Density Profile of a Field-Reversed Configuration´s n = 2 Rotational Instability VIA Tomographic Inversion of FRX-L´S Multichord Optical Interferometer

Summary form only given. An FRC gains angular momentum over time, eventually resulting in an n=2 instability (invariant under rotation by 180 degrees). This is of tentative concern for magnetized target fusion applications since compression by a solid liner may drive the mode further from stability, assuming adiabatic compression, angular momentum conservation, and an idealized model of the coupled effects of gyroviscosity and sheared flow on the Rayleigh-Taylor instability. To benchmark future simulations needed to clarify the mode dynamics, density profiles are generated by tomographic inversion of interferometer data. To accomplish this, data is used from a fanned array of 8 chords which record line integrated density at several times over a period when the mode has saturated and rotates roughly as a rigid body. The FRC in the rotating frame, then, may be interpreted as having a fixed profile scanned at different angles, permitting full 2-D inversion. Corrections due to acoustic noise, refractive changes in the quartz vacuum vessel walls, and a gradual density decay over a rotation period are made. Inversion is then accomplished by minimizing the chi ² deviation of the corrected data from the line integrated inferred density profile (as quantified on a rotating square mesh) plus a "regularizing" scalar functional that increases in value as the spatially integrated finite-element Laplacian magnitude of the density distribution increases. The latter term is needed to avoid high spatial frequency artifacts that would otherwise dominate the inversion