Power balance analysis at the L-H transition in JET-ILW NBI-heated deuterium plasmas

Abstract The understanding of the physics underlying the L-H transition has strong implications for ITER and DEMO. In many tokamaks, including JET, it has been observed that, at a particular plasma density, ne,min, the power necessary to access H-mode PL-H is minimum. In the present work L-H transitions of JET deuterium plasmas heated by neutral beam injection (NBI) are studied for the first time by means of a power balance analysis to characterize the main contributions in the transition, through integrated transport modelling. In the analysed pulses, we do observe a minimum of the L-H power threshold in density, indicating the presence of density branches and of ne,min. Electron and ion heat fluxes at the transition are estimated separately. The electron/ion equipartition power results to be in favour of the ions, as shown by QuaLiKiz quasilinear gyrokinetic simulations, which predict a larger ion transport that causes Te > Ti. The resulting edge ion heat flux also shows a clear change of slope below ne,min, similarly to ASDEX-Upgrade NBI pulses [Ryter F. et al 2014 Nucl. Fusion 54 083003]. JET NBI data are compared to radio-frequency heated ASDEX-Upgrade and Alcator C-mod pulses [Schmidtmayr M. et al 2018 Nucl. Fusion 58 056003], showing a different trend of the power coupled to ions at the L-H transition with respect to the linearity observed in the radio-frequency heated plasmas. The presence of ne,min and the role of the ion heat flux is discussed in the paper, although it seems not possible to explain the presence of a PL-H minimum in density by a critical ion heat flux and by the equipartition power for the JET NBI-heated plasmas analysed.