Volume 115, Number 1, July 2016
|Number of page(s)||6|
|Section||Physics of Gases, Plasmas and Electric Discharges|
|Published online||08 August 2016|
Combined action of phase-mixing and Landau damping causing strong decay of geodesic acoustic modes
1 Max-Planck-Institut für Plasmaphysik - Garching, Germany
2 ENEA C. R. Frascati - Via E. Fermi 45, CP 65-00044 Frascati, Italy
3 Institute for Fusion Theory and Simulation, Zhejiang University - Hangzhou, PRC
Received: 1 May 2016
Accepted: 13 July 2016
We report evidence of a new mechanism able to damp very efficiently geodesic acoustic mode (GAM) in the presence of a nonuniform temperature profile in a toroidally confined plasma. This represents a particular case of a general mechanism that we have found and that can be observed whenever the phase-mixing acts in the presence of a damping effect that depends on the wave number kr. Here, in particular, the combined effect of the Landau and continuum damping is found to quickly redistribute the GAM energy in phase-space, due to the synergy of the finite orbit width of the passing ions and the cascade in wave number given by the phase-mixing. This damping mechanism is investigated analytically and numerically by means of global gyrokinetic simulations. When realistic parameter values of plasmas at the edge of a tokamak are used, damping rates up to 2 orders of magnitude higher than the Landau damping alone are obtained. We find in particular that, for temperature and density profiles characteristic of the high confinement mode, the so-called H-mode, the GAM decay time becomes comparable to or lower than the nonlinear drive time, consistently with experimental observations (Conway G. D. et al., Phys. Rev. Lett., 106 (2011) 065001).
PACS: 52.65.Tt – Gyrofluid and gyrokinetic simulations / 52.35.Lv – Other linear waves / 52.35.Fp – Electrostatic waves and oscillations (e.g., ion-acoustic waves)
© EPLA, 2016
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