Turbulence analysis of an experimental flux-rope plasma

Abstract

Abstract. We have previously generated elongated Taylor double-helix flux rope plasmas in the SSX MHD wind tunnel. These plasmas are remarkable in their rapid relaxation (about one Alfvén time) and their description by simple analytical Taylor force-free theory despite their high plasma β and high internal flow speeds. We report on the turbulent features observed in these plasmas including frequency spectra, autocorrelation function, and probability distribution functions of increments. We discuss here the possibility that the turbulence facilitating access to the final state supports coherent structures and intermittency revealed by non-Gaussian signatures in the statistics. Comparisons to a Hall-MHD simulation of the SSX MHD wind tunnel show similarity in several statistical measures.

Magnetohydrodynamic turbulence: Observation and experiment

“…Turbulence never absolutely reflects these symmetries, for example, the flow direction away from the Sun is special in the solar wind. We find that for the SSX plasma wind tunnel, [5][6][7][8] there are extended periods during which the turbulence is approximately stationary, homogeneous, and isotropic. The as yet unproven general ergodic theorem states that time averages are the same as ensemble averages, assuming the fluctuations are stationary (the ergodic theorem has been proven under certain conditions 9 ).…”

Observation of Turbulent Intermittency Scaling with Magnetic Helicity in an MHD Plasma Wind Tunnel

“…A possible connection to a physical mechanism was established through soft x-ray and ion Doppler spectrometer measurements, which suggested the intermittency is related to the spatial size of reconnection sites in the plasma. However, given the limitations of the current diagnostics, definitive conclusions cannot be made but do provide an impetus for further comparison to simulation in this experimental configuration [17] as well as more exploration as to the effects of helicity on the turbulence [14]. Finally, since helicity observations have been made in many of the same turbulent plasmas that exhibit intermittency [30][31][32], a link between helicity and turbulent intermittency may be a useful metric for understanding turbulence in both space and experiment, including possible comparisons to the variation of intermittency as a function of solar distance [33] as well as differences between confinement regimes in fusion devices.…”

“…Since the magnetic helicity of a plasma is reflective of the twistedness or knottedness of the magnetic fields, a scan of magnetic helicity can be used to vary the magnetic field structure and potentially modify the character of current sheets in the plasma. A novel experiment developed on the MHD wind-tunnel configuration of the Swarthmore Spheromak Experiment (SSX) [16,17] explores this possible relationship between the observed intermittency in magnetic fluctuations and the magnetic helicity of the plasma. Given the nature of the plasma source on the SSX, a magnetic helicity injection can be very finely controlled, and thus resulting changes in turbulent characteristics-including both spectra and intermittency-can be carefully examined.…”

Permutation entropy and statistical complexity analysis of turbulence in laboratory plasmas and the solar wind

“…The solar wind is highly variable but there are two broad types: fast wind (V > 600 km/s) which is emitted from open coronal field lines and is typically low density (<5 protons/cm 3 ), has few large scale structures and has high amplitude but less developed turbulence, and slow wind, (V < 500 km/s) which is typically found in the ecliptic plane and originates from more complex coronal magnetic topology and is denser and more structured than the fast wind with more evolved but lower amplitude turbulence [26,27]. Here we use multiday long intervals of a fast wind stream (January [14][15][16][17][18][19][20][21]2008) and a slow wind stream (January [24][25][26][27][28][29]2010) with large scale magnetic fluctuations on the order of 10 nT.…”