Connecting the separatrix operational space (SepOS) framework to separatrix and turbulence measurements in DIII-D L and H mode plasmas

The separatrix operational space (SepOS) framework [1] for tokamak confinement mode classification is compared with separatrix and turbulence measurements in DIII-D L and H mode plasmas, showing partial quantitative agreement for the L-H transition boundary and qualitative agreement for the fluctuation characteristics within each confinement mode, including shots with both favorable and unfavorable grad-B drift directions. Experimentally-measured line-integrated magnetic and density fluctuations yield power spectra, amplitude ratios, and cross-phases within each of the confinement modes that are compared with predictions from a kinetic eigenvalue analysis. It is found that the radial line-integrated fluctuation measurements are qualitatively consistent with the temporal evolution predicted by the linear analysis (i.e., low-frequency amplitude ratio in L-mode decreases with increasing density and cross-phase magnitude is close to 90º). Notably, the similarity of the separatrix parameters and power spectra in Type-III ELMy H-mode and high n_e (post-H) L-mode suggests a common origin for these fluctuations. The research represents the first partially successful attempt to apply the SepOS methodology to DIII-D plasmas, connecting variation in the separatrix parameters to the suppression and onset of L-mode turbulence.

This work is supported by US DOE under DE-FC02-04ER54698 and DE-SC0019004.

[1] T. Eich, et. al., 2021 Nucl. Fusion 61 086017

[2] J. Chen, et al., 2024 Nucl. Fusion 64 086054