Z_eff dependence of the L-H power threshold in ITER-similar-shape hydrogen and deuterium plasmas, and implications for reducing P_LH in ITER

Review of a database of >500 DIII-D L-H transitions near ITER Pre-Fusion Power-Operation (PFPO-1) conditions (ITER-similar shape (ISS), low collisionality, near-balanced beam torque) has found a strong reduction of the L-H transition power threshold with increasing effective ion charge (Z_eff) in both hydrogen and deuterium plasmas. This result suggests that up to 50% reduction of the L-H power threshold may be possible in ITER hydrogen plasmas by modest seeding of light impurities (Z_eff~1.7). This trend was found after investigating different physics mechanisms (electron non-adiabaticity, ExB shear, collisionality) that can contribute to increasing thermal transport with smaller isotope mass [1]. Comparison of thermal fluxes to predictions from TGLF quasi-linear gyro-fluid simulations and CGYRO gyro fluid modeling in hydrogen and deuterium indicate the difference in heat flux is attributed to the different carbon content, with increased Z_eff in D stabilizing ITG/TEM turbulence by increasing the critical ion temperature gradient [2,3] via main ion dilution and Landau damping by impurities.

[1] Belli, E. Physics of Plasmas (2019): 082305

[2] G.M. Staebler et al 2021 Nucl. Fusion 61 116007

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