Using the linear gyrokinetic code LIGKA, we study the structure of the continuous spectra of shear Alfvén waves (SAW) and ion sound waves (ISW) in a high-beta JT-60U tokamak plasma and look for evidence of Alfvén acoustic couplings or mode conversion. Here, is the complex local eigenfrequency, ρ is a radial coordinate, and we consider waves with low toroidal mode number . We focus on the frequency range between the beta-induced and toroidicity-induced Alfvén frequency gaps. The real frequencies of the gyrokinetic ISW continua are remarkably similar to MHD results. The kinetic damping rates are of order for , and reduce to when the temperature ratio is raised to . It is shown that SAW and ISW continua can be simultaneously excited with an antenna and that the global response of the ISWs is significantly enhanced when the on-axis beta value is raised from to 3.6% while keeping . In contrast, when the ion temperature is increased such that , ISW branches become undetectable in spite of higher . At the same time, a large part of the SAW continuum is locally destabilized by ion temperature gradients and a set of discrete global modes was found, some of which are weakly damped or unstable and interpreted as kinetic beta-induced Alfvén eigenmodes. It is estimated that the kinetic damping of such low-n Alfvénic modes contributes much more to the anomalous bulk ion heating than the excitation of nearby ISW continua, so that Alfvén acoustic couplings in the frequency band are irrelevant within the scope of the model used.
Theoretical studies of low-frequency Alfvén modes in tokamak plasmas