Theory, control, and use of a fast ion axial bounce mode on C-2W

In the C-2W[1] device at TAE Technologies, beam-injected fast ions can excite axisymmetric waves in field-reversed configuration (FRC) and magnetic mirror plasmas. Dedicated experiments in mirror plasmas demonstrate the wave frequency is set by the fast ion axial bounce motion (see related poster by S. Karbashewski), suggesting the magnetic fluctuations may be caused by clumping in axial bounce phase.

To test this hypothesis, a model using a clumped distribution of current rings which bounce axially in a background magnetic field was developed. This model reproduces the main features observed in the experiment including harmonics with alternating axial parity and decreasing amplitude outside the turning point. The ratio of the axial and radial perturbation observed on magnetic probes near the wall is approximately matched after accounting for vessel image currents.

The drive for this mode is sensitive to the fast ion axial bounce period distribution. Staggering the voltages of the 8 neutral beam injectors reduced the mode amplitude by about 75% in an FRC plasma. Energy confinement time was not improved, indicating this mode does not adversely affect plasma performance.

The internal field in high-beta plasmas deviates substantially from the vacuum field, and C-2W lacks an internal field measurement. The frequency of this mode may serve as an additional constraint on reconstructed equilibria enabling the plasma state to be determined with greater fidelity.

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