Understanding the L-H transition isotope effect in DIII-D*

Recent database and gyro-kinetic analysis of DIII-D plasmas in low-confinement mode (L-mode) just before transition to high-confinement mode (H-mode) have identified decreased carbon impurity content as the dominant isotope effect responsible for increasing the L-H power threshold (P_LH) in hydrogen at ITER-relevant low collisionality. Increased Z_eff in deuterium, due to enhanced (mass dependent) physical and chemical sputtering of graphite (carbon) from diverter and main chamber tiles, is found to increase the ITG critical gradient, stabilizing ITG modes [1]. Gyro-fluid and gyro-kinetic simulations capture the observed impurity isotope effect and its influence on electron and ion heat transport and power loss, via the TGLF and CGYRO codes, respectively. Edge simulations identify subdominant, intrinsic main ion mass effects due to electron non-adiabaticity [2] and differences in normalized E⨉B shear, that also contribute significantly to the isotope scaling of P_LH. At high collisionality, no isotope effect is observed, as previously documented [3]. Measurements of density and electron temperature fluctuations using the Beam Emission Spectroscopy (BES) and Correlation Electron Cyclotron Emission (CECE) diagnostics, respectively, were found to agree well with turbulence predictions from flux matched CGYRO simulations at ρ = 0.7, 0.9, & 0.95. This comparison was made possible via advanced synthetic diagnostics. The observed P_LH reduction with low-Z impurity dilution opens the important prospect of improving H-mode access in ITER hydrogen plasmas via Ne or N seeding.

[1] S. Migliuolo 1992 Nucl. Fusion 32 1331

[2] Belli, E. A. et al 2020 PRL 125.1 015001

[3] Z. Yan et al. 2017 Nucl. Fusion 57, 126015