In contrast to hydrodynamic turbulence, which is generally expected to be spatially isotropic, MHD turbulent structures are expected to be highly elongated along the background magnetic field. Anisotropy has been measured in solar wind turbulence, and is inferred from observations of pulsar scintillation.
Fully developed magnetic turbulence with this strong anisotropy has now been measured in the reversed field pinch laboratory plasma device MST. The observed magnetic turbulence has much larger kperp than kpar with respect to the background magnetic field, a feature that is consistent with Alfvenic turbulence. Furthermore, a linkage between large-scale tearing modes that act as a stirring mechanism and the small-scale magnetic fluctuations has been demonstrated through the variation of the tearing mode amplitudes. This nonlinear cascade has both power-law and exponential-law spectral features, which are consistent with inertial and dissipation ranges, respectively, in MHD turbulence theories. It is possible that the dissipation associated with the observed exponential decrease in the small-scale fluctuations is related to the strong collisionless ion heating observed in these plasmas.
Power spectra for the magnetic turbulence measured at several radii in the edge of MST reversed field pinch plasmas. The background magnetic field is strongly sheared, and the turbulence maintains anisotropy locally as the field line orientation changes. From Y. Ren, A.F. Almagri, G. Fiksel, S.C. Prager, J.S. Sarff, and P.W. Terry.
These results may help improve our understanding of how anisotropic magnetic turbulent cascades form and evolve in astrophysical plasmas, and how they can heat background particles.