Abrupt excitation of energetic-particle-driven geodesic acoustic mode in the large helical device

Energetic-particle-driven geodesic acoustic mode (EGAM) is being investigated in the Large Helical Device (LHD). In this article, two topics based on observation in the LHD are presented. One is the observation of the EGAM with the frequency higher than the ordinary GAM frequency. The previous theory and experiments show that the EGAM has a frequency lower than the ordinary GAM frequency. Newly extended theory and numerical simulation have clarified that the difference in the frequency comes from the difference in the velocity distribution function of energetic particles. Another topic is the abrupt excitation of a GAM during the evolution of EGAM. As the frequency of the EGAM approaches twice the conventional GAM frequency, another GAM is excited abruptly. The experimental results indicate that the abrupt excitation of the GAM is attributed to the EGAM. A theoretical model, in which kinetic nonlinearity and fluid coupling between the GAM and the EGAM are taken into account, has successfully reproduced the experimental results. The nonlinearity in the model predict the subcritical instability of the GAM, and a threshold observed in the amplitude of the GAM seems to correspond to the necessary magnitude of the seed for the subcritical instability. Recently developed numerical simulation takes into account the kinetic coupling, which is ignored in the above theoretical model, between the EGAM and the abruptly excited GAM, and indicates that the kinetic coupling can trigger the GAM. The observation and the analysis demonstrate one experimental path for investigating the trigger problem of the onset of the abrupt phenomena.