Characterization of L-H transition density branches in JET D-T plasmas through a power balance analysis

The dependence of L-H transition power threshold on plasma density is well documented and captured by the "ITPA 2008 scaling"[1] for high density D plasmas. In view of ITER operations, several studies with different H isotopes resulted in a 1/A mass dependence of the threshold [2]. Of particular interest for ITER H-mode access at low auxiliary power is the existence of a minimum L-H power threshold in density (n_{e,min}), observed also in JET with metallic wall (ILW) [3]. Below n_{e,min} (low-density branch), the power threshold is seen to increase again. The existence of n_{e,min} has been characterized in JET for different plasma species (H, D, T, He) [4]. In AUG and C-mod experiments with dominant electron heating, the appearance of n_{e,min} has been explained by the existence of an ion heat flux threshold, linear in density, to be exceeded for H-mode access. The inefficiency of electron-ion coupling in the low-density branch would then require increasing power to reach the ion-power threshold. Nevertheless, the linearity of the ion heat flux below n_{e,min} at the transition has not been observed in JET NBI-heated D plasmas [5]. In the recent JET D-T campaign, specific L-H transition experiments have been performed, including a plasma density scan, using NBI heating, at fixed magnetic field and plasma current. These experiments enables the characterization of density branches and n_{e,min} existence in D-T [6]. The data collected confirms the existence of n_{e,min} region in D-T plasmas. Interpretative transport simulations have been run to analyze D-T plasma behavior just before LH transition. Thanks to transport modelling, a power balance analysis is carried out to evaluate the power terms contributing to the transition, separating the power coupled to ions and to electrons. The current contribution, after characterizing density branches in JET, NBI-heated D-T plasmas, presents the results of the power balance analysis and the dependence of the ion heat flux on D-T plasma density. The results are compared to JET D plasmas and to evidences from other fusion devices.