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

ビアワーゲ, アンドレアス  ,  Lauber, Philipp

57pp.116063-1 - 116063-21 , 2018-05
Description
Using the linear gyrokinetic code LIGKA, we study the structure of the continuous spectra Ω(ρ) = ω(ρ) + iγ(ρ) of shear Alfvén waves (SAW) and ion sound waves (ISW) in a high­-beta JT­60U tokamak plasma and look for evidence of Alfvén acoustic couplings or mode conversion. Here, Ω is the complex local eigenfrequency, ρ is a radial coordinate, and we consider waves with low toroidal mode number n = 3 . We focus on the frequency rangeω[BAE] <= ω <= ω[TAE] between the beta­-induced and toroidicity­-induced Alfvén frequency gaps. The real frequencies ω(ρ) of the gyrokinetic ISW continua are remarkably similar to MHD results. The kinetic damping rates are of order −γ/ω ~ 30% for Te /Ti ≈ 1.7 , and reduce to 15% when the temperature ratio is raised to Te /Ti ≈ 4.8 . It is shown that SAW and ISW continua can be simultaneously excited with an antenna and that the global response of theISWs is significantly enhanced when the on­-axis beta value is raised from β0 = 1.7% to 3.6% while keeping Te /Ti > 1 . In contrast, when the ion temperature is increased such that Te /Ti ≈ 0.4 , ISW branches become undetectable in spite of higher β0 . 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 dampingof 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 ω[BAE] <= ω <= ω[TAE] are irrelevant within the scope of the model used.

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