SSX MHD Plasma Wind Tunnel

A new turbulent plasma source at the Swarthmore Spheromak Experiment (SSX) facility is described. The MHD wind tunnel configuration employs a magnetized plasma gun to inject high-beta plasma into a large, well-instrumented, vacuum drift region. This provides unique laboratory conditions approaching that in the solar wind: there is no applied background magnetic field in the drift region and has no net axial magnetic flux; the plasma flow speed is on the order of the local sound speed (M ~ 1), so flow energy density is comparable to thermal energy density; and the ratio of thermal to magnetic pressure is of order unity (plasma β ~ 1) so thermal energy density is also comparable to magnetic energy density. Results presented here and referenced within demonstrate the new capabilities and show how the new platform is proving useful for fundamental plasma turbulence studies

An LQR controller design approach for a Large Gap Magnetic Suspension System (LGMSS)

Two control approaches for a Large Gap Magnetic Suspension System (LGMSS) are investigated and numerical results are presented. The approaches are based on Linear Quadratic Regulator (LQR) control theory and include a nonzero set point regulator with constant disturbance input and an integral feedback regulator. The LGMSS provides five degree of freedom control of a cylindrical suspended element which is composed of permanent magnet material. The magnetic actuators are air core electromagnets mounted in a planar way

Temporal and Spatial Turbulent Spectra of MHD Plasma and an Observation of Variance Anisotropy

The nature of MHD turbulence is analyzed through both temporal and spatial magnetic fluctuation spectra. A magnetically turbulent plasma is produced in the MHD wind-tunnel configuration of the Swarthmore Spheromak Experiment (SSX). The power of magnetic fluctuations is projected into directions perpendicular and parallel to a local mean field; the ratio of these quantities shows the presence of variance anisotropy which varies as a function of frequency. Comparison amongst magnetic, velocity, and density spectra are also made, demonstrating that the energy of the turbulence observed is primarily seeded by magnetic fields created during plasma production. Direct spatial spectra are constructed using multi-channel diagnostics and are used to compare to frequency spectra converted to spatial scales using the Taylor Hypothesis. Evidence for the observation of dissipation due to ion inertial length scale physics is also discussed as well as the role laboratory experiment can play in understanding turbulence typically studied in space settings such as the solar wind. Finally, all turbulence results are shown to compare fairly well to a Hall-MHD simulation of the experiment.Comment: 17 pages, 17 figures, Submitted to Astrophysical Journa

Possible Signatures Of Dissipation From Time-Series Analysis Techniques Using A Turbulent Laboratory Magnetohydrodynamic Plasma

The frequency spectrum of magnetic fluctuations as measured on the Swarthmore Spheromak Experiment is broadband and exhibits a nearly Kolmogorov 5/3 scaling. It features a steepening region which is indicative of dissipation of magnetic fluctuation energy similar to that observed in fluid and magnetohydrodynamic turbulence systems. Two non-spectrum based time-series analysis techniques are implemented on this data set in order to seek other possible signatures of turbulent dissipation beyond just the steepening of fluctuation spectra. Presented here are results for the flatness, permutation entropy, and statistical complexity, each of which exhibits a particular character at spectral steepening scales which can then be compared to the behavior of the frequency spectrum

Temporal And Spatial Turbulent Spectra Of MHD Plasma And An Observation Of Variance Anisotropy

The nature of magnetohydrodynamic (MHD) turbulence is analyzed through both temporal and spatial magnetic fluctuation spectra. A magnetically turbulent plasma is produced in the MHD wind tunnel configuration of the Swarthmore Spheromak Experiment. The power of magnetic fluctuations is projected into directions perpendicular and parallel to a local mean field; the ratio of these quantities shows the presence of variance anisotropy which varies as a function of frequency. Comparisons among magnetic, velocity, and density spectra are also made, demonstrating that the energy of the turbulence observed is primarily seeded by magnetic fields created during plasma production. Direct spatial spectra are constructed using multi-channel diagnostics and are used to compare to frequency spectra converted to spatial scales using the Taylor hypothesis. Evidence for the observation of dissipation due to ion inertial length scale physics is also discussed, as well as the role laboratory experiments can play in understanding turbulence typically studied in space settings such as the solar wind. Finally, all turbulence results are shown to compare fairly well to a Hall-MHD simulation of the experiment

Observation Of Turbulent Intermittency Scaling With Magnetic Helicity In An MHD Plasma Wind Tunnel

The intermittency in turbulent magnetic field fluctuations has been observed to scale with the amount of magnetic helicity injected into a laboratory plasma. An unstable spheromak injected into the MHD wind tunnel of the Swarthmore Spheromak Experiment displays turbulent magnetic and plasma fluctuations as it relaxes into a Taylor state. The level of intermittency of this turbulence is determined by finding the flatness of the probability distribution function of increments for magnetic pickup coil fluctuations B(t). The intermittency increases with the injected helicity, but spectral indices are unaffected by this variation. While evidence is provided which supports the hypothesis that current sheets and reconnection sites are related to the generation of this intermittent signal, the true nature of the observed intermittency remains unknown

Turbulence Analysis Of An Experimental Flux-Rope Plasma

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 Alfven time) and their description by simple analytical Taylor force-free theory despite their high plasma beta 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