Anatomy of a field-reversed configuration
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 Physics of Plasmas ARTICLE
 scitation.org/journal/php
 Anatomy of a field-reversed configuration
Cite as: Phys. Plasmas 27, 112508 (2020); doi: 10.1063/5.0022663 Submitted: 3 August 2020 . Accepted: 27 October 2020 . Published Online: 19 November 2020
a)
      L. C. Steinhauer, AFFILIATIONS
T. Roche,   and J. D. Steinhauer
 TAE Technologies, Rancho Santa Margarita, California 92688, USA
a)Author to whom correspondence should be addressed: madscientistnw@gmail.com ABSTRACT
A reconstructor tool is developed for fast computation of fully two-dimensional equilibria of field-reversed configurations (FRCs) that are consistent with routine magnetic data from experiments. This tool fosters physical interpretation of multiple FRC properties. The physics model is a rotating fluid that also captures several realistic kinetic effects. The tool allows both FRC solutions and high-b mirror solutions (no closed magnetic flux) a bifurcation, dependent on the input data. A major conclusion is that FRCs can exist only within a limited shape domain, i.e., combinations of plasma radius and length. These limitations reflect the requirement of equilibrium force balance and tearing stability. Inspection of a considerable range of experiments shows that the shape domain reflects actual operational boundaries. Working from timeline data from an experiment the tool finds the evolving two-dimensional structure plus the time histories of critical properties such as trapped flux, plasma energy, and plasma current. These timelines offer clues about underlying stability and transport properties not contained within the equilibrium model itself. Properties of traditional FRCs as well as modern, neutral-beam driven FRCs are explored, and specific experimental shots are reconstructed.
Published under license by AIP Publishing. https://doi.org/10.1063/5.0022663
  I. INTRODUCTION A. Preview
This paper develops an automated tool for fast reconstruction of the structure and properties of field-reversed configurations (FRCs) starting from common experimental data. This tool has been used for theoretical analysis to explore the nature of individual equilibria and suggest physical interpretations. It also has been applied for automated reconstruction of a sequence of equilibria, drawing directly from an experimental database.
Typical FRCs are elongated, axisymmetric plasmas composed of a closed-field core and an open-field periphery. The latter includes the scrape-off layer (SOL) alongside the core and spindle-like jets, extend- ing from each end of it. The core is simply-connected; no solid struc- ture passes through its heart. As such an FRC can be housed in a linear confinement vessel. The periphery acts as a natural divertor connecting to remote solid surfaces of unrestricted area. The plasma is relatively remote from vessel walls both alongside and at the ends; the separatrix-to-wall gap is comparable to the core radius itself. As such an FRC is a free-floating object surrounded by a broad, low-density, magnetic-field-rich buffer. The average b (plasma pressure/magnetic pressure) is very high because the field is primarily poloidal. Other toroidal plasmas are stabilized by a strong toroidal field that acts as a somewhat rigid skeletal structure but restricts it to relatively low-b. By contrast FRCs are stabilized, as it were, by cartilage instead of bone,
Phys. Plasmas 27, 112508 (2020); doi: 10.1063/5.0022663 Published under license by AIP Publishing
relying on the stiffening influence of finite ion gyroradius. High-b and linear structure give FRCs notable advantages for magnetic engi-
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neering. The history and physics of FRCs were reviewed elsewhere. Illumination of plasma detail (structure, critical values) is a challenge for all dense, high-temperature plasmas because of limited diagnostics and the need to locate some of them at a safe distance from the plasma core. Innovative diagnostic tools have been developed over the years, yet they never seem to yield enough information to confidently nail down critical properties of interest. Thus, the task of reconstructing experimental details must rely heavily on a respectable plasma model and an expeditious computational algorithm to process the data. This challenge is doubly difficult in high-b plasmas where approximations useful in low-b systems do not apply. At high-b, the host magnetic structure is much more strongly shaped by the presence of a plasma. Details of particular interest are the actual dimensions of the core (radius, length), the trapped magnetic flux, the proportions of current and inventory in the core relative to that in the periphery, and stability properties associated with the equilibrium. The trapped flux is a property deep inside the core (at the O-point) so that inferring it is problematic. Inferring radial and axial dimensions and trapped flux as
well as other properties calls for a reliable reconstruction technique. The reconstruction tool developed here is called “Grushenka.” Its rationale is to draw from a small set of readily available inputs from experiment on one hand and place it in conversation with a
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