Tearing mode observation and stabilization in C-2W

The C-2W device (also known as 'Norman') at TAE Technologies has proven successful at generating stable, long-lived field-reversed configuration plasmas (FRCs) with record temperatures. [1] We report on the observation of axisymmetric periodic deformations of the FRC in external magnetic field and internal electron density measurements in C-2W that appear consistent with the tearing mode. [2] Effects of this mode on particle confinement based on bolometry measurements are presented. Importantly, the mode is found to stabilize when the FRC separatrix radius is close enough to the conducting vessel wall. The empirical stabilization condition for these fluctuations as a function of the ratio of the FRC radius to that of the vessel wall is demonstrated to agree well with theoretical values. Finally, the consequences of these observations for the hypothesis that the tearing mode mediates FRC equilibrium are discussed.