Observation and interpretation of tornado modes coupled to near-axis Alfven cascade eigenmodes in JET sawtoothing plasmas

The coupling between upwards frequency sweeping modes and tornado modes observed in a set of JET sawtoothing discharges is investigated. The frequency sweeping modes are identified as near-axis Alfven cascade (AC) eigenmodes. These ACs are associated with a very flat but monotonic q-profile near the magnetic axis, in contrast to the usual reversed-shear scenarios. The frequency evolution of the modes during gradual decrease of the q-profile is numerically reproduced and the transition from cascade modes to tornado modes is demonstrated. The transition occurs for specific values of q on-axis given by q (0) = (n - 1/2)/n and is marked by the characteristic tip of the tornado modes' frequency evolution pattern observed in spectrograms. Based on this result a novel MHD spectroscopy technique is proposed to track the evolution of q (0) in the post sawtooth crash regime with high accuracy by using tornado mode tips as MHD markers, the only requirement being that the toroidal mode numbers are measured. Additionally, calculation of the resonant interaction between the modes and an ICRH-heated hydrogen minority population shows the ions transferring their energy to the modes, thus contributing to the mode drive.