Modification of the Alfvén spectrum by 3D density inhomogeneities

Alfvén eigenmodes driven unstable by energetic particles are routinely observed in tokamak plasmas. The most frequently observed Alfvén eigenmodes are gap modes, which consist of poloidal harmonics coupled by inhomogeneity in the magnetic field. Further coupling can be introduced by 3D inhomogeneities in the plasma density, for example, transiently during the assimilation of injected pellets. The material deposited by the pellet temporarily breaks the toroidal and poloidal symmetry of the density profile, causing additional coupling of poloidal and toroidal harmonics. This additional coupling modifies the Alfvén continuum and discrete eigenmode spectrum.

In JET, we observe significant changes to Alfvén eigenmodes during pellet injection [1]. From observed changes in the frequency of the Alfvén eigenmodes, information about the changes in the plasma density due to pellets can be inferred. To use Alfvén eigenmodes for MHD spectroscopy of pellet injected plasmas, we generalised the 3D MHD codes Stellgap [2] and AE3D [3] to incorporate 3D density profiles. Hence, we obtain the Alfvén continuum and discrete eigenmode spectrum for tokamak plasmas with pellet injection. We compare this numerical work to analytical results of mode coupling due to density inhomogeneities.