Compressional Alfven Eigenmodes on MAST

Magnetic fluctuations at frequencies omega less than or similar to omega(ci) driven by neutral-beam injection heating and identified as compressional Alfven eigenmodes (CAEs) have been observed on MAST. The measured toroidal mode numbers are in the range 4 < vertical bar n vertical bar < 10 and waves rotate in both co- and counter-current directions. The frequency variation is consistent with an Alfvenic 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 similar to 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 omega = omega(ci) and omega(ci)/2 consistently with previous observations on START and DIII-D suggesting that the CAEs exist in two distinct ranges of k(parallel to). Calculations of CAEs suggest that the modes are localized at r/a similar to 0.5. The modes form within a potential well due to the variation of (nq/kappa rho)(2), and are not directly influenced by variations in nu(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(perpendicular to) and 1/upsilon(A)