Development of a spatially resolved polarization spectroscopic diagnostic for magnetic field measurements in the Caltech jet experiments*

The Caltech coaxial magnetized plasma gun experiment exhibits a well-defined sequence of phenomena that have been studied in detail. This sequence consists of formation of a self-collimated, MHD-driven plasma jet that then undergoes a sequence of distinct instabilities where each member of the sequence provides the destabilizing environment for the next. In this sequence, the jet first forms, then the jet undergoes a kink instability, the kink destabilizes a Rayleigh-Taylor instability (RTI), and then the RTI destabilizes a kinetic instability that is postulated to be a collisional generalization of a Buneman instability. Determination of this sequence has involved many different diagnostics and in particular a 13-channel polarimeter was constructed [1] that measured and verified the magnetic field structure of a nitrogen plasma jet prior to onset of the instability sequence. This stable-jet measurement was based on passive viewing of Zeeman splitting of an optical line. We describe here the construction of a new polarimeter designed to have improved spatial resolution that will enable measurements of the magnetic field when the jet is undergoing the instability sequence. The new polarimeter has 64 spatial channels with spatial resolution (distance between channels) adjustable from 0.3 to 1.5 mm. The polarimeter will use the Zeeman splitting of N II and Ar II. A preliminary goal will be to measure the evolution of the magnetic field profile during the growth of kink and Rayleigh-Taylor instabilities in argon and nitrogen plasma jets. It is ultimately intended to measure the magnetic field profile when a jet undergoes the kinetic instability observed to cause a transient burst of hard x-rays.

[1] Shikama, T. and P. M. Bellan, Rev. Sci. Instrum. 84(2), 023507 (2013), https://doi.org/10.1063/1.4793403