Analysis of beam-driven instabilities near the ion cyclotron frequency in DIII-D plasmas

Frontier Science experiments in 2018 and 2021 studied instabilities above and below the ion cyclotron frequency Ω_i by injecting ten different neutral beam populations into L-mode plasmas with different thermal compositions of H, D, and 3He at five different values of toroidal field. Magnetic loops [1] diagnose mode properties. Below Ω_i, modes are more unstable at low field and the amplitude increases with increasing H concentration. Above Ω_i,the modes are stronger at high field and increasing H concentration shifts the dominant harmonic to higher values. Instability is driven more strongly for on-axis neutral beam injection (which creates a more centrally peaked beam ion profile) than for off-axis injection. Database results are compared with multi-species extensions of formulas by Lestz [2] below Ω_i and with Cook’s magnetoacoustic code [3] above Ω_i.

[1] G.H. DeGrandchamp et al., RSI 92 (2021) 033543.

[2] J.B. Lestz et al., NF 61 (2021) 086016.

[3] J.W.S. Cook, PPCF 64 (2022) 115002.