Dependence of ion-cyclotron range of frequencies instabilities on species mix and fast-ion distribution: I. Stability

Ion cyclotron emission (ICE) driven by fast ions has the potential to be a reactor relevant diagnostic of confined and escaping fast ions; moreover, controlled experiments in magnetic fusion devices can shed light on similar magnetospheric instabilities below and above the ion cyclotron frequency f_ci. Dedicated DIII-D experiments studied instabilities below f_ci and at cyclotron harmonics l f_ci by injecting thirteen different neutral beam populations into plasmas with different thermal compositions of H, D, and 3He at five different values of toroidal field. High-frequency magnetic loops diagnose mode properties. Each beam type is individually injected for an approximate slowing-down time (~100 ms), resulting in a database of 2529 conditions that are analyzed in frequency bands corresponding to ICE harmonics and < f_ci global Alfvén eigenmodes. The sub-cyclotron modes are more unstable at low field and in plasmas with large H concentration. For D injection, first harmonic ICE at f_cD is more unstable in hydrogen plasmas but, for H injection, the converse is true (ICE at f_cH is more unstable in D), suggesting that thermal same-species ions damp fundamental ICE. ICE stability also depends upon the pitch-angle anisotropy gradient and upon the ‘bump-on-tail’ gradient ∂f/∂v. Although the frequency of unstable ICE in these plasmas always occurs near the central cyclotron frequency, the precise value also depends on the Alfvén speed and beam direction, consistent with the idea that instability occurs for modes that simultaneously satisfy ω ≃ 2πlf_ci, the magnetosonic dispersion relation, and the Doppler-shifted resonance condition.. Authors: Heidbrink, W. W. [University of California, Irvine, CA (United States)] (ORCID:0000000269428043); DeGrandchamp, G. H. [Lawrence Livermore National Laboratory (LLNL), Livermore, CA (United States)] (ORCID:0000000213639570); Lestz, J. B. [General Atomics, San Diego, CA (United States)] (ORCID:0000000269751537); Thome, K. E. [General Atomics, San Diego, CA (United States)] (ORCID:0000000248013922); Vincena, S. [University of California, Los Angeles, CA (United States)]. DOE Contract: FC02-04ER54698; SC0018270; SC0020337; SC0021201. Subjects: electromagnetic ion cyclotron instabilities; equatorial noise; global Alfvén eigenmodes; ion cyclotron emission