Linear and Nonlinear Kinetic Alfv\'en Wave Physics in Cylindrical Plasmas

Abstract

Kinetic Alfv\'en Waves (KAWs) are generated in magnetized space and laboratory plasmas due to a continuous shear Alfv\'en wave (SAW) spectrum and, unlike SAWs, are characterized by microscale perpendicular structures of the order of the thermal ion Larmor radius. This has important consequences on heating, acceleration and transport processes connected with KAWs. Historically, KAWs generation by mode conversion of SAWs in laboratory plasmas and their strong damping/absorption right after SAW mode conversion have been investigated for plasma heating. Here, we focus on the opposite limit: a mode converted KAW weakly absorbed in a periodic magnetized plasma cylinder. We show that a KAW may be excited as resonant cavity mode in the region between the magnetic axis and the SAW resonant layer generated externally by an antenna launcher; this process is qualitatively similar to mode converted electron Bernstein waves. In this way, large amplitude KAWs may be generated time asymptotically with relatively small coupled antenna power. This case has little or no relevance for plasma heating but interesting nonlinear implications for plasma equilibrium. In particular, we demonstrate that KAWs may generate convective cells (CCs) by modulational instability, that a consequence of plasma nonuniformity is the azimuthal symmetry breaking due to plasma diamagnetic effects, that the modulational instability growth rate is enhanced over the corresponding uniform plasma limit, that the unstable parameter space is extended, and that the cylindrical geometry causes a complex interplay between nonlinearity and nonuniformity. As a result, we show that it is possible to control the CC radial structures and the corresponding parallel electric field generation not only by means of the antenna frequency but also by fine tuning of its amplitude.Comment: 117 pages, 42 figure