Developing Plasma Spectroscopy and Imaging Diagnostics to Understand Astrophysically-Relevant Plasma Experiments: Megameters, Femtometers, and Everything in Between

Citation

Marshall, Ryan Scott (2020) Developing Plasma Spectroscopy and Imaging Diagnostics to Understand Astrophysically-Relevant Plasma Experiments: Megameters, Femtometers, and Everything in Between. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/gfcd-4q50. https://resolver.caltech.edu/CaltechTHESIS:05082020-085730717

Abstract

One of the main attractions of using laboratory experiments as a proxy to study solar and astrophysical plasmas is the ability to build diagnostics that directly measure things. This cannot be done on actual solar and astrophysical plasmas as they are either i) extremely distant, ii) in an extreme environment, or iii) both. Fortunately, the lack of intrinsic scales in the MHD equations means that a plasma created in the laboratory with similar β, S, and magnetic topology will evolve similarly to its astrophysical analogs. Thus the use of diagnostics in the laboratory to understand the evolution of laboratory plasmas can assist in understanding complicated astrophysical plasma dynamics.

This thesis is broken up into three main areas. The first is about the development of and results from two new custom X-ray scintillator detectors and a CMOS camera repurposed into an X-ray spectrometer mounted on the Caltech Astrophysical Jet Experiment. Next, water-ice grain growth in a cold dusty plasma is quantified by analyzing the frames in a movie recorded by an ultra-high-speed camera. Finally, the development of and results from a custom, motorized Laser-Induced Fluorescence diagnostic that measures the temperature and flow speed of neutral argon atoms in the dusty plasma experiment are presented.

Two custom-built X-ray scintillator detectors mounted on the jet experiment detect a burst of hard X-rays establishing that this burst occurs simultaneously with a fast magnetic reconnection event taking place in the T = 2 eV plasma. A repurposed windowless CMOS camera acting as an X-ray spectrometer confirms the burst consists of non-mono-energetic photons around 6 keV energy. This magnetic reconnection event is triggered after the jet undergoes an ideal MHD kink instability which accelerates the jet laterally inducing a fast-growing secondary Rayleigh-Taylor instability. The Rayleigh-Taylor instability causes the ideal MHD treatment of the jet to be violated when it pinches the jet diameter past c/ω_pi causing it to break apart. As it breaks apart, a burst of hard X-rays are detected. These findings lead to the conclusion that an inductive electric field arises at the location of the reconnection event that accelerates a small fraction of electrons to keV energy despite the plasma being so collisional that acceleration is unexpected. This theory leads to the hypothesis that the fine structure of solar prominences consists of many Litz-wire like strands of plasma each on the order of a few ion skin depths in diameter, as opposed to the traditional picture of one monolithic arch.

Analysis of a high-speed video of ice grains growing from 20 to 80 µm inside the dusty plasma experiment leads to the conclusion that the charged ice grains in the experiment grow via accretion of water molecules. The video challenges the common astrophysical assumption that the dusts in dusty plasmas are spherical as they are clearly seen to be elongated, fractal structures in the movie. Another commonly made assumption is that the grains grow via agglomerating collisions and that this results in the grains having a power law dependence on radius. Video of the grains in the Caltech experiment shows a log-normal dependence and absolutely no evidence of agglomerating collisions; or even a case of two grains approaching with a large relative velocity, and then scattering. It is believed that the grains have a large negative charge resulting in strong mutual repulsion and this, combined with their nearly non-existent relative velocities due to undergoing oscillatory motion by a relatively coherent wave, prevents them from agglomerating. This combined with a detailed study of Coulomb repulsion between the grains leads to the conclusion that direct accretion of water molecules is likely the dominant contribution to the observed ice grain growth.

Lastly, a Laser-Induced Fluorescence diagnostic has been developed for the dusty plasma experiment. Whereas the first two projects rely on passive detection instruments, the LIF diagnostic actively uses a pump beam to excite atoms in the plasma, and then detects the resulting emission. The diagnostic is motorized and automated with Labview so that the plasma volume can be scanned in three dimensions. Argon neutral temperature is measured to be slightly above room temperature on the Caltech experiment and the PK4 experimental setup at Baylor University. Challenges such as the lack of absolute calibration of diode lasers and wavelength drift due to slight changes in ambient room conditions are overcome to measure sub-linewidth bulk neutral flow speeds on the order of 1-2 m/s with resolution on the order of 2/3 of a meter per second. The competing influences of a density gradient and wavelength dependent absorption broadening mechanism are separated and quantified. High-speed video shows that introducing an argon flow to a cloud of ice grains causes the cloud of ice grains to move and change shape. This motion is analyzed and found to show agreement with neutral LIF flow measurements. Surprisingly, when the flow ceases, the ice grain cloud reverts to its original location and shape.

Thesis Files