Gyrokinetic analysis of low-n shear Alfven and ion sound wave spectra in a high-beta tokamak plasma

Using the linear gyrokinetic code LIGKA, we study the structure of the continuous spectra Omega(rho) = omega (rho) + i gamma (rho) of shear Alfvn waves (SAW) and ion sound waves (ISW) in a highbeta JT-60U tokamak plasma and look for evidence of Alfvn acoustic couplings or mode conversion. Here, Omega is the complex local eigenfrequency, rho is a radial coordinate, and we consider waves with low toroidal mode number n = 3. We focus on the frequency range omega(BAE) less than or similar to omega less than or similar to omega(TAE) between the beta-induced and toroidicity-induced Alfvn frequency gaps. The real frequencies omega(rho) of the gyrokinetic ISW continua are remarkably similar to MHD results. The kinetic damping rates are of order -gamma/omega similar to 30% for Te/Ti approximate to 1.7, and reduce to 15% when the temperature ratio is raised to T-e/T-i approximate to 4.8. 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 beta(0) = 1.7% to 3.6% while keeping T-e/T-i > 1. In contrast, when the ion temperature is increased such that T-e/T-i approximate to 0.4, ISW branches become undetectable in spite of higher beta(0). 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 Alfvn eigenmodes. It is estimated that the kinetic damping of such low-n Alfvnic modes contributes much more to the anomalous bulk ion heating than the excitation of nearby ISW continua, so that Alfvn acoustic couplings in the frequency band omega(BAE) less than or similar to omega less than or similar to omega(TAE) are irrelevant within the scope of the model used.