Diagnosing the MHD stability of an FRC with neutral beam shine through on C-2W

Heating, current drive, and partial fueling from neutral beam injection are essential to sustainment of C-2W field-reversed configuration (FRC) plasmas.¹ Eight beams are injected off-axis, just outside of the axial center of the FRC. After traversing the plasma, the uncaptured components of each beam interface with a secondary electron emission (SEE) detector array installed in the beam dump and the shine through can be measured. Since beam capture is dependent on plasma density, any changes in density within the beam path are reflected in the measurement. Due to the geometry of beam injection, changes outside of the FRC core can be monitored. During the n = 1 and 2 instabilities, the plasma wobbles and the density of the plasma in the beam path changes. Without effective stability control, it has been seen that when beam shine through reaches a minimum, the FRC plasma appears to start shrinking and the n = 1 instability grows, after which the FRC dies out due to the instability or keeps rotating with somewhat reduced/saturated mode growth.

[1] M.W. Binderbauer et al., AIP Conf. Proc. 1721, 030003 (2016).