Investigation of large amplitude UHR localized oscillation and its influence on reduction of direct X-B mode conversion efficiency for high-β NSTX in nonlinear regime

Abstract

The electron Bernstein wave (EBW) is typically the only wave in the electron cyclotron (EC) range that can be applied in spherical tokamaks for heating and current drive (H&CD). Spherical tokamaks (STs) operate generally in high-regimes, in which the usual EC ordinary (O) and extraordinary (X) modes are cut off. But, nowadays, ever increasing needs and capabilities in heating of current and future magnetic fusion plasmas motivate expansion of understanding for high power sources and approaching the nonlinear regime. Among many motivating experiments, one of the most relevant and actively studied in the regime for EBW heating is National Spherical Torus Experiment (NSTX). As we previously investigated the existence of EBWs at nonlinear regime1, undoubtedly the next step would be the discovering and also explaining the probable nonlinear phenomena which are predicted to be occurred within the high levels of injected power. In this paper, we demonstrate a very special type of large amplitude electron plasma oscillation known as localized upper hybrid (UH) mode. It is shown that the mutual synergetic interaction of EBW and the localized UH mode can significantly shift the resonance layer about Δx∼0.9mm compared to the linear theory prediction and consequently can explain the considerable reduction of conversion value around 35% observed in our modelling. This reduction is due to scale up of density scale length, L n , at the new UHR location followed by increase of Budden parameter, η, which varied from 0.18 predicted by linear aspect to 0.4 in new position of UHR layer obtained by our modelling. Moreover, the parametric instabilities and dispersion of localized UH mode, approximately 7mm due to the finite electron temperature account, are also observed which have an important contribution in reduction of conversion efficiency2.