Henry James Oliver, Institute for Fusion Studies, UT-Austin

"Modification of the Alfven wave spectrum by pellet injection"

Abstract: Pellets of frozen deuterium ice are injected into tokamak plasmas to refuel the plasma core. Pellets typically break down within a few milliseconds, but the assimilation of this deposited material is not well understood. Existing diagnostics can not resolve the 3D evolution of the plasma density on the necessary timescales. We explore whether MHD spectroscopy can reveal information about the dynamics of the pellet material. Alfvén eigenmodes driven unstable by energetic particles are ubiquitous in tokamak plasmas. In JET, the frequencies of the Alfvén eigenmodes drop dramatically when a pellet is injected. From the dependence of the wave frequency on the plasma density, we can infer the initial density of the pellet ablation cloud. From the evolution of the wave frequency with time, we can determine the time at which the plasma density is poloidally homogeneous.