Anatomy of plasma turbulence

Abstract

Turbulence is a state of fluids and plasma where nonlinear interactions including cascades to finer scales take place to generate chaotic structure and dynamics1. However, turbulence could generate global structures2, such as dynamo magnetic field, zonal flows3, transport barriers, enhanced transport and quenching transport. Therefore, in turbulence, multiscale phenomena coevolve in space and time, and the character of plasma turbulence has been investigated in the laboratory4,5,6,7,8,9,10 as a modern and historical scientific mystery. Here, we report anatomical features of the plasma turbulence in the wavenumber–frequency domain by using nonlinear spectral analysis including the bi-spectrum11. First, the formation of the plasma turbulence can be regarded as a result of nonlinear interaction of a small number of irreducible parent modes that satisfy the linear dispersion relation. Second, the highlighted finding here, is the first identification of a streamer (state of bunching of drift waves12,13) that should degrade the quality of plasmas for magnetic confinement fusion14,15. The streamer is a poloidally localized, radially elongated global structure that lives longer than the characteristic turbulence correlation time, and our results reveal that the streamer is produced as the result of the nonlinear condensation, or nonlinear phase locking of the major triplet modes.