Characterizing Intermittent Turbulent Wave Kinetics and Three-Wave Coupling in Dipole-Confined Plasma*

Plasmas confined by a dipole magnetic field exhibit interchange and entropy mode turbulence causing bursty intermittent transport of particles and energy [1].  On the Collisionless Terrella Experiment (CTX), this turbulence is dominated by low-frequency, long-wavelength modes with amplitudes and phases that vary chaotically in time [2]. We present a new paradigm for characterizing this turbulence by measuring the time-evolution of the fluctuation power spectrum and the instantaneous bispectrum using the continuous wavelet transform [3,4] and computing the statistical properties of turbulent wave kinetics. We observe that both the fluctuation power and the energy transfer by three-wave coupling, or bispectrum, between these fluctuations can be intermittent. When antenna are used to actively launch waves into the turbulence, the intermittency of the driven waves decreases, while the intermittency of other waves increases. Similarly, application of active feedback [5] to amplify the turbulence decreases the intermittency of the wave energy, while suppressing feedback increases this intermittency. Measurements based on this new paradigm show that the transfer of wave energy to larger and smaller scales in a turbulent plasma is not steady but occurs in short and intense bursts, analogous to the better-known short bursts of particle transport in magnetized plasma.

 

[1] B.A. Grierson, et al, Phys Rev Lett 105, 205004 (2010).

[2] B.A. Grierson, et al, Phys Plasmas 16, 055902 (2009).

[3] C. Torrence and G.P. Compo, Bulletin of the American Meteorological Society, 61-78 (1998).

[4] B.P. Van Milligen, et al, Phys Rev Lett 74, 395 (1995).

[5] T.M. Roberts, et al, Phys Plasmas 22, 055702 (2015).