The role of non-resonant collision dissipation in lower hybrid current driven plasmas

In this paper the role of non-resonant collision absorption (NRCA) of lower hybrid (LH) wave power is analysed using a combined ray-tracing Fokker–Planck model. The analysis shows how much and under what conditions NRCA affects the current drive efficiency (CDE) and the effective ability of LH wave to penetrate into the plasma thus generating a fast electron tail. In all LH experiments the CDE is shown to increase as a function of the volume-averaged electron temperature up to a saturation level. Such dependence can be attributed to NRCA, at the plasma periphery, that turns out to be more relevant, the lower the plasma temperature. Furthermore, NRCA is shown to be a key issue in interpreting recent results on LHCD experiments on FTU, where a new regime is found of high-density discharges with a hot edge, where clear signs of fast electron tail generation are present. In standard high-density discharges with a cold edge conversely, LHCD effects are not observed. The numerical calculations reported here clearly show that the difference between these two regimes can be entirely attributed to the difference in NRCA at the plasma periphery. In conclusion, the paper shows that collision absorption can be responsible for the degradation of the CDE in low-temperature plasmas and can completely prevent the penetration of LHCD in high-density cold edge plasmas.