Dynamics of an Arched Magnetically-Twisted Current-Carrying Plasma

Citation

Wongwaitayakornkul, Pakorn (2020) Dynamics of an Arched Magnetically-Twisted Current-Carrying Plasma. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/6Q9D-JH07. https://resolver.caltech.edu/CaltechTHESIS:03042020-114816620

Abstract

Experimental and numerical studies of a dense magnetically-twisted plasma and their applications to solar plasmas are the subject of this dissertation. In the corona, plasma lies in a low-beta, high Lundquist number regime, meaning that it is magnetically dominated and the magnetic fields are well frozen into the plasma. Understanding the dynamics of these plasmas help us predict and prevent damage from future catastrophic solar eruption events. In situ measurements from satellite and ground-based observation provide limited information that is not controllable nor reproducible. The research objective in this thesis is to produce a miniature-scaled plasma with the same dimensionless parameters as the space plasmas. Along with numerical simulation, theoretical study, and observational data, the laboratory plasma can give novel insights into the physics of solar plasma.

First, an experimental dip on a flux rope, previously thought to be caused by a kink instability, is discussed and explained. We find that the apex cusp is in fact caused by the differential acceleration due to a non-uniform density. The pileup density results from a nonlinear interaction of the neutral gas. This result introduces a new method to impose effective gravity on the arched plasma and explains the suppression of kink instability. Second, a model for a morphology of CME and its shock driving mechanism is investigated. In the experiment, the chamber is prefilled with neutral gas, leading to an observation of a density cavity. Because the plasma is flux conserving, injecting a current into the plasma induces an opposite eddy current in front of the flux rope. The two opposing currents repel and leave a low density region in between. This feature is often observed in CMEs. We propose this mechanism to be the model of the CME 3-part structure formation. The opposite eddy current acts as a current piston driving an MHD perturbation/shock, which is often observed on the sun as an EUV wave.

A Magnetic Rayleigh-Taylor instability has been observed in the arched plasma loop. For the first time, the magnetic effect of the MRT instability is shown when the wavelength observed depends on the initial magnetic field initially injected into the system. In several years of working with the experiment in the Bellan plasma group, I designed and constructed several diagnostics, such as Langmuir probes, magnetic probes, and a coded aperture camera. Together with fast multi-images camera and spectroscopy techniques, plasma parameters are measured and compared to verify the models.

The 3D MHD numerical simulation was performed using the supercomputer from the Los Alamos National Laboratory. The initial condition and injection routines were modified to appropriately replicate the experiment. The code has been significant in improving our understanding of the physical phenomena we observed in the experiment. We attain a proper initial distribution of the mass density and the initial and injected current density. In addition to simulating an arched flux rope experiment, we use this tool to replicate MHD instabilities detected in the astrophysical jet experiment. Specifically, both a sausage-to-kink and kink-to-Rayleigh-Taylor instability have been reproduced using the numerical simulation. Each process thins the plasma current channel to be below the ion skin depth. The kinetic effect then gives rise to magnetic reconnection. An anomalous resistivity is added to simulate this process.

In conclusion, an interdisciplinary approach, through experimental, numerical, observational, and theoretical studies, is presented. It improves our understanding of the underlying mechanism for solar eruptions. A magnetically-twisted current-carrying flux rope, once formed, could exhibit dips and cavity. Its evolution could a drive shock and instabilities, which ultimately cause particle acceleration.

Thesis Files