Visualization of Fast Ion, Phase-space Flow Driven by Alfvén Instabilities

Fast ion migration across a broad range of phase space, induced by multiple Alfvén Eigenmodes (AE) is measured for the first time by a novel imaging neutral particle analyzer [1] combined with a newly developed neutral beam modulation technique in the DIII-D tokamak. The unprecedented phase space resolution enabled by the INPA reveals details of fast ion transport physics and has identified three key features: (1) The transport promptly occurs immediately following neutral beam ionization, forming a phase space ‘hole’ at the injected energy and the radial location of minimum safety factor (q_min). (2) Fast ions move radially outward towards the plasma peripheral region at a reduced energy, where the thermalization process is much faster than that at the birth location. As a result, a pile-up of fast ions, i.e., larger population than neoclassical predictions, is observed, when the edge AE amplitude is reduced. (3) Phase-space tomography of the INPA data [2] reveals the formation of a high-energy tail of fast ions, with energies exceeding the injection energy of beams in the plasma core, interior to the q_min location, representing inward transport of the fast ions by AEs. Moreover, the migration is also found to affect the temporal evolution of AE amplitudes and radial structures as predicted by nonperturbative modeling.The measured flow pattern can be qualitatively interpreted by nonlinear hybrid kinetic-Magnetohydradynamic code (MEGA). A direct comparison to the calculated phase-space islands along the phase-space flow streamline highlights the key role of the pitch angle scattering on the formation of phase space flow.

 

[1] X.D. Du et al., Nucl. Fusion 58, 082006 (2018).

[2] X.D. Du et al., Nucl. Fusion 60, 112001 (2020).