Axial structures of the Alfvén ion cyclotron mode in the GAMMA10 tandem mirror

Axial structures of the Alfvén ion cyclotron (AIC) mode have been investigated in the GAMMA10 tandem mirror. The AIC mode is a microinstability driven by an anisotropy in the velocity space, and is excited as an axial eigenmode in the GAMMA10. There are several discrete peaks in the frequency range of ω/Ω ci ∼0.8–0.9, where Ω ci is the ion cyclotron frequency at the midplane of the central cell. A driving term of the AIC mode is theoretically given by β ⊥ ( T ⊥ / T // ) 2 . Here, β ⊥ is a perpendicular beta value and T ⊥ / T // is a temperature anisotropy (an ion temperature ratio between perpendicular and parallel to the magnetic field line). In GAMMA10, the AIC mode is observed in the range of β ⊥ ( T ⊥ / T // ) 2 ≥0.3 (which is much smaller than the theoretical prediction of β ⊥ ( T ⊥ / T // ) 2 ≥3.52). This is thought to be related to the wave structure in the axial direction. The frequency ω and axial wave number k // of each mode changed with β ⊥ and T ⊥ / T // . As β ⊥ and T ⊥ / T // increased, the AIC frequency range became wide and the AIC wave changed from a propagating wave to a standing wave.