@article{oai:nifs-repository.repo.nii.ac.jp:00010906,
 author = {IDO,  Takeshi and ITOH,  Kimