Time-dependent simulation of lower hybrid current drive in JET discharges

In this paper we report on simulations of lower hybrid current drive (LHCD) in JET closely comparing the simulation results to the available experimental data. The simulations are performed all over the relevant discharge duration by ASTRA. The LHCD module, FRTC, is based on a standard ray-tracing Fokker-Planck model. The purpose of the paper is to understand the present LHCD experiments issues within the limit of the LH linear propagation model. These issues are: (i) analysis of non-resonant collisional absorption (NRCA) of LH wave power in the main JET plasma during the current ramp-up phase and in steady-state (SS) scenarios, (ii) the lack of penetration of LHCD in high-density plasmas, (iii) current diffusion during the LHCD-assisted current ramp-up and (iv) assessment of the current profile alignment in JET SS discharges in the presence of LHCD. In recent experiments from FTU, JET and C-MOD, LHCD effects at high plasma density are either completely absent or less than expected. It has been shown, both in FTU and ALCATOR-C-MOD, that NRCA of LH wave power can be responsible for that. Indeed NRCA is estimated to be small in JET plasmas, at least in the main heating phase and therefore it is not expected to be responsible for the lack of penetration of LHW in high-density JET plasmas, however here we show for the first time that it can be effective during the early phase of the current ramp-up, when the plasma is still collisional. On the contrary it is suggested that the reduction of LHCD effects at high density may be attributed at least partially to the loss of accessibility of the n spectrum effectively launched into the plasma. Furthermore it is shown that the linear propagation model provide very broad and stable LH current density profiles, with no need to include any non-linear spectral broadening. The current diffusion during the LHCD-assisted current ramp-up is investigated and a careful comparison between the simulated q-profiles and the measured ones is performed over time. The implications of the observed difference are discussed. Finally the important question of the alignment of all the current profile components is analysed in JET SS discharges at high βN.