Diagnosing magnetic islands using soft X-ray emission in lithium-walled spherical tokamak plasmas

The Lithium Tokamak eXperiment-𝛽 (LTX-𝛽) aims to study improved plasma performance resulting from lithium coatings on its plasma-facing components (PFCs). Previous results from the device show that progressively adding more solid lithium to the PFCs can reduce recycling coefficients to values as low as 0.5. The recycling estimates were based on a poloidal ex-vessel Lyman-alpha array that had limited coverage of the device edge. Historical results from a toroidal soft X-ray array have been analyzed through simulations to possibly diagnose the presence of magnetic islands and their effects on tearing mode activity. After simulating an emissivity profile of a 2:1 magnetic island within the tokamak, forward ray-tracing was used to find the intersections of diode sightlines through the magnetic island. From there, emissivity parameters are varied across an optimization algorithm in order to best match the simulated data to the real data collected from the previous campaign. This new approach also allows for the refinement of previous simulated emissivity models, and ultimately, the identification and pinpointing of magnetic islands in real-time when combined with soft X-ray detectors. This exercise will facilitate real-time control of tearing activities and disruption prediction in the near future. A planned poloidal detector design that utilizes a re-entrant system will also be presented, along with any initial data collected from the LTX-𝛽 campaign.