94 research outputs found
On scattering and damping of Toroidal Alfven eigenmode by drift wave turbulence
No full textWe demonstrate analytically that, in toroidal plasmas, scattering by drift
wave turbulence could lead to appreciable damping of toroidal Alfven eigenmodes
via generation of short-wavelength electron Landau damped kinetic Alfven waves.
A corresponding analytic expression of the damping rate is derived, and found
to be, typically, comparable to the linear drive by energetic particles. The
implications of this novel mechanism on the transport and heating processes in
burning plasmas are also discussed.Comment: submitted (April 2022
On beat-driven and spontaneous excitations of zonal flows by drift waves
No full textUsing the slab plasma as a paradigm model, we have derived analytically equations for the nonlinear generation of zero-frequency zonal flows by electron drift waves including, on the same footing, both the beat-driven and spontaneous excitations. It is found that the beat-driven zonal flow tends to reduce the frequency mismatch between the electron drift waves and, thereby, contributes to a significant O(1) enhancement of the modulational instability drive and lowering its threshold. Implications to tokamaks plasmas as well as drift-wave soliton formation are also discussed
Nonlinear Electron Phase‐Space Dynamics in Spontaneous Excitation of Falling‐Tone Chorus
No full textAbstract Whistler mode chorus, characterized by frequency chirping, is an important type of waves in planetary magnetospheres. To investigate the role of nonlinear wave particle interactions in excitation of chorus, analysis of electron phase space dynamics is required. While electron phase hole associated with rising tone chorus has been well studied, the phase space structure corresponding to falling tone chorus is less understood. Here, we investigate in detail the electron phase space dynamics in a spontaneously generated falling tone chorus with a parabolic type magnetic field, where field strength decreases away from the equator. We demonstrate the formation and evolution of electron phase space clump from downstream to upstream regions, and show that the variation of frequency chirping rate is caused by the movement of the source region. The results are consistent with the recently proposed Trap‐Release‐Amplify model, and should be useful to understand the mechanism of chorus frequency chirping
Nonlinear drift-wave and energetic particle long-time behaviour in stellarators: solution of the kinetic problem
No full textWe propose a theoretical scheme for the study of the nonlinear interaction of drift-wave-like turbulence and energetic particles in stellarators. The approach is based on gyrokinetics, and features a separation of time and scales, for electromagnetic fluctuations, inspired by linear ballooning theory. Two specific moments of the gyrokinetic equation constitute the main equations of the system, which requires a full kinetic nonlinear solution. This is found iteratively, expanding in the smallness of the bounce-average radial drift frequency, and nonlinear drift frequency, compared with the inverse time scales of the resonantly interacting energetic particles. Our analysis is therefore valid for neoclassically optimised stellators. The resummation of all iterative and perturbative nonlinear kinetic solutions is discussed in terms of Feynman diagrams. Particular emphasis is put on the role of collisionlessly undamped large-scale structures in phase space, the kinetic equivalent of zonal flows, i.e. phase-space zonal structures, and on wave-like fluctuations generated by energetic particles
Gyrokinetic theory for particle and energy transport in fusion plasmas
No full textA set of equations is derived describing the macroscopic transport of particles and energy in a thermonuclear plasma on the energy confinement time. The equations thus derived allow studying collisional and turbulent transport self-consistently, retaining the effect of magnetic field geometry without postulating any scale separation between the reference state and fluctuations. Previously, assuming scale separation, transport equations have been derived from kinetic equations by means of multiple-scale perturbation analysis and spatio-temporal averaging. In this work, the evolution equations for the moments of the distribution function are obtained following the standard approach; meanwhile, gyrokinetic theory has been used to explicitly express the fluctuation induced fluxes. In this way, equations for the transport of particles and energy up to the transport time scale can be derived using standard first order gyrokinetics. © 2018 EURATOM
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)
Frequency Chirping of Electromagnetic Ion Cyclotron Waves in Earth's Magnetosphere
No full textElectromagnetic ion cyclotron waves are known to exhibit frequency chirping,
contributing to the rapid scattering and acceleration of energetic particles.
However, the physical mechanism of chirping remains elusive. Here, we propose a
new model to explain the chirping and provide direct observational evidence for
validation. Our results relate the frequency chirping of the wave to both the
wave amplitude and magnetic field inhomogeneity for the first time. The general
applicability of the model's underlying principle opens a new path toward
understanding the frequency chirping of other waves.Comment: 8 pages, 3 figure
The mixed Wentzel-Kramers-Brillouin-full-wave approach and its application to lower hybrid wave propagation and absorption
No full textThe mixed Wentzel-Kramers-Brillouin (WKB)-full-wave approach for the calculation of the 2D mode structure in tokamak plasmas is further developed based on our previous work [A. Cardinali and F. Zonca, Phys. Plasmas 10, 4199 (2003) and Z. X. Lu et al., Phys. Plasmas 19, 042104 (2012)]. A new scheme for theoretical analysis and numerical implementation of the mixed WKB-full-wave approach is formulated, based on scale separation and asymptotic analysis. Besides its capability to efficiently investigate the initial value problem for 2D mode structures and linear stability, in this work, the mixed WKB-full-wave approach is extended to the investigation of radio frequency wave propagation and absorption, e. g., lower hybrid waves. As a novel method, its comparison with other approaches, e.g., WKB and beam tracing methods, is discussed. Its application to lower hybrid wave propagation in concentric circular tokamak plasmas using typical FTU discharge parameters is also demonstrated. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4798408]http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcApp=PARTNER_APP&SrcAuth=LinksAMR&KeyUT=WOS:000317295200016&DestLinkType=FullRecord&DestApp=ALL_WOS&UsrCustomerID=8e1609b174ce4e31116a60747a720701Physics, Fluids & PlasmasSCI(E)EI8ARTICLE3null2
Shear Alfvén wave continuous spectrum within magnetic islands
No full textThe radial structure of the continuous spectrum of shear Alfven waves is calculated in this paper within the separatrix of a magnetic island. Geometrical effects due to the noncircularity of the flux surface's cross section are retained to all orders. On the other hand, only curvature effects responsible for the beta-induced gap in the low-frequency part of the continuous spectrum are kept. Modes with different helicity from that of the magnetic island are considered. The main result is that, inside a magnetic island, there is a continuous spectrum very similar to that of tokamak plasmas, where a generalized safety factor q can be defined and where a wide frequency gap is formed, analogous to the ellipticity induced Alfven eigenmode gap in tokamaks. The presence of this gap is due to the strong eccentricity of the island cross section. The importance of the existence of such a gap is recognized in potentially hosting magnetic island induced Alfven eigenmodes (MiAE). Due to the frequency dependence of the shear Alfven wave continuum on the magnetic island size, the possibility of utilizing MiAE frequency scalings as a novel magnetic island diagnostic is also discussed. (C) 2010 American Institute of Physics. [doi:10.1063/1.3531689
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