Eigenmode excitation of Alfvén ion cyclotron instability

In the GAMMA 10 tandem mirror [M. Inutake et al., Phys. Rev. Lett. 55, 939 (1985)], plasmas with a high beta value (ratio of plasma pressure to magnetic pressure) are produced by using an ICRF (ion cyclotron range of frequency) heating. Magnetic field fluctuations identified as an Alfvén ion cyclotron (AIC) mode are excited with strong dependence on a perpendicular beta value and a temperature anisotropy T⊥/T∥ (a ratio between the temperatures perpendicular and parallel to the magnetic field line). Spatial structures and excitation conditions of the AIC mode are experimentally studied. AIC modes are spontaneously excited as axial eigenmodes, of which boundary is determined by the reflection point in anisotropic plasmas. The AIC mode in the GAMMA 10 is observed well below the theoretically predicted threshold.