Towards a Direct Internal Magnetic Field Measurement of a Translating Field-Reversed Configuration Plasma on C-2W

In TAE Technologies' current experimental device, C-2W (also called ”Norman’’) [1], record breaking, advanced beam-driven field reversed configuration (FRC) plasmas are produced and sustained in steady state utilizing variable energy neutral beams (15 - 40 keV, total power up to 20 MW), expander divertors, end bias electrodes, and an active plasma control system. Direct measurements of the FRC magnetic field topology are useful for validation of theoretical models and simulations but present an experimental challenge, as diagnostic probes generally disrupt plasma performance and suffer damage in the energetic core. In an effort to measure the internal magnetic field with minimal degradation of both probe and plasma, a novel experiment has been developed in which the programmable C-2W magnet system is utilized to rapidly translate an FRC past a custom insertable B-dot array. The array measures the axial and radial components of the magnetic field at four simultaneous locations along a radial axis and can be inserted via a motorized actuator to any position from the vessel wall to within the FRC null radius. In situ corrections for small rotational misalignments in the B-dot windings are achieved via calibration against a vacuum field eddy current model. In a preliminary deployment, the probe detects deformation in the poloidal field structure of the scrape-off layer that is consistent with a simple equilibrium FRC model. Diagnostic and experimental considerations for probe survivability, acquisition electronics protection, and future direct measurement of field reversal inside the null radius are addressed.

[1] Gota et al, "Overview of C-2W: High-temperature, steady-state beam-driven field-reversed configuration plasmas", Nucl. Fusion 61 106039 (2021).