High-Energy-Density Field-Reversed Configurations for Sub-Microsecond Magnetized Target Fusion
Sporer, Brendan
2023
Abstract
Magnetized target fusion (MTF) has gained popularity in recent decades as potentially a more economic route to fusion gain greater than unity in the laboratory. The magnetized liner inertial fusion (MagLIF) program on the Z-machine (20~MA, 100~ns) at Sandia National Laboratories has successfully integrated and demonstrated the principles of MTF, and several other organizations are pursuing MTF approaches to fusion energy. The ``field-reversed configuration", or FRC, is a popular magnetized plasma target, favored for its closed-field lines and high average plasma beta (ratio of particle pressure to magnetic field pressure), among other attributes. The work detailed in this thesis explores the nascent idea of producing and compressing high-energy-density (HED), centimeter-scale FRCs on the Z-machine, for the purposes of large fusion yield and other fundamental plasma/code-benchmarking studies like magnetic reconnection. In practice, this would be done using bias coils and liner compression hardware similar to that used for the MagLIF program. The textit{in-situ} formation concept for the FRC requires axial field production by both external coils and a helically slotted (i.e. ``AutoMag"-type) liner. To inform FRC physics at this novel hydrodynamic scale, a platform has been developed on the MAIZE linear transformer driver (1~MA, 100~ns) to form high-applied-field (10--15~T), solenoidal FRCs with physics similar to those desired for compression on the Z-facility. To the author's knowledge, this represents the first time FRCs have been formed using two separate field sources (external slow coils and a helical fast coil) as well as on a linear transformer driver system. The claim of FRC formation on MAIZE will be justified by analysis of visible light imaging, magnetic probe data, and the appearance of expected FRC instabilities. The high-density FRCs are studied for stability and lifetime, and compared with simulation and the predictions found in the existing FRC literature. As predicted by historical experiments, it appears that low-density FRCs are more well-formed and stable than higher-density ones. Furthermore, the density limit for successful formation does not seem to be related to gross stability, but to non-uniformities in Z-discharge pre-ionization (ZPI) of the high-pressure deuterium gas fill. Alternative methods of pre-ionization are recommended to be explored. As expected with no active stabilization, the wobble and rotational instabilities appear in MAIZE FRCs with small radius. Large radius FRCs do not show rotational instability; it is theorized that the asymmetry and finite azimuthal fields inherent to solenoidal fields near the coil stabilize the rotational instability. To complement the experimental effort on MAIZE, numerical simulations were run using the two-dimensional axisymmetric version of the resistive magnetohydrodynamics (MHD) code, Kraken. The code was initialized with an analytic FRC profile and used to study relaxation and decay, including two treatments of anomalous resistivity..... not completeDeep Blue DOI
Subjects
field-reversed configuration magnetized target fusion FRC plasma MagLIF implosion
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