Center for Magnetic Self Organization
in Laboratory and Astrophysical Plasmas
High velocity turbulent MHD wind tunnel on SSX
We designed a four-foot extension to SSX to give us over 2 meters for injected plasma plumes to flow and evolve. Typical flow speeds are 50 km/s with densities and temperatures in the 1015 cm-3 and 20 eV ranges respectively. Mean magnetic fields are about 0.25 T. Our main observation is the relaxation of the turbulence to a single twisted helical structure with kz = 2.2 m-1. Magnetic fluctuations are measured with high resolution probe arrays (4 mm spacing) and using 96 channels of new digitizers at 65 MHz and 14 bits. This allows us to study fluctuations above the ion cyclotron frequency (> 3 MHz) with a dynamic range of gauss to tesla. Preliminary fluctuation spectra show a f-5/3 dependence for about a decade, followed by a steeper spectrum at frequencies above the ion cyclotron frequency.
Evolution of the fluctuation spectrum over time, showing the growing dominance of a narrow band of wavenumbers. The horizontal line is the wavenumber of the lowest energy state which the plasma can attain while conserving magnetic helicity. Right: The red curve outlines a fieldline in the preferred final state; the blue points are data points.