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

摘要

Using the linear gyrokinetic code LIGKA, we study the structure of the continuous spectra Q(p) = Lj(p) + i-y(p) of shear Alfven waves (SAW) and ion sound waves (ISW) in a high-beta JT-60U tokamak plasma and look for evidence of Alfven acoustic couplings or mode conversion. Here, Ω is the complex local eigenfrequency, p is a radial coordinate, and we consider waves with low toroidal mode number n = 3. We focus on the frequency range ω_(BAE)≤ ω≤ω_(TAE) between the beta-induced and toroidicity-induced Alfven frequency gaps. The real frequencies ω(p) of the gyrokinetic ISW continua are remarkably similar to MHD results. The kinetic damping rates are of order -γ/ω ~ 30% for T_e/T_i ≈ 1.7, and reduce to 15% when the temperature ratio is raised to T_e/T_i ≈ 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 β_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 ≈ 0.4, ISW branches become undetectable in spite of higher β_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 Alfven eigenmodes. It is estimated that the kinetic damping of such low-n Alfvenic modes contributes much more to the anomalous bulk ion heating than the excitation of nearby ISW continua, so that Alfven acoustic couplings in the frequency band ω_(BAE)≤ ω≤ω_(TAE) are irrelevant within the scope of the model used.