Systematic study of toroidicity-induced Alfvén eigenmodes at low-q discharges in JT-60U

Toroidicity-induced Alfvén eigenmodes (TAE) were studied systematically in low-q discharges in JT-60U. Low-n TAE modes appeared outside the q = 1 surface at a low internal inductance (li), while high-n multiple TAE modes appeared inside the q = 1 surface at high internal inductance. In addition, the low-n multiple TAE modes were observed outside the q = 1 surface in very low-li plasmas (li ∼ 1.0), while the bi-directional TAE modes were observed inside the q = 1 surface in high-li (li ∼ 1.25) and relatively high electron density plasmas (n_e > 2 × 10¹⁹ m^-3). The high-n multiple TAE modes observed inside the q = 1 surface are much more harmful for energetic ion confinement than the low-n TAE modes observed outside the q = 1 surface.