Compressional alfvén eigenmodes on MAST

Magnetic fluctuations at frequencies ω ≲ ω_ci driven by neutral-beam injection heating and identified as compressional Alfvén eigenmodes (CAEs) have been observed on MAST. The measured toroidal mode numbers are in the range 4 < |n| < 10 and waves rotate in both co- and counter-current directions. The frequency variation is consistent with an Alfvénic scaling, and modes are elliptically polarized with a significant magnetic field component aligned parallel to the equilibrium field. Frequency clustering of modes occurs on three frequency scales. At the finest scale there are multiple modes each separated by a constant frequency ∼10-20 kHz; this is shown to be a result of modulation by low-frequency tearing modes. A larger scale frequency splitting exists in the range 100-150 kHz; these have consecutive toroidal mode numbers and are in agreement with numerical modelling. Finally, modes exist at frequencies close to ω = ω_ci and ω_ci/2 consistently with previous observations on START and DIII-D suggesting that the CAEs exist in two distinct ranges of k _∥. Calculations of CAEs suggest that the modes are localized at r/a ∼ 0.5. The modes form within a potential well due to the variation of (nq/κρ)², and are not directly influenced by variations in v_A. This is distinct from observations based on ion cyclotron emission in conventional aspect ratio tokamaks which indicate that CAE modes occur closer to the plasma edge and that their existence relies on a competition between k_⊥ and 1/v_A.