A Statistical Analysis of Disruptions in C-2W

In TAE's C-2W machine [1], a high-beta field-reversed configuration (FRC) is sustained within a magnetic mirror. Various mechanisms can cause rapid (<1 ms) collapse of the high-beta FRC state into a lower-beta mirror state, herein referred to as a plasma disruption. While damage from disruptions in an FRC reactor would be greatly reduced when compared with a tokamak reactor, avoiding and/or controlling them is still an important operational goal.

The large number of plasma discharges (>10,000) in a recent configuration of C-2W permits a statistical analysis on the nature of disruptions. Probability density functions are generated for many system diagnostics and plasma parameters at multiple times leading up to disruptions. It is found that the majority of C-2W disruptions result from a fast growing, low frequency "wobble" or "shift" mode (even parity in Bz) with n=1 character. Growth of the mode is associated with the loss of plasma rotation by end-biasing, a known key ingredient to sustain high-beta plasmas. The classic FRC tilt mode is NOT suspected due to sub-Alfvénic identification of the mode on magnetics and interferometry, precedent decrease of the S*/E parameter, and the loss of plasma radially rather than axially. Evidence of this claim is presented along with potential strategies to avoid the observed disruptions.

[1] H. Gota et al., Nucl. Fusion 61, 106039 (2021).