Dynamics of Plasma Structures Interacting with External and Self-Generated Magnetic Fields

Citation

Yun, Gunsu Soonshin (2008) Dynamics of Plasma Structures Interacting with External and Self-Generated Magnetic Fields. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/9CMD-C377. https://resolver.caltech.edu/CaltechETD:etd-07242007-162442

Abstract

Plasmas interacting with external and self-generated magnetic fields often develop a long tubular structure of nearly uniform cross section. Such long collimated plasma tubes have been observed in a variety of contexts ranging from astrophysical plasma jets (10¹⁵–10²² m) to solar coronal loops (10⁷–10⁸ m). Remarkably, much smaller-sized plasmas (0.1–1 m) produced by the Caltech planar spheromak gun develop collimated structures bearing a striking resemblance to these natural plasma tubes. This thesis presents experimental observations of gun-produced plasma tubes that support a recently-proposed magnetohydrodynamic (MHD) pumping model as a universal collimation mechanism. For any flared flux tube carrying a finite axial current, the model predicts (i) magnetic pumping of plasma particles from a constricted region into a bulged region and (ii) tube collimation if the flow slows down at the bulged region, leading to accumulation of mass and thus concentrating the azimuthal magnetic flux frozen in the mass flow (i.e., increasing the pinch force). Time- and space-resolved spectroscopic measurements of gun-produced plasmas show (i) suprathermal Alfvenic flow (30–50 km/s), (ii) large density amplification from ~10¹⁷ to ~10²² m^-3 in an Alfvenic time scale (5–10 µs), and (iii) flow slowing down and mass accumulation at the flow front, the place where the tube collimation occurs according to high-speed camera imaging. These observations are consistent with the predictions of the MHD pumping model, and thus the model offers valuable insight into the formation mechanism of laboratory, solar, and astrophysical plasma structures.

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