Accurate determination of the plasma boundary-especially the Last Closed Flux Surface (LCFS)-is crucial for understanding equilibrium, optimizing confinement, and achieving effective control in magnetic confinement devices. Conventional detection methods, such as magnetic reconstructions (e.g., EFIT) and passive optical diagnostics, have limitations during transient or low-emission conditions. We present an innovative plasma boundary identification technique that uses boron powder injection in the EXL-50U spherical torus device. When the boron particles ablate at the plasma boundary, they emit a strong localized line radiation of boron in the visible spectral range, serving as a clear, real-time marker for LCFS detection. By combining precise camera calibration with boron-filter-based contrast enhancement, our method provides high spatial accuracy and fast response with minimal hardware complexity. We describe the principles of the technique, experimental setup, and data processing workflow, and compare its performance to established boundary detection methods. Lastly, we explore the potential of active boron injection for advanced plasma control in future high-performance fusion devices, emphasizing its ability to meet the demanding requirements of long-pulse and reactor-grade plasmas.
Guo et al. (Tue,) studied this question.