Experimental Scaling of Compressional Alfvén Eigenmode Stability

Compressional Alfvén eigenmodes (CAE) are relatively ubiquitous in NSTX plasmas with nominal toroidal field strength up to 4.65 kG, but are largely absent in the NSTX upgrade plasmas with toroidal fields of 5.9 kG. In this paper we show that CAE are seen with frequencies 0.4 ω_ci < ω_CAE ≤ 1.2 ω_ci , that toroidal mode numbers range from 6 ≤ n ≤ 17, calculations with the CAEk linear code find that 0.5 < | k_par/k_perp | < 1, and that for plasmas with toroidal field greater than about 3.8 kG V_beam/V_Alfvén < 4. Empirically the prevalence of CAE activity increases for V_beam/V_Alfvén > 2 which is consistent with the relative lack of CAE in the NSTX-U plasmas from the 2016 campaign which typically have V_beam/V_Alfvén < 2. The onset threshold is consistent with prior modeling [1]. There is a weak positive scaling of toroidal mode number with toroidal field. We further explore the nature of the fast-ion resonances and the coupling to low-frequency kink modes by linking CAEk with a full-orbit code. We find that the CAE primarily interact with beam fast-ions with energies below ≈ 30 keV and that there are typically several strong resonances for each mode. As of yet, no theoretical modeling has provided an explanation for the lack of the fundamental poloidal harmonic, nor for the range of toroidal mode numbers that were seen.

[1] LESTZ, J B, GORELENKOV, N N, BELOVA, E V, et al., Analytic stability boundaries for compressional and global Alfvén eigenmodes driven by fast ions. II. Interaction via Landau resonance, Phys. Plasmas 27 (2020) 022512.