of ion acoustic waves explored using fast imaging decompositions

Abstract

Fast camera imaging is used to study ion acoustic waves propagating azimuthally in a magnetized plasma column. The high speed image sequences are analyzed using Proper Orthogonal Decomposition and 2D Fourier Transform, allowing to evaluate the assets and differences of both decomposition techniques. The spatio-temporal features of the waves are extracted from the high speed images, and highlight energy exchanges between modes. Growth rates of the modes are extracted from the reconstructed temporal evolution of the modes, revealing the influence of ion-neutral collisions as pressure increases. Finally, the nonlinear interactions between modes are extracted using bicoherence computations, and show the importance of interactions between modes with azimuthal wave numbers $m$, $m-1$ and $-1$, with $m$ an integer.