Turbulent plasmas appear naturally in the magnetospheres of magnetized planets like Jupiter and Earth and, also, in the laboratory where plasma is confined by strong electromagnets. To better understand and predict the behavior of these turbulent events, it’s necessary to first understand the nonlinear structures in magnetized plasmas which directly affect the transport and confinement of those plasmas. One such nonlinearly excited structure is a streamer. Streamers are produced by the coupling between turbulence and mediating modes.

A study published in Physics of Plasmas shows that both the mediator and streamer have higher harmonic components than noted in previous studies, produced by self-coupling. The researchers report that by subtracting the mediator’s waveform from the signal, they obtain the streamer’s envelope. As with solitary waves, the height increases inversely with the width squared for both the streamer and the mediator. The height and localization of the streamer and mediator both increase with the magnetic field strength.

The researchers used a magnetized linear cylindrical plasma with a cooled vacuum vessel that had built-in probes allowing them to measure the ion saturation current and floating potential of the plasma turbulence. They applied conditional averaging to the resulting signal as a method to extract the nonlinear characteristics of the streamer structure. Conditional averaging had the advantage of being able to select harmonic components in addition to the fundamental one, confirming that the mediator and the streamer have higher harmonic components.

Source: “Extraction of nonlinear waveform in turbulent plasma,” by F. Kin, K. Itoh, A. Fujisawa, Y. Kosuga, M. Sasaki, T. Yamada, S. Inagaki, S.-I. Itoh, T. Kobayashi, Y. Nagashima, N. Kasuya, H. Arakawa, K. Yamasaki, and K. Hasamada, Physics of Plasmas (2018). The article can be accessed at https://doi.org/10.1063/1.5027124.