Tomographic and centroid reconstructions of plasma emission on C-2W via enhanced 300-channel bolometry system

Abstract

The C-2W experimental device at TAE Technologies utilizes neutral beam injection and edge biasing to sustain long-lived, stable field reversed configuration (FRC) plasma. An ongoing effort is under way to optimize the electrode biasing system, which provides boundary control to stabilize the FRC. To this end, tomography offers a powerful and non-invasive technique as tomographic reconstruction of the FRC emission profile provides an important assessment of global stability. Recently, a new signal acquisition system was implemented on a bolometer array dedicated to tomography on C-2W, significantly enhancing the signal-to-noise of the collected data. The array consists of 300 simultaneously digitized photodiode channels that respond to a broad range of wavelengths, from soft x-ray to near-infrared, as well as energetic particles, yielding 180 unique lines of sight that intersect a toroidal plane of the FRC near the mid-plane. Utilizing the collected photo-signals from a set of plasma discharges in which the electrode biasing was intentionally terminated mid-shot, time-resolved reconstruction of the plasma emissivity is achieved via pixel-based 1D and 2D tomographic algorithms, revealing sharply annular profiles with a clear magnetohydrodynamic (MHD) mode structure. In addition, reconstruction of the plasma center-of-emission trajectories via a centroid algorithm applied to the same set of discharges demonstrates a cyclical plasma wobble. Crucially, both the tomography reconstruction and centroid reconstruction indicate an n = 1 toroidal mode that reverses from the electron diamagnetic direction to the ion diamagnetic direction and grows in amplitude after bias termination, qualitatively consistent with the expected stabilizing effect of electrodes.