Negative triangularity plasma shapes and the Alfvén slow-magnetosonic wave coupling

The coupling between the Alfvén and slow magnetosonic waves can be modified by the plasma shaping. This can have a profound impact on the available number of eigenmodes, their stability, and potential impact on tokamak performance. Alfvén waves form a continuous spectrum with gaps when different poloidal harmonics have the same frequency. In finite pressure plasmas, slow magnetosonic waves can couple to the Alfvén waves and modify the continuous spectrum, especially at the lower frequencies as was shown in [1] using ideal MHD modeling. The coupling is governed by the geodesic magnetic curvature which depends on the shape of the magnetic field. The possibilities to manipulate the low frequency eigenmode spectrum, from zero up to the TAE frequency, are explored by varying the ellipticity and triangularity of the plasma shape. Simulations indicate that both ellipticity and triangularity change the eigenmode spectrum but changes in triangularity are more effective. For a DIII-D like plasma it was found that at moderate negative triangularity (-0.4) the number of potential eigenmodes was minimal. These simulation results can be tested experimentally during the upcoming negative triangularity experiments in DIII-D.

[1] C.Z. Cheng et al. Phys. Plasmas 26 082508 (2019)