Orbit-based analysis of nonlinear energetic ion dynamics in tokamaks. II. Mechanisms for rapid chirping and convective amplification

• Published in: Physics of plasmas Vol. 23; no. 4
• Main Authors: Bierwage, Andreas, Shinohara, Kouji
• Format: Journal Article
• Language: English
• Published: Melville American Institute of Physics 01.04.2016
• Subjects: 70 PLASMA PHYSICS AND FUSION TECHNOLOGY; AMPLIFICATION; BEAMS; COMPARATIVE EVALUATIONS; Evolution; FLUCTUATIONS; GEOMETRY; Ion dynamics; JT-60U TOKAMAK; KEV RANGE 100-1000; Magnetic resonance; Nonlinear analysis; Nonlinear dynamics; NONLINEAR PROBLEMS; ONE-DIMENSIONAL CALCULATIONS; Orbital mechanics; Orbital resonances (celestial mechanics); PARTICLE TRACKS; PARTICLES; PHASE SPACE; Plasma physics; Power transfer; RESONANCE; Resonant frequencies; SHEAR; SPECTRA; TAIL IONS; Tokamak devices; Variation
• ISSN: 1070-664X; 1089-7674
• DOI: 10.1063/1.4947034

The nonlinear interactions between shear Alfvén modes and tangentially injected beam ions in the 150–400 keV range are studied numerically in realistic geometry for a JT-60U tokamak scenario. In Paper I, which was reported in the companion paper, the recently developed orbit-based resonance analysis method was used to track the resonant frequency of fast ions during their nonlinear evolution subject to large magnetic and electric drifts. Here, that method is applied to map the wave-particle power transfer from the canonical guiding center phase space into the frequency-radius plane, where it can be directly compared with the evolution of the fluctuation spectra of fast-ion-driven modes. Using this technique, we study the nonlinear dynamics of strongly driven shear Alfvén modes with low toroidal mode numbers n = 1 and n = 3. In the n = 3 case, both chirping and convective amplification can be attributed to the mode following the resonant frequency of the radially displaced particles, i.e., the usual one-dimensional phase locking process. In the n = 1 case, a new chirping mechanism is found, which involves multiple dimensions, namely, wave-particle trapping in the radial direction and phase mixing across velocity coordinates.