F. Palermo

Combined action of phase-mixing and Landau damping causing strong decay of geodesic acoustic modes

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 k r . 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).

Transport barriers associated to the resonant interaction between trapped particle modes triggered by plasma polarization injection

The Letter explores the parametric excitation of low-frequency zonal flow induced by the beating of collisionless trapped ion and trapped electrons modes (CTIMs and CTEMs) and its feedback on the formation of transport barrier (TB). The zonal flow generation and the associated intermittent transport are investigated using a semi-Lagrangian gyrokinetic Vlasov simulations based on a Hamiltonian reduction technique, where both the fastest scales (cyclotron and bounce motions) are gyro-averaged. The model confirms the possibility to trigger TB by means of polarization effects, recently observed in global gyrokinetic simulations by A. Strugarek et al, Phys. Rev. Lett. 111 (2013) 145001.

Cross-code gyrokinetic verification and benchmark on the linear collisionless dynamics of the geodesic acoustic mode

The linear properties of the geodesic acoustic modes (GAMs) in tokamaks are investigated by means of the comparison of analytical theory and gyrokinetic numerical simulations. The dependence on the value of the safety factor, finite-orbit-width of the ions in relation to the radial mode width, magnetic-flux-surface shaping, and electron/ion mass ratio are considered. Nonuniformities in the plasma profiles (such as density, temperature, and safety factor), electro-magnetic effects, collisions, and the presence of minority species are neglected. Also, only linear simulations are considered, focusing on the local dynamics. We use three different gyrokinetic codes: the Lagrangian (particle-in-cell) code ORB5, the Eulerian code GENE, and semi-Lagrangian code GYSELA. One of the main aims of this paper is to provide a detailed comparison of the numerical results and analytical theory, in the regimes where this is possible. This helps understanding better the behavior of the linear GAM dynamics in these different...

Decay of geodesic acoustic modes due to the combined action of phase mixing and Landau damping

Geodesic acoustic modes (GAMs) are oscillations of the electric field whose importance in tokamak plasmas is due to their role in the regulation of turbulence. The linear collisionless damping of GAMs is investigated here by means of analytical theory and numerical simulations with the global gyrokinetic particle-in-cell code ORB5. The combined effect of the phase mixing and Landau 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. When plasma parameters characteristic of realistic tokamak profiles are considered, the GAM decay time is found to be an order of magnitude lower than the decay due to the Landau damping alone, and in some cases of the same order of magnitude of the characteristic GAM drive time due to the nonlinear interaction with an ion-temperature-gradient (ITG) mode. In particular, the radial mode structure evolution in time is investigated here an...

Radial acceleration of geodesic acoustic modes in the presence of a temperature gradient

The global dynamics of geodesic acoustic modes (GAMs) is studied analytically and by means of gyrokinetic simulations for several equilibria with flat and nonuniform profiles. In particular, the effects of phase mixing via the continuum spectrum in the presence of a temperature gradient are investigated. We show that the frequency of GAM is not constant but can evolve in time because of the increase of the radial wavenumber. As a consequence, also the radial velocity of GAM increases in time. Thus, this study reduces the discrepancy between the linear theory and the experiments, in which strong velocities of GAM are generally observed. An estimate of phase and group velocity is given for simulations performed with experimental parameter values.