Study of fast-ion-driven toroidal Alfvén eigenmodes impacting on the global confinement in TCV L-mode plasmas (original) (raw)
Related papers
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.
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.
Nuclear Fusion, 2010
A theory is developed from which it follows that energetic-ion-driven instabilities can, first, channel the energy of the energetic ions outside the region where these ions are located and, second, considerably affect the electron heat flux across the magnetic field. A new mechanism of the frequency chirping is revealed. Namely, it is shown that instabilities caused by the energetic ions can influence the plasma rotation, in which case the development of the instabilities results in variation of the Doppler shift in time. It is concluded that a key factor responsible for the mentioned phenomena is the local imbalance of the wave emission by energetic ions and the wave absorption by electrons along the radius. On the basis of the developed theory, experiments on the stellarator Wendelstein 7-AS and the spherical torus NSTX, where effects of Alfvénic activity on the plasma temperature were observed, are considered.
Beam anisotropy effect on Alfvén eigenmode stability in ITER-like plasmas
Nuclear Fusion, 2005
This work studies the stability of the toroidicity-induced Alfvén eigenmodes (TAE) in the proposed ITER burning plasma experiment, which can be driven unstable by two groups of energetic particles, the 3.5 MeV α-particle fusion products and the tangentially injected 1 MeV beam ions. Both species are super-Alfvénic but they have different pitch angle distributions and the drive for the same pressure gradients is typically stronger from co-injected beam ions as compared with the isotropically distributed α-particles. This study includes the effect of anisotropy of the beam ion distribution function on TAE growth rate directly via the additional velocity space drive and indirectly in terms of the enhanced effect of the resonant particle phase space density. For near parallel injection TAEs are marginally unstable if the injection aims at the plasma centre, where the ion Landau damping is strong, whereas with the off-axis neutral beam injection the instability is stronger with the growth rate near 0.5% of the TAE mode frequency. In contrast, for perpendicular beam injection TAEs are predicted to be stabilized in nominal ITER discharges.
1999
Resonant Toroidal AlfvŽn Eigenmodes (RTAEs) excited by neutral beam ions are observed in the region of the internal transport barrier in enhanced reverse shear (ERS) plasmas on TFTR. These modes occur in multiples of the same toroidal mode number in the range n=2-4 and appear as highly localized structures near the minimum in the q-profile with frequency near to that expected for TAEs. Unlike regular TAEs, these modes are observed in plasmas where the birth velocity of beam ions is well below the fundamental or sideband resonance condition. Theoretical analysis indicates that the Toroidicity induced AlfvŽn Eigenmode (TAE) does not exist in these discharges due to strong pressure gradients (of the thermal and fast ions) which moves the mode frequency down into the lower AlfvŽn continuum. However a new non-perturbative analysis (where the energetic particles are allowed to modify the mode frequency and mode structure) indicates that RTAEs can be driven by neutral beam ions in the weak magnetic shear region of ERS plasma, consistent with observations on TFTR. The importance of such modes is that they may affect the alpha particle heating profile or enhance the loss of energetic alpha particles in an advanced tokamak reactor where large internal pressure gradients and reverse magnetic shear operation are required to sustain large bootstrap current.
Impact of fast ions on a trapped-electron-mode dominated plasma in a JT-60U hybrid scenario
Nuclear Fusion, 2020
The impact of fast ions on a trapped electron mode (TEM) is extensively analysed by linear and nonlinear gyrokinetic simulations for a JT-60U plasma at high β and low magnetic shear using the Gene code in local approximation. For the first time, it is shown that TEM-induced turbulent transport may remain unaffected by the steep fast ion pressure profile generated by the Neutral Beam Injection. Unlike recent observations of ion temperature gradient (ITG)-induced turbulent transport reductions due to fast ions, TEMdominated systems could act differently in the presence of a significant fast ion population. The possible role of zonal flows as a saturation mechanism is analyzed, showing that their weak impact in the reported JT-60U scenario might lead to different behaviour of fast ions with respect to the ITG-dominated discharges. It is also shown that Alfvénic shear modes are destabilized at low wave numbers (n 8). They are identified as drift Alfvén waves destabilized by ITG, which is a form of Alfvénic ITG instabilities. Deep numerical analysis provides the physical parameter range in which ITG-driven BAEs are stabilized. These results open the way to new possibilities of tailoring future experimental scenarios in order to benefit from transport reduction by fast ions.
Linear gyrokinetic simulation of high-n toroidal Alfven eigenmodes in a burning plasma
2010
A hybrid gyrokinetic ions/massless fluid electron model is used to study the stability of high-n toroidal Alfvén eigenmodes �TAEs� in ITER �M. Shimada et al., Nucl. Fusion 47, S1 �2007��. The hybrid model has been implemented in the particle-in-cell turbulence simulation code GEM �Y. Chen and S. E. Parker, J. Comput. Phys. 220, 839 �2007��. The adequacy of the hybrid model for simulating TAEs has been previously demonstrated �J. Lang et al., Phys. Plasmas 16, 102101 �2009�� by comparing the simulated TAE mode frequency and structure with an eigenmode analysis, and the thermal ion kinetic damping effect with analytic theory. By using a global particle-in-cell code the effects of large orbit width and nonlocal mode structures can be accurately included. Damping rate due to numerical filtering is carefully monitored, and convergence with respect to particle number, grid resolution, etc., is thoroughly tested. The simulations show that the most unstable modes in ITER lie in the rage of 10� n � 20. Thermal ion pressure effect and alpha particle nonperturbative effect are important in determining the mode radial location and stability threshold. The thermal ion Landau damping rate and radiative damping rate from the simulations are compared with analytical estimates. The thermal ion Landau damping is the dominant damping mechanism. Plasma elongation has a strong stabilizing effect on the alpha driven TAEs. The central alpha particle pressure threshold for the most unstable n = 15 mode is about � � �0� = 0.7% for the fully shaped ITER equilibrium.