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Current sheet dynamics during driven magnetic reconnection

  • Magnetic reconnection is a ubiquitous phenomenon observed in a wide range of magnetized plasmas from magnetic confinement fusion devices to space plasmas in the magnetotail. The process enables the release of accumulated magnetic energy by rapid changes in magnetic topology, heating the plasma in the vicinity of the reconnection site, generating fast particles and allowing a wealth of instabilities to grow. This thesis reports on the results from a newly constructed linear, cylindrical and modular guide field reconnection experiment with highly reproducible events, VINETA.II. A detailed analysis of the reconnecting current sheet properties on a macroscopic and microscopic scale in time and space is presented. In the experiment, four parallel axial wires create a figure-eight in-plane magnetic field with an X-line along the central axis, as well as an axial inductive field that drives magnetic reconnection. Particle-in-cell simulations show that the axial current is limited by sheaths at the boundaries and that electrostatic fields along the device axis always set up in response to the induced electric field. Current sheet formation requires an additional electron current source, realized as a plasma gun, which discharges into a homogeneous background plasma created by a rf antenna. The evolution of the plasma current is found to be dominantly set by its electrical circuit. The current response to the applied electric field is mainly inductive, which in turn strongly influences the reconnection rate. The three-dimensional distribution of the current sheet is determined by the magnetic mapping of the plasma gun along the sheared magnetic field lines, as well as by radial cross-field expansion. This expansion is due to a lack of equilibrium in the in-plane force balance. Resistive diffusion of the magnetic field by E=η j is found to be by far insufficient to account for the high reconnection rate E=-dΨ/dt at the X-line, indicating the presence of large electrostatic fields which do not contribute to dissipative reconnection. High-frequency magnetic fluctuations are observed throughout the current sheet which are compared to qualitatively similar observations in the Magnetic Reconnection Experiment (MRX, Princeton). The turbulent fluctuation spectra in both experiments display a spectral kink near the lower hybrid frequency, indicating the presence of lower hybrid type instabilities. In contrast to the expected perpendicular propagation of mainly electrostatic waves, an electromagnetic wave is found in VINETA.II that propagates along the guide field and matches the whistler wave dispersion. Good correlation is observed between the local axial current density and the fluctuation amplitude across the azimuthal plane. Instabilities driven by parallel drifts can be excluded due to the large required drift velocities or low resulting phase velocities that are not observed. It is instead suggested that a perpendicular, electrostatic lower hybrid mode indeed exists that resonantly excites a parallel, electromagnetic whistler wave through linear mode conversion. The resulting fluctuations are found to be intrinsic to the localized current sheet and are independent of the slower reconnection dynamics. Their amplitude is small compared to the in-plane fields, and have a negligible contribution to anomalous resistivity through momentum transport in the present parameter regime.
  • Ein neues, lineares, zylindrisches und modulares Experiment zur magnetischen Rekonnexion, VINETA.II, wurde verwendet um die makroskopischen und mikroskopischen Eigenschaften der rekonnektierenden Stromschicht zu untersuchen. Neben der technischen Entwicklung des Experiments, unter anderem dem Design und Bau einer miniaturisierten Bogenentladung, der Plasmakanone, wurde die zeitliche Abhängigkeit des Plasmastromes von den globalen Randbedingungen, die zeitliche und räumliche Entwicklung der dreidimensionalen Stromschicht sowie die Eigenschaften hochfrequenter Magnetfeldfluktuationen untersucht. Letztere wurden mit weiteren Messungen am Magnetic Reconnection Experiment (MRX) in Princeton verglichen, und die Spektren und Dispersionseigenschaften verglichen.

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Author: Adrian von Stechow
Title Additional (German):Stromschichtdynamik in getriebener magnetischer Rekonnexion
Advisor:Dr. Olaf Grulke, Prof. Dr. Thomas Klinger
Document Type:Doctoral Thesis
Date of Publication (online):2015/06/30
Granting Institution:Ernst-Moritz-Arndt-Universität, Mathematisch-Naturwissenschaftliche Fakultät (bis 31.05.2018)
Date of final exam:2015/03/19
Release Date:2015/06/30
GND Keyword:Magnetische Rekonnexion, Plasmadiagnostik, Plasmaphysik
Faculties:Mathematisch-Naturwissenschaftliche Fakultät / Institut für Physik
DDC class:500 Naturwissenschaften und Mathematik / 530 Physik