Design of a multichord optical interferometer with an axial fiber-optic probe to measure electron density in a field-reversed configuration

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 time-resolved 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 be used to probe different axial locations simultaneously. 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.