Experimental study of current-driven turbulence during magnetic reconnection

Abstract

Magnetic reconnection is an important process in magnetized plasmas ranging from the laboratory to astrophysical scales. It enables the release of magnetic energy believed to power solar flares and magnetospheric substorms. Reconnection also controls the evolution of the topology of the magnetic field, enabling deleterious instabilities, such as the sawtooth instability in fusion experiments, to transport plasma across the experiment's minor radius. Notably, simple estimates of the finite reconnection rate due to classical resistivity fail to explain the fast and explosive nature of reconnection observed in these systems. A major goal of reconnection research is to determine which mechanisms enable "fast" reconnection to occur. This thesis studied the fluctuations arising in the plasma during magnetic reconnection experiments on the Versatile Toroidal Facility (VTF), with a primary goal of testing whether "anomalous resistivity" due to micro-instabilities can speed the reconnection process. Fluctuations were studied using impedance-matched, high-bandwidth Langmuir probes. Strong, broadband fluctuations, with frequencies extending from near the lower-hybrid frequency [fLH = (fcefci)1=2] to the electron cyclotron frequency fce were found to arise during the reconnection events. Based on frequency and wavelength measurements, lower-hybrid waves and Trivelpiece-Gould waves were identied. The lower-hybrid waves appear to be driven by strong perpendicular drifts or gradients which arise due to the reconnection events; an appealing possibility is strong temperature gradients.

(cont.) The Trivelpiece-Gould modes were found to result from kinetic, bump-on-tail instability of a runaway electron population energized by the reconnection events. Nonlinear, spiky turbulence was also observed, and attributed to the creation of "electron phase-space holes," a class of nonlinear solitary wave known to evolve from a strong beam-on-tail instability. Overall, these instabilities were found to be a consequence of reconnection, specifically the strong energization of electrons, leading to steep gradients in both coordinate- and velocity-space. However, it was not established that these modes had a strong feedback on the reconnection process: fluctuation power varied strongly between discharges and was observed to systematically trail the reconnection events. Finally, crude estimates (using quasi-linear theory) of the anomalous resistivity due to these modes did not appear large enough to substantially impact the reconnection process.