On the vertical stability of DIII-D discharges with strong negative triangularity

Negative triangularity (NT) plasmas feature larger Shafranov shifts and more elongated inner flux surfaces than their positive triangularity counterparts, increasing the drive for n=0 modes associated with vertical stability. Interpretive modeling shows that coupling with a non-conformal vessel wall can reduce the growth rates of this instability and help to enable the control of diverted NT discharges on DIII-D [1, 2]. However, due to limitations of the poloidal field coils on DIII-D, increased beta reduces the controllable parameter space for DIII-D NT plasmas, necessitating the development of more advanced control algorithms in the absence of improved hardware. The addition of diagnostic noise and power supply tuning to the TokSys model are needed to capture the time dependent behavior of DIII-D NT discharges, which is able to reproduce experimentally measured growth rates with errors of no more than ∼20%. Implications of these findings for NT reactor design are also discussed.

[1] J. Song et. al. Nucl. Fusion 61 096033 (2021)

[2] A. O. Nelson et. al. Plasma Phys. Control. Fusion 65 044002 (2023)