2024 – Investigation of Turbulence Stabilization in Negative Triangularity Plasmas

Investigation of Turbulence Stabilization in Negative Triangularity Plasmas

2024 Research Campaign, Negative Triangularity

Purpose of Experiment

Core-edge integration remains an open question for future Fusion Pilot Plant (FPP) design concepts. H-mode concepts suffer from the untenable heat-loads from Edge Localized Modes (ELMs) while ELM-free designs suffer from a lack of robustness or degraded energy confinement. Negative Triangularity (NT) offers a uniquely robust ELM-free edge solution, with promising turbulence properties. Understanding the shaping effects of negative triangularity on turbulence is critical to predicting NT performance as an FPP concept. Turbulence fluctuations have been seen to be reduced in NT plasmas in TCV and DIII-D across a variety of plasma parameters. Dedicated turbulence experiments are required to isolate the stabilizing effect of negative triangularity. This experiment will seek to shed insight into how core and edge fluctuations are reduced by NT and what determines the edge performance in NT plasmas.

Experimental Approach

2D Beam Emission Spectroscopy (BES), Ultra-Fast CHarge Exchange Recombination Spectroscopy (UF-CHERS), and Charge eXchange Imaging (CXI) will be utilized to measure the low to intermediate wavenumber density turbulence characteristic of Trapped Electron Modes (TEM) in the edge and Ion Temperature Gradient (ITG) modes in the core. Fluctuation level, correlation lengths, correlation times, ion temperature fluctuations, and toroidal rotation fluctuation measurements can be obtained. BES velocimetry allows the 2D velocity field to be inferred. Doppler Back Scattering (DBS) will measure intermediate wavenumber density turbulence and Correlation ECE (CECE) will measure low-k electron temperature fluctuations. To test the turbulence stabilization model/prediction, triangularity will be scanned from -0.2 to +0.2 in the Lower Single Null (LSN) hybrid shelf shape at constant injected power, torque, and plasma current to isolate the effect of shaping. These sweeps will be repeated for four different power compositions: Low neutral beam power below the L-H power threshold will be used to keep the plasma in L-mode in the Positive Triangularity (PT) phases, isolating turbulent dynamics without an H-mode shear layer. Higher beam power will be used to capture the shaping-induced transitions from the H-mode to the ELM-free NT-edge. Finally, two mixes of beam and RF power with different amounts of beam power will be used to lower collisionality and linearly destabilize TEM turbulence via increases to electron temperature. In each power composition, a radial scan of the fluctuation diagnostics’ locations from will be performed by repeating discharges to construct an extended radial profile.

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