Speaker: Dr. Peter O’Shea (General Fusion): Magnetic reconstruction on laboratory plasmas is a standard tool at General Fusion. While development of a polarimeter progresses, our reconstructions on laboratory based plasma injectors rely solely on edge magnetic (“Bdot”) probes. On plasma experiments built for field compression (PCS) tests, the number and locations of Bdot probes is limited by mechanical constraints. Additional information about the magnetic structure of our plasmas, especially near the core, is needed. Fortunately we have been able to infer much about the q profiles in our Spector plasmas by using passive MHD spectroscopy. The coaxial helicity injection (CHI) process of forming our compact toroid (CT) plasmas naturally generates very hollow current profiles. This causes reverse shear magnetic configurations in our early plasmas. Central Ohmic heating naturally peaks the temperature and thus current profiles as our plasmas evolve in time. This peaking of the current profile leads to a simultaneous reduction of the core safety factor (q(0)) and a reduction in the reverse shear. As the central, non-monotonic q profile hits rational flux surfaces, we observe on both edge magnetic probes and soft X-ray diagnostics transient magnetic reconnection events (MRE’s) due to the double tearing mode. Modal analysis and observations of the changes in these MRE’s as we change the currents in our plasmas allows us to infer the q surfaces involved in each burst. Many plasma discharges have several MRE’s in succession allowing us to estimate the continuous evolution of the core q profile in our shots. This information greatly enhances our certainty of the overall q profile when combined with edge magnetic probes.
Peter O’Shea, Michel Laberge, Alex Mossman, Meritt Reynolds, Aaron Froese. Passive MHD Spectroscopy For Augmenting Magnetic Reconstructions on Spherical Tokamak Plasmas at General Fusion. Exploratory Plasma and Fusion Research Workshop, Vancouver, Canada. August 3, 2017