Multi-spectral Imaging of a Two Species Plasma Jet to Examine Braiding & Complex Topological Structures

The Caltech plasma jet experiment is an MHD-driven plasma for the majority of its duration, which implies a conservation of flux & consequently helicity. Helicity is frequently a better constant of motion for MHD plasmas because its dissipation time far exceeds the dissipation time for the magnetic energy, yet it can be quite difficult to measure. By using two gas species in the Caltech MHD Jet Experiment, the braiding of the jet can be examined through simultaneous measurements of two well separated spectral lines, from each gas species respectively. This is accomplished by using an optical relay network which creates two identical images, via a beamsplitter and off-axis focusing network, of the plasma on a single fast camera frame. Each image copy can be optically bandpass filtered for spectral lines of interest. Movies of Ar and N plasmas will be made using 10nm bandpass filters for 568nm and 667nm, which are N-II and Ar-II lines respectively. We will attempt to examine other mixtures which have a mass ratio closer to 1. Preliminary measurements of Ar and N plasmas indicate the two gas species begin well separated, but approach a well-mixed state long before the plasma dissipates, when it is in a stable regime. We will examine the braiding of the Ar flux ropes & N flux ropes, and present an interpretation of helicity conservation in the jet with relation to any observed instabilities. This work has been supported by NSF award no. 2403814.