A1fven Eigenmodes in Spherical Tokamaks (original) (raw)
Related papers
1991
Toroidal Alfvtn eigenmodes are shown to be resonantly destabilized by both circulating and trapped energetic ions/alpha particles. In particular, the energetic circulating ions are shown to resonate with the mode not only at the AlfvCn speed (v,), but also at one-third of this speed, while for trapped ions, the dominant instability mechanism is shown to be due to the resonance between the precessional magnetic drift and the wave. Implications of the theory for present and future tokamaks are discussed. With the advent of next-generation fusion experiments which will focus on thermonuclear self-heating, it has become imperative to assess the potential of collective instabilities instigated by alpha particles. There are two classes of instabilities that are believed to be of serious concern to alpha-particle confinement: kinetic ballooning modes (KBM)'*2 and toroidal AlfvCn eigenmodes (TAE). lT3 These modes deserve special scrutiny because they are discrete in character [akBM e weip and arAE= v,/2qR, where o*;~ k,p,u,,/Lpi, Ls ' =-d In pi/dr is the bulk ion pressure scale length, pi=VtJni is the ion Larmor radius, VA=B/(h?lJti)
Evolution of toroidal Alfvén eigenmode instability in Tokamak Fusion Test Reactor
Physics of Plasmas, 1997
The nonlinear behavior of the Toroidal Alfvén Eigenmode (TAE) driven unstable by energetic ions in the Tokamak Fusion Test Reactor (TFTR) [Phys. Plasmas 1, 1560 (1994)] is studied. The evolution of instabilities can take on several scenarios: a single mode or several modes can be driven unstable at the same time, the spectrum can be steady or pulsating and there can be negligible or anomalous loss associated with the instability. This paper presents a comparison between experimental results and recently developed nonlinear theory. Many features observed in experiment are compatible with the consequences of the nonlinear theory. Examples include the structure of the saturated pulse that emerges from the onset of instability of a single mode, and the decrease, but persistence of, TAE signals when the applied rf power is reduced or shut off.
Frontiers in Physics, 2023
Following recent observations of unstable Toroidal Alfvén Eigenmodes (TAEs) in a counter-current Neutral Beam Injection (NBI) scenario developed in TCV, an indepth analysis of the impact of such modes on the global confinement and performance is carried out. The study shows experimental evidence of nondegradation of ion thermal confinement despite the increasing of auxiliary power. During such an improved confinement period, Toroidal Alfvén Eigenmodes (TAEs) driven by fast ions generated through Neutral Beam Injection (NBI) are found unstable. Together with the TAEs, various instabilities associated with the injection of the fast neutrals are observed by multiple diagnostics, and a first characterization is given. Nonlinear wave-wave couplings are also detected through multi-mode analysis, revealing a complex picture of the stability dynamics of the TCV scenario at hand. The measurements provided by a short-pulse reflectometer corroborate the identification and radial localization of the instabilities. A preliminary, but not conclusive, analysis of the impact of TAEs on the amplitude of the electron density fluctuations is carried out. Local flux-tube gyrokinetic simulations show that the dominant underlying instabilities in the absence of fast ions are Trapped Electron Modes (TEM), and that these modes are effectively suppressed by zonal flows. Attempts to simulate the simultaneous presence of fast-ion driven TAEs and TEM turbulence show that elongated streamers develop up to the full radial extent of the flux-tube domain, thereby invalidating the local assumption and indicating that a global approach is mandatory in these TCV plasmas.
Observation of Alfvén Eigenmodes driven by off-axis neutral beam injection in the TCV tokamak
Plasma Physics and Controlled Fusion, 2020
Fast-particle driven Alfvén Eigenmodes (AEs) have been observed in low-collisionality discharges with off-axis neutral beam injection (NBI), electron cyclotron resonance heating (ECRH) and a reduced toroidal magnetic field. During NBI and ECRH, Toroidicity induced Alfvén Eigenmodes (TAEs) appear in frequency bands close to 200 kHz, and chirping modes are observed at about 40 kHz and 80 kHz that are likely Energetic-Particle-Induced Geodesic Acoustic Modes (EGAMs). When turning off ECRH in the experiment, those beam-driven modes disappear which can be explained by a modification of the fast-ion slowing down distribution. In contrast, coherent fluctuations close to the frequency of the beam driven TAEs are present throughout the experiment. The modes have the same toroidal mode number as the beam-driven ones and are even observed during ohmic plasma conditions. This clearly demonstrates that they are not caused by fast particles and suggests an alternative drive, such as turbulence. The mode-induced fast-ion transport has been found to be weak and marginal in terms of the fast-ion diagnostic sensitivities. Measurements of the plasma stored energy, neutron rates, neutral particle fluxes and fast-ion D-alpha spectroscopy show good agreement with neoclassical modelling result from TRANSP. This is further supported by a similarly good agreement between measurement and modelling in cases with and without ECRH and therefore with and without the modes. Instead, a significant level of charge exchange losses are predicted and observed which generate a bump-on-tail fast-ion distribution function that can provide free energy to EGAMs.
