Beam modulation and bump-on-tail effects on Alfvén eigenmode stability in DIII-D

Van Zeeland, M. A.; Bardoczi, L.; Gonzalez-Martin, J.; Heidbrink, W. W.; Podesta, M.; Austin, M.; Collins, C. S.; Du, X. D.; Duarte, V. N.; Garcia-Munoz, M.; Munaretto, S.; Thome, K. E.; Todo, Y.; Wang, X.

doi:10.1088/1741-4326/abf174

Beam modulation and bump-on-tail effects on Alfvén eigenmode stability in DIII-D

Journal Article · Fri May 14 00:00:00 EDT 2021 · Nuclear Fusion

DOI:https://doi.org/10.1088/1741-4326/abf174· OSTI ID:1784041

^[1]; Bardoczi, L. ^[2]; Gonzalez-Martin, J. ^[3]; ^[4]; ^[5]; Austin, M. ^[6]; Collins, C. S. ^[2]; ^[2]; ^[5]; ^[7]; ^[2]; ^[2]; ^[8]; Wang, X. ^[9]

General Atomics, San Diego, CA (United States); General Atomics, Energy & Advanced Concepts, DIII-D
General Atomics, San Diego, CA (United States)
Univ. of Seville (Spain)
Univ. of California, Irvine, CA (United States)
Princeton Plasma Physics Lab. (PPPL), Princeton, NJ (United States)
Univ. of Texas, Austin, TX (United States)
Univ. de Sevilla (Spain)
National Inst. for Fusion Science, Gifu (Japan)
Max-Planck-Inst. fur Plasmaphysik, Garching (Germany)

Beam modulation effects on Alfvén eigenmode stability have been investigated in a recent DIII-D experiment and show that variations in neutral beam modulation period can have an impact on the beam driven Alfvén eigenmode spectrum and resultant fast ion transport despite similar time-averaged input power. The experiment was carried out during the current ramp phase of L-mode discharges heated with sub-Alfvénic 50–80 kV deuterium neutral beams that drive a variety of Alfvén eigenmodes unstable. The modulation period of two interleaved beams with different tangency radii was varied from shot to shot in order to modify the relative time dependent mix of the beam pitch angle distribution as well as the persistence of a bump-on-tail feature near the injection energy (a feature confirmed by imaging neutral particle analyzer measurements). As the beam modulation period is varied from 7 ms to 30 ms on/off (typical full energy slowing down time of τ_slow ≈ 50 ms at mid-radius), toroidicity-induced Alfvén eigenmodes (TAEs) located in the outer periphery of the plasma become intermittent and coincident with the more tangential beam. Core mode activity changes from reversed shear Alfvén eigenmodes (RSAEs) to a mix of RSAE and beta-induced Alfvén eigenmodes. Discharges with 30 ms on/off period do not have a persistent bump-on-tail feature, have the lowest average mode amplitude and least fast ion transport. Furthermore, detailed analysis of an individual TAE using TRANSP kick modeling (Monte Carlo evolution of the distribution function with probabilistic 'kicks' by the AEs) and the resistive MHD code with kinetic fast ions, MEGA, find no strong role of energy gradient drive due to bump-on-tail features. Instead, the observed TAE modulation with interleaved beams is likely a pitch angle dependent result combined with slowing down of the tangential beam between pulses. For the conditions investigated, bump-on-tail contributions to TAE drive were found to be 5% or less of the total drive at any given time.

View Accepted Manuscript (DOE)

Research Organization:: General Atomics, San Diego, CA (United States)

Sponsoring Organization:: USDOE Office of Science (SC), Fusion Energy Sciences (FES); USDOE Office of Science (SC), Basic Energy Sciences (BES). Scientific User Facilities Division

Grant/Contract Number:: FC02-04ER54698; AC02-09CH11466; AC05-00OR22725

OSTI ID:: 1784041

Alternate ID(s):: OSTI ID: 1807716
OSTI ID: 23129765

Journal Information:: Nuclear Fusion, Journal Name: Nuclear Fusion Journal Issue: 6 Vol. 61; ISSN 0029-5515

Publisher:: IOP ScienceCopyright Statement

Country of Publication:: United States

Language:: English

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