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Title: Laboratory study of the stability of solar-relevant, arched, line-tied magnetic flux ropes
Authors: Alt, Andrew
Advisors: Ji, Hantao
Contributors: Astrophysical Sciences—Plasma Physics Program Department
Subjects: Plasma physics
Issue Date: 2022
Publisher: Princeton, NJ : Princeton University
Abstract: Coronal mass ejections (CMEs) are some of the most energetic and violent events in our solar system. The prediction and understanding of CMEs is of particular importance due to the impact they can have on Earth-based satellite systems, and in extreme cases, ground-based electronics. CMEs often occur when long-lived magnetic flux ropes (MFRs) anchored to the solar surface destabilize and erupt away from the Sun. One potential cause for these eruptions is an ideal magnetohydrodynamic (MHD) instability such as the kink or torus instability. These instabilities have long been studied in axisymmetric fusion devices, such as tokamaks, where the instability criteria are given in terms of the edge safety factor, qa, and equilibrium magnetic field decay index, ns, respectively. The study of these instabilities as they pertain to the Sun has been conducted by both remote observations and numerical simulations. However, both of these approaches have their limitations. By creating MFRs in the laboratory where their properties can be controlled and diagnostics can be placed in-situ, some of these difficulties can be overcome. The experiments presented in this thesis allow for new physics insights that are required for better predictions of space weather events but are difficult to obtain otherwise. The torus instability onsets when the decay index is above a critical value, ns > ncr. However, extending the torus instability theory to non-toroidally symmetric line-tied systems such as MFRs has proved challenging. Because of this, there has been a long-standing discrepancy in the literature about the onset criteria (particularly the value of ncr) for the torus instability. We have investigated these criteria in a laboratory setting by creating arched, line-tied MFRs inside the Magnetic Reconnection Experiment (MRX) at the Princeton Plasma Physics Laboratory (PPPL) and have applied a theoretical model of the torus instability. Our model describes an MFR as a partial torus with foot points anchored to a conducting surface and numerically calculates various magnetic forces on it. This calculation yields better predictions of the critical decay index which take into account the specific parameters of the MFR. This thesis also describes a systematic methodology to properly translate our laboratory results to their solar counterparts, provided that the MFRs have sufficiently small edge safety factor, or equivalently, large enough twist. After this translation, our model predicts that ncr in solar conditions falls near ncr ∼ 0.9 and within a larger range of ncr ∼ (0.7, 1.2) depending on the parameters. The methodology of translating laboratory MFRs to their solar counterparts enables quantitative investigations of CME initiation through laboratory experiments. Previous experiments on MRX revealed a class of MFRs that were torus-unstable but kink-stable, which failed to erupt. These “failed-tori” went through a process similar to Taylor relaxation where the toroidal current was redistributed before the eruption ultimately failed. In this thesis, we have investigated this behavior through additional diagnostics that measure the current distribution at the foot points and the energy distribution before and after an event. These measurements indicate that ideal MHD effects are sufficient to explain the energy distribution changes during failed torus events. This excludes Taylor relaxation as a possible mechanism of current redistribution during an event. A new model that only requires non-ideal effects in a thin layer above the electrodes is presented to explain the observed phenomena. This thesis expands on our understanding of the stability of MFRs and the mechanism behind the failed torus through the improved prediction of the torus instability and through new diagnostics to measure the energy inventory and current profileat the foot points.
Type of Material: Academic dissertations (Ph.D.)
Language: en
Appears in Collections:Plasma Physics

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