Alfvén eigenmodes (AEs) are driven unstable by on-axis neutral beam (NB) injection in DIII-D reversed magnetic shear discharges #166496 and #159243, a high-beta steady-state demonstration discharge and an L-mode current ramp discharge, respectively. The experimental results are used to validate the ideal MHD code with kinetic extensions NOVA-K and the gyro-Landau-fluid code TGLFEP. Both codes predict low toroidal mode number (n = 1–2) toroidal Alfvén eigenmode (TAE) instability in the former discharge and low-n (n = 2–6) reverse shear Alfvén eigenmode (RSAE) instability in the latter discharge, consistent with observations. Discrepancies between the two codes are discussed. The same two codes are also employed to predict linear AE instabilities driven by α particles in the China fusion engineering test reactor (CFETR). For the CFETR reversed shear q profile, the growth rates of AEs oscillate with increasing toroidal mode number, depending on the dominant AE type, and the RSAEs near a rational surface always have larger growth rates than TAEs. AEs are found to be further destabilized by the addition of 500 keV off-axis injected NB. The effects of q-profile variation on AE stability in CFETR are also investigated and the results show that raising qmin can further destabilize RSAEs, while increasing q0 with fixed qmin, to create stronger negative magnetic shear, is stabilizing. Increasing plasma density is beneficial for AE stability due to reduced fast ion drive because of shorter slowing down time and higher electron collisional damping.
Core-localized Alfvénic modes driven by energetic ions in HL-2A NBI plasmas with weak magnetic shears