Ducted kinetic Alfvén waves in plasma with steep density gradients

Abstract

Given their high plasma density (n ∼ 1013 cm−3), it is theoretically possible to excite Alfvén waves in a conventional, moderate length (L ∼ 2 m) helicon plasma source. However, helicon plasmas are decidedly inhomogeneous, having a steep radial density gradient, and typically have a significant background neutral pressure. The inhomogeneity introduces regions of kinetic and inertial Alfvén wave propagation. Ion-neutral and electron-neutral collisions alter the Alfvén wave dispersion characteristics. Here, we present the measurements of propagating kinetic Alfvén waves in helium helicon plasma. The measured wave dispersion is well fit with a kinetic model that includes the effects of ion-neutral damping and that assumes the high density plasma core defines the radial extent of the wave propagation region. The measured wave amplitude versus plasma radius is consistent with the pile up of wave magnetic energy at the boundary between the kinetic and inertial regime regions.