The Electron Cyclotron Emission Imaging (ECEI) diagnostic was used to observe a finite-n interchange mode structure in the edge of negative triangularity shaped plasmas on DIII-D. At a small negative triangularity ( = −0.2), the plasma is in the H-mode with ELMs that are triggered by a low-n interchange mode. At a larger negative triangularity ( = −0.4) and low NBI power (2 MW), a dithering oscillation is observed that is triggered by a low-n interchange mode, whereas at higher NBI power (>2 MW), the edge reverts to L-mode and the low-n interchange mode is present continuously. In all cases, the edge pressure gradient is clamped when the interchange mode is present. It is concluded that the low-n interchange mode prevents the plasma from transitioning to H-mode at a large negative triangularity. This agrees with linear BOUT++ simulations which suggest that the interchange-type MHD can be a resistive ballooning mode whereby resistivity can significantly increase the finite-n ballooning mode growth rate. The absence of H-mode at large negative triangularity can, thus, be explained by the excitation of low-n pressure driven resistive ballooning modes in the plasma edge.
Understanding the negative triangularity ELM trigger and ELM free state on DIII-D with ECE-imaging
Note: This paper is part of the Special Collection: Papers from the 64th Annual Meeting of the APS Division of Plasma Physics.
Note: Paper GI2 6, Bull. Am. Phys. Soc. 67 (2022).
G. Yu, Z. Li, G. Kramer, F. Scotti, A. O. Nelson, A. Diallo, C. Lasnier, M. E. Austin, X. Qin, Y. Chen, Y. Zheng, Y. Zhu, N. C. Luhmann; Understanding the negative triangularity ELM trigger and ELM free state on DIII-D with ECE-imaging. Phys. Plasmas 1 June 2023; 30 (6): 062505. https://doi.org/10.1063/5.0144711
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