3D Magnetic Reconnection in SSX: Experimental Results

We present the results of a large (N > 1000) series of experiments that investigate the dynamical merging of large aspect-ratio, high magnetic Reynolds number (R_m ≈ 1000) plasmas in the Swarthmore Spheromak Experiment (SSX) device. In SSX, two plasmas evolve through a highly conducting copper flux conserver with an aspect ratio of L/r ≈ 10 before colliding near the device midplane. During evolution, plasmas twist into relaxed helical plumes (plectonemes), with typical velocity v = 30 km/s, density n_e = 0.5 × 10²² m^-3, proton temperature T_i = 65 eV, and magnetic field strength B = 0.4 T. As the plasmas collide and merge, measurements of interest are the following: line-averaged plasma ion temperature acquired via Ion-Doppler Spectroscopy; fluid-scale vector magnetic field (distance between probes ≈ 36 × ρ_i ) acquired via 2D 4x4 grid array of B-dot probes; and line-averaged plasma density acquired via Helium-Neon Mach-Zehnder & Heterodyne interferometry (see Ayla Çimen et al this session), all located near the device midplane in the magnetic reconnection region.

We observe appreciable ion heating, density pile-up, and magnetic energy transfer consistent with magnetic reconnection. Here we investigate particular plasma merging events, both of varieties which are indicative of general statistical observations and those which are anomalous. Finally, we present observed differences between experimental results and well-known 2D models of magnetic reconnection.