Measurement of edge turbulence in negative triangularity plasmas with phase contrast imaging on DIII-D

Sweeps of the upper triangularity (δ_U in [-0.4,-0.2]) in negative-triangularity [1] discharges have been performed with fixed lower triangularity (δ_L=0.2) with a variety of plasma parameters , providing a platform for the study of the edge turbulence. The Phase Contrast Imaging diagnostic [2] provides measurements of line-integrated edge turbulence with good wavenumber resolution and wide frequency bandwidth and is especially sensitive to the turbulence in the sheared edge region of high-performance plasmas. The spectra of the ion-scale turbulence seen in negative triangularity discharges, even without auxiliary heating, is similar to that seen in plasmas with significant edge shear such as H-modes rather than L-mode plasmas. Two instability regimes were identified, corresponding to pure ohmic heating and moderate beam heating. With no auxiliary heating, the turbulence both above and below the midplane was sensitive to the upper triangularity. The edge turbulence in plasmas with moderate beam heating was independent of the upper triangularity. This demonstrates that the good performance edge observed in negative-triangularity plasmas is robust and persists over a variety of parameters and turbulence regimes.

[1] Marinoni et al 2021 Nucl. Fusion 61 116010

[2] Davis et al 2016 Rev. Sci. Instrum. 87 11E117