Nonlinear simulations of beam-driven compressional Alfvén eigenmodes in NSTX

Abstract

Results of 3D nonlinear simulations of neutral-beam-driven compressional Alfvén eigenmodes (CAEs) in the National Spherical Torus Experiment (NSTX) are presented. Hybrid MHD-particle simulations for the H-mode NSTX discharge (shot 141398) using the HYM code show unstable CAE modes for a range of toroidal mode numbers, n=4−9, and frequencies below the ion cyclotron frequency. It is found that the essential feature of CAEs is their coupling to kinetic Alfvén wave (KAW) that occurs on the high-field side at the Alfvén resonance location. High-frequency Alfvén eigenmodes are frequently observed in beam-heated NSTX plasmas, and have been linked to flattening of the electron temperature profiles at high beam power. Coupling between CAE and KAW suggests an energy channeling mechanism to explain these observations, in which beam-driven CAEs dissipate their energy at the resonance location, therefore significantly modifying the energy deposition profile. Nonlinear simulations demonstrate that CAEs can channel the energy of the beam ions from the injection region near the magnetic axis to the location of the resonant mode conversion at the edge of the beam density profile. A set of nonlinear simulations show that the CAE instability saturates due to nonlinear particle trapping, and a large fraction of beam energy can be transferred to several unstable CAEs of relatively large amplitudes and absorbed at the resonant location. Absorption rate shows a strong scaling with the beam power.

Analytic stability boundaries for compressional and global Alfvén eigenmodes driven by fast ions. II. Interaction via Landau resonance

“…Furthermore, such modes would have been even more difficult to excite in HYM simulations, as their drive is strongly enhanced by coupling to the compressional mode, and this coupling is underestimated in the HYM model due to the absence of thermal plasma two-fluid effects (see Ref. 32 for a detailed description of the simulation model).…”

Suppression of Alfvén Modes on the National Spherical Torus Experiment Upgrade with Outboard Beam Injection

“…GAE are shear waves spatially localized near the minimum in the Alfvén continuum, with a frequency just below the minimum, ω < ⎤ Amin = min[k || (r) V Alfvén (r) ]. GAE have been implicated in electron thermal transport [7,8] and fast ion redistribution [9]. GAE are counter-propagating and excited through a Doppler-shifted cyclotron resonance [6].…”

Energetic-particle-modified global Alfvén eigenmodes

“…Nonperturbative modifications of the mode characteristics could alter the thresholds in number of overlapping modes and mode amplitudes required to generate the level of diffusion necessary to explain the experimental observations. While compressional modes have received more attention for their potential to channel energy away from the core to the edge through mode conversion to kinetic Alfvén waves 23,24 , GAEs also couple to KAWs in principle 21,22 and may also contribute. At least in the case of GAE-KAW mode conversion, the simulation results presented here suggest that nonperturbative inclusion of the energetic particles should be further explored for a more accurate description of that coupling in application to energy channeling in fusion conditions.…”

“…This endeavor is complicated by the fact that the GAEs, the counter-propagating modes especially, may interact with the continuum and excite a kinetic Alfvén wave, inferred through the presence of a well-localized δE fluctuation on the high field side and coincident short-scale modulation of the δB ⊥ mode structure near this region. The coupling of the KAW with the compressional mode in HYM simulations was studied in depth in a recent publication 24 , which identified key signatures of the KAW in the simulation which can also be leveraged in the case of the GAEs. Some of the more dramatic changes in mode structure can be attributed to gradual suppression or excitation of KAW features, which has dominant δB ⊥ polarization just as the GAEs do.…”

“…One involves the stochastization of electron orbits induced by the presence of many overlapping modes of sufficient amplitude 20 . The other is an energy-channeling mechanism where a core-localized CAE or GAE mode converts to a kinetic Alfvén wave (KAW) at the Alfvén resonance location, which damps efficiently on electrons, effectively redirecting neutral beam power from the core to the arXiv:1708.00498v3 [physics.plasm-ph] 1 May 2018 edge [21][22][23][24] . These mechanisms have been demonstrated numerically, though their quantitative predictions do not presently reproduce the experimental anomaly.…”