1,721,255 research outputs found
Short wavelength geodesic acoustic mode excitation by energetic particles
No full textTaking the collisionless damping of the geodesic acoustic mode (GAM) in the short wavelength limit originally investigated in [Z. Qiu et al., Plasma Phys. Controlled Fusion 51, 012001 (2009)] as an example, the physics processes underlying wave particle resonances in the short wavelength limit are clarified. As an illustrative application, GAM excitation by energetic particles in the short wavelength limit is investigated assuming a single pitch angle slowing-down fast ion equilibrium distribution function. Conditions for this energetic particle-induced GAM to be unstable are discussed. © 2018 Author(s)
Effects of energetic particles on zonal flow generation by toroidal Alfvén eigenmode
No full textGeneration of zonal flow (ZF) by energetic particle (EP) driven toroidal Alfvén eigenmode (TAE) is investigated using nonlinear gyrokinetic theory. It is found that nonlinear resonant EP contribution dominates over the usual Reynolds and Maxwell stresses due to thermal plasma nonlinear response. ZF can be forced driven in the linear growth stage of TAE, with the growth rate being twice the TAE growth rate. The ZF generation mechanism is shown to be related to polarization induced by resonant EP nonlinearity. The generated ZF has both the usual meso-scale and micro-scale radial structures. Possible consequences of this forced driven ZF on the nonlinear dynamics of TAE are also discussed. © 2016 Author(s)
Symmetry breaking of ion temperature gradient mode structure: From local to global analysis
No full textThe 2D mode structure symmetry breaking of the ion temperature gradient mode is studied analytically and numerically based on the mixed initial-value-eigenvalue approach, the Gyrokinetic PIC code ORB5 and the gyrokinetic Eulerian code GKW. The radial propagation and amplitude variation, in terms of the real and imaginary parts of the generalized "tilting parameter" θk, are treated in the framework of the Mode Structure Decomposition approach. The radial symmetry breaking is shown to be intimately coupled to the parallel symmetry breaking. In particular, I m { θ k } can lead to the increase in the local growth rate and the absolute value of the real frequency, as well as to parallel symmetry breaking. The complex θk can be measured in global simulations and accounted for in local simulations. This provides a way to include global effects related to the mode radial propagation and the intensity gradient in local descriptions and can be important for the study of momentum transport. © 2017 EURATOM
Nonlinear Decay and Plasma Heating by a Toroidal Alfvén Eigenmode
No full textWe demonstrate theoretically that a toroidal Alfvén eigenmode (TAE) can parametrically decay into a geodesic acoustic mode and kinetic TAE in a toroidal plasma. The corresponding threshold condition for the TAE amplitude is estimated to be |δB/B0|∼O(10-4). Here, δB and B0 are, respectively, the perturbed magnetic field of the pump TAE and the equilibrium magnetic field. This novel decay process, in addition to contributing to the nonlinear saturation of energetic-particle or α-particle driven TAE instability, could also contribute to the heating as well as regulating the transports of thermal plasmas. © 2018 American Physical Society
Kinetic effects of thermal ions and energetic particles on discrete kinetic BAE mode generation and symmetry breaking
No full textThe kinetic effects of both thermal ions and energetic particles (EPs) on global Alfvén mode generation and structure symmetry breaking are important for the understanding of resonant excitation and EP transport in tokamak plasmas. In this work, the formulation for the study of the beta induced Alfvén eigenmode (BAE) driven by EPs is derived for weakly coupled poloidal harmonics with finite Larmor radius (FLR) and finite orbit width (FOW). Agreement between the theoretical model and corresponding XHMGC simulations is demonstrated. It is shown that including FLR and FOW can lead to the global mode structure broadening. The non-perturbative effects of the energetic particles on global mode structure symmetry breaking is demonstrated. In particular, when the energetic particle drive shifts away from the mode rational surface, volume averaged parallel and radial wave numbers are generated in the plasma as a demonstration of EPs' role on mode structure symmetry breaking. The connection of the mode structure symmetry breaking with experimental observations using ECEI and with particle/momentum transport are discussed. © EURATOM 2018
On fast radial propagation of parametrically excited geodesic acoustic mode
No full textThe spatial and temporal evolution of parametrically excited geodesic acoustic mode (GAM) initial pulse is investigated both analytically and numerically. Our results show that the nonlinearly excited GAM propagates at a group velocity which is, typically, much larger than that due to finite ion Larmor radius as predicted by the linear theory. The nonlinear dispersion relation of GAM driven by a finite amplitude drift wave pump is also derived, showing a nonlinear frequency increment of GAM. Further implications of these findings for interpreting experimental observations are also discussed. © 2015 AIP Publishing LLC
Excitation of kinetic geodesic acoustic modes by drift waves in nonuniform plasmas
No full textEffects of system nonuniformities and kinetic dispersiveness on the spontaneous excitation of Geodesic Acoustic Mode (GAM) by Drift Wave (DW) turbulence are investigated based on nonlinear gyrokinetic theory. The coupled nonlinear equations describing parametric decay of DW into GAM and DW lower sideband are derived and then solved both analytically and numerically to investigate the effects on the parametric decay process due to system nonuniformities, such as nonuniform diamagnetic frequency, finite radial envelope of DW pump, and kinetic dispersiveness. It is found that the parametric decay process is a convective instability for typical tokamak parameters when finite group velocities of DW and GAM associated with kinetic dispersiveness and finite radial envelope are taken into account. When, however, nonuniformity of diamagnetic frequency is taken into account, the parametric decay process becomes, time asymptotically, a quasi-exponentially growing absolute instability. © 2014 AIP Publishing LLC
On nonlinear geodesic acoustic modes in tokamak plasmas
No full textIt is shown that, in tokamak plasmas, finite drift/banana-orbit width (FOW) effects play crucial roles in the nonlinear evolution of kinetic/geodesic acoustic modes (KGAM/GAM). In particular, it is found that, in contrast to the negligible second-harmonic generation, KGAM/GAM can generate appreciable zero-frequency zonal flow (ZFZF) due to the FOW effects. On the other hand, ZFZF is found to have negligible effects on the dynamics of GAM/KGAM. This route of generating ZFZF has important implications to the nonlinear dynamics of zonal flows and, consequently, drift wave turbulences. © CopyrightEPLA, 2014
Theory on excitations of drift Alfvén waves by energetic particles. I. Variational formulation
No full textA unified theoretical framework is presented for analyzing various branches of drift Alfvén waves and describing their linear and nonlinear behaviors, covering a wide range of spatial and temporal scales. Nonlinear gyrokinetic quasineutrality condition and vorticity equation, derived for drift Alfvén waves excited by energetic particles in fusion plasmas, are cast in integral form, which is generally variational in the linear limit; and the corresponding gyrokinetic energy principle is obtained. Well known forms of the kinetic energy principle are readily recovered from this general formulation. Furthermore, it is possible to demonstrate that the general fishbone like dispersion relation, obtained within the present theoretical framework, provides a unified description of drift Alfvén waves excited by energetic particles as either Alfvén eigenmodes or energetic particle modes. The advantage of the present approach stands in its capability of extracting underlying linear and nonlinear physics as well as spatial and temporal scales of the considered fluctuation spectrum. For these reasons, this unified theoretical framework can help understanding experimental observations as well as numerical simulation and analytic results with different levels of approximation. Examples and applications are given in Paper II [F. Zonca and L. Chen, "Theory on excitations of drift Alfvén waves by energetic particles. II. The general fishbone-like dispersion relation," Phys. Plasmas 21, 072121 (2014)]. © 2014 EURATOM
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