112,312 research outputs found
Transport theory of phase space zonal structures
No full textWe adopt gyrokinetic theory to extract the phase space zonal structure from the flux surface averaged particle response, that is, the nonlinear response that is undamped by collisionless processes. We argue that phase space zonal structures are a proper definition for the nonlinear distortion of the plasma reference state and, thus, of the generally non-Maxwellian neighboring nonlinear equilibria consistent with toroidal symmetry breaking fluctuations. Evolution equations for phase space zonal structures are derived and discussed, along with the corresponding density and energy transport equations. It is shown that this approach is consistent with the usual evolution of macroscopic plasma profiles under the action of fluctuation induced fluxes, when the deviation of the reference state from local Maxwellian response is small. In particular, the present results recover those of a previous article [M. V. Falessi and F. Zonca, Phys. Plasmas 25, 032306 (2018)], where transport equations holding on the reference state length scale have been derived using the moment approach introduced in the classical review work by Hinton and Hazeltine
Shear Alfvén and acoustic continuum in general axisymmetric toroidal geometry
No full textThe equations describing the continuous spectrum of shear Alfvén and ion sound waves propagating along magnetic field lines are introduced and solved in the ballooning space for general geometry in the ideal MHD limit. This approach is equivalent to earlier analyses by Chu et al. 1992 [Phys. Fluids B 4, 3713 (1992)], but the present formulation in the ballooning space allows us to readily extend it to include gyrokinetic and three-dimensional equilibrium effects. In particular, following Chen and Zonca 2017 [Phys. Plasmas 24, 072511 (2017)], the MHD limit is adopted to illustrate the general methodology in a simple case, and the equations are solved within the framework of Floquet and Hill's equation theory. The connection of shear Alfvén and ion sound wave continuum structures to the generalized plasma inertia in the general fishbonelike dispersion relation is also illustrated and discussed. As an application, the continuous frequency spectrum is calculated for a reference equilibrium of the Divertor Tokamak Test facility. The results are compared with those obtained by the MARS code adopting the standard methodology, demonstrating excellent agreement
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
Spontaneous excitation of geodesic acoustic mode by toroidal Alfvén eigenmodes
No full textSpontaneous nonlinear excitation of geodesic acoustic mode (GAM) by toroidal Alfvén eigenmodes (TAE) is studied within the framework of gyrokinetic theory. The dispersion relation for the parametric decays of a pump TAE mode into a TAE lower sideband and a GAM is derived. It is shown that, in the ideal MHD first stability region, the condition for spontaneous excitation of GAM by TAEs is ω2 0 > V2 A/(4q2R2 0), in which, ω0 is the pump TAE real frequency, V A is the Alfvén speed, q is the safety factor and R0 is the torus major radius. The corresponding threshold condition is also derived and suggests the decay process as an effective saturation mechanism for TAE. © Copyright EPLA, 2013
An XFEM/DG approach for fluid-structure interaction problems with contact
No full textsummary:In this work, we address the problem of fluid-structure interaction (FSI) with moving structures that may come into contact. We propose a penalization contact algorithm implemented in an unfitted numerical framework designed to treat large displacements. In the proposed method, the fluid mesh is fixed and the structure meshes are superimposed to it without any constraint on the conformity. Thanks to the Extended Finite Element Method (XFEM), we can treat discontinuities of the fluid solution on the mesh elements intersecting the structure. The coupling conditions at the fluid-structure interface are enforced via a discontinuous Galerkin mortaring technique, which is a penalization method that ensures the consistency of the scheme with the underlining problem. Concerning the contact problem, we consider a frictionless contact model in a master/slave approach. By considering the coupled FSI-contact problem, we perform some numerical tests to assess the sensitivity of the proposed method with respect to the discretization and contact parameters and we show some examples in the case of contact between a flexible body and a rigid wall and between two deformable structures
Electron fishbone simulations in tokamak equilibria using XHMGC
No full textThis paper discusses the first nonlinear numerical simulation results of fishbone excitation by a magnetically trapped supra-thermal electron population with pressure profile peaked on axis due to, e.g. electron cyclotron resonance heating. The precession resonance underlying the linear instability is clearly identified. Meanwhile, mode saturation is shown to occur because of the secular resonant particle motion, pumping particles out of the radial region where the wave-particle power exchange is initially localized. The mode nonlinear saturation is accompanied by downward frequency chirping, due to phase-locking of the mode with resonant particles, consistent with the 'mode particle pumping' mechanism. © 2013 IAEA, Vienna
Nonlinear radial envelope evolution equations and energetic particle transport in tokamak plasmas
No full textThis work provides a general description of the self-consistent energetic particle phase space transport in burning plasmas, based on nonlinear gyrokinetic theory. The self consistency is ensured by the evolution equations of the Alfvénic fluctuations by means of nonlinear radial envelope evolution equations, while energetic particle fluxes in the phase space are explicitly constructed from long-lived phase space zonal structures, which are undamped by collisionless processes. As a result, this work provides a viable route to computing fluctuation induced energetic particle transport on long time scales in realistic tokamak plasmas
Evidence of 'two plasmon' decay of energetic particle induced geodesic acoustic mode
No full textSecondary low frequency mode generation by energetic particle induced geodesic acoustic mode (EGAM) observed in LHD experiment is studied using nonlinear gyrokinetic theory. It is found that the EGAM frequency can be significantly higher than local geodesic acoustic mode (GAM) frequency in low collisionality plasmas, and it can decay into two GAMs as its frequency approaches twice GAM frequency, in a process analogous to the well-known two plasmon decay instability. The condition for this process to occur is also discussed
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