Linear and nonlinear properties of moderate-toroidal-number (n) shear-Alfvén modes in tokamaks are investigated by using a hybrid MHD-particle simulation code, which solves the coupled set of MHD ͑magnetohydrodynamic͒ equations for the electromagnetic fields and gyrocenter Vlasov equation for a population of energetic ions. The existence of unstable toroidal Alfvén eigenmodes ͑TAE's͒ and their kinetic counterpart is shown for low values of the energetic-ion pressure gradient. Above a certain threshold value, the energetic particle continuum mode ͑EPM͒ is destabilized, with growth rate fast increasing with increasing energetic-particle pressure gradient. The threshold shows an inverse dependence on n. High-n EPM's could then be unstable in realistic plasma conditions. Neglecting MHD nonlinearities, for the sake of simplicity, it is shown that nonlinear TAE saturation appears to be due to the trapping of resonant energetic ions in the potential well of the wave. Saturation of the EPM occurs instead because of a macroscopic outward displacement of the energetic-ion population, with potentially dramatic consequences on ␣-particle confinement; such conclusions are not modified by the inclusion of MHD nonlinearities.
Kinetic thermal ions effects on Alfvénic fluctuations in tokamak plasmas
Adopting the theoretical framework for the generalized fishbone-like dispersion relation, an extended hybrid magnetohydrodynamics gyrokinetic simulation model has been derived analytically by taking into account both thermal ion compressibility and diamagnetic effects in addition to energetic particles kinetic behaviors. The extended model has been used for implementing an eXtended version of Hybrid Magnetohydrodynamics Gyrokinetic Code (XHMGC) to study thermal ion kinetic effects on Alfvénic modes driven by energetic particles, such as kinetic beta induced Alfvén eigenmode in tokamak fusion plasmas. It is shown, both analytically and by numerical simulations, that, in the presence of thermal ion kinetic effects, the beta induced Alfvén eigenmode (BAE) -shear Alfvén wave continuous spectrum can be discretized into radially trapped eigenstates known as kinetic BAE (KBAE). While the thermal ion compressibility gives rise to finite BAE accumulation point frequency, the discretization occurs via the finite Larmor radius and finite orbit width effects. Simulations and analytical theories agree both qualitatively and quantitatively. Simulations also demonstrate that KBAE can be readily excited by the finite radial gradients of energetic particle profiles.
Nonlinear interplay of Alfvén instabilities and energetic particles in tokamaks
Plasma Physics and Controlled Fusion, 2017
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Energetic Particle Mode Dynamics in Tokamaks
18th IAEA Fusion …, 2000
Energetic Particle Modes [1] (EPM) are strongly driven oscillations excited via wave-particle resonant interactions at the characteristic frequencies of the energetic ions , ω tE , ω BE and/or ω dE , i.e., respectively the transit frequency for circulating particles and the bounce and precessional drift frequencies for trapped ions. A sharp transition in the plasma stability at the critical EPM excitation threshold has been observed by nonperturbative gyrokinetic codes in terms of changes in normalized growth rate (γ/ω A , with ω A = v A /qR 0 ), real frequency (ω r /ω A ) and parallel wave vector (k qR 0 ) both as α = −R 0 q 2 β [3, 4] of the thermal plasma and that, α E [3, 5, 6], of fast ions are varied. The present work further explores theoretical aspects of EPM excitations by spatially localized particle sources, possibly associated with frequency chirping, which can radially trap the EPM in the region where the free energy source is strongest. Results of a nonperturbative 3D Hybrid MHD Gyrokinetic code are also presented to emphasize that nonlinear behaviors of EPM's are different from those of Toroidal Alfvén Eigenmodes (TAE) and Kinetic TAE (KTAE) [8] and that particle losses and mode saturation are consistent with the mode-particle pumping model [9] (particle radial convection). Results of theoretical analyses of nonlinear EPM dynamics are also presented and the possible overlap with more general nonlinear dynamics problems is discussed.