Lower hybrid wave coupling and propagation in the presence of nonthermal edge electrons

Abstract

In order to improve our understanding of lower hybrid current drive experiments in high power regimes, where fast electrons (up to few keV) are generated in front of the lower hybrid (LH) grill, an analysis of LH wave coupling and propagation is carried out using a formulation of the dielectric tensor that incorporates the fast electron population (Thot≲5 keV). It is found that both the real and imaginary parts of the dielectric tensor are modified for waves with |N∥|≳10, which can interact with the fast electrons through Landau damping. Consequently high N∥ waves can be absorbed within less than 1 cm, or even cut-off at the grill–plasma interface. Antenna–plasma coupling code results show that the power available to accelerate electrons in front of the grill (launched power with |N∥|>10) decreases in the presence of fast electrons. Also, despite strong modifications of the dielectric tensor, and of the power transfer to high N∥ waves, the presence of fast electrons (Thot≲5 keV) leads to only moderate changes of antenna reflectivity and directivity.