Design of a Multichord Optical Interferometer with an Axial Fiber-Optic Probe to Measure Electron Density in a Field-Reversed Configuration

Abstract

We present the design of a four-chord laser interferometer system operating at 633 nm that will measure the electron density of field-reversed configurations (FRCs) produced by the magnetized target fusion (MTF) experiment at the Air Force Research Laboratory. The design is a modified version of an eight-chord system previously used to provide timeresolved information about the spatial distribution of electron density in a similar FRC experiment.' With the current system, a fanned array of laser beams will probe the plasma through the FRC midplane along four different chords, and the optical phase shift of each beam relative to a reference beam will be used to infer the line integrated electron density. In addition, a new feature of our design will be the option of diverting any or all of the four probe beams into single-mode optical fibers whose collimated outputs can used to probe different axial locations simultaneously.2 This fiber-optic probe beam modification will enable us to place the interferometer system's optical table at a safe distance from the MTF-FRC experiment when destructive tests involving plasma compression by a solid metal liner imploded by the Shiva Star capacitor bank are attempted.1. E. L. Ruden et al., "Multichord optical interferometry of FRX-L's field reversed configuration," Rev. Sci. Instrum. 77, 103502 (2006). 2. L. M. Smith et al., "A fiber-optic interferometer for in situ measurements of plasma number density in pulsed-power applications," Rev. Sci. Instrum. 74, 3324 (2003).