Visualization of Fast Ion Phase-space Flow in Plasmas Well-below, Near and Well-above Alfv ́en eigenmode Stability Threshold in Tokamak

• Published in: Nuclear fusion
• Main Authors: Du, Xiaodi, Heidbrink, William W., Van Zeeland, Michael A., Gonzalez-Martin, Javier, Austin, Max E., Yan, Zheng, McKee, George R.
• Format: Journal Article
• Language: English
• Published: IAEA IOP Publishing 24.02.2023
• ISSN: 0029-5515; 1741-4326

Abstract Transport of fast ions along certain local phase space paths, referred to as fast ion phase- space flow, has been systematically measured by an imaging neutral particle analyzer (INPA) in three plasma regimes, which are well-below, near, and well-above the Alfv ́en eigenmode stability threshold [X.D. Du et al., Physical Review Letters 127, 235002 (2021)]. (1) In plasmas well-below the AE stability threshold, fast ions are well-confined on passing particle orbits without noticeable transport over the phase space. The observed INPA images agree well with the synthetic INPA images, using the fast ion distribution predicted by neoclassical theory. (2) In plasmas near the AE stability threshold, INPA images in the presence of AE activity moderately deviate from those without AE activity. The image difference can be well interpreted by AE-driven, phase-space fast ion flow. Paths of this flow over the velocity space (or streamlines) is reconstructed by the intersection lines of curved E′ and μ surfaces, referred to as E′ & μ line (where E′ ≡ E − (ω/n)Pζ ; E, Pζ and μ are the energy, canonical toroidal momentum and magnetic moment of ions; ω and n are the angular frequency and toroidal mode number of AEs, respectively). Resonant fast ions move radially inward by gaining energy and move radially outward by losing energy and the trajectory well aligns with the E′ & μ lines that pass through the mode resonances near the injection energy of neutral beams. (3) In plasmas well-above the AE stability threshold, fast ion phase-space dynamics show additional features. Fast ions are transported out of the birth positions so promptly along the streamlines that the slowing-down process from the injection energy is not observable, exhibiting strong critical gradient behavior at local phase space. As a result, the increase of electron temperature is very small, in spite of an increase of beam power by ∼ 45%. It should be emphasized that the directions of phase-space transport, induced by AEs with different frequencies, structures and mode numbers, do not largely differ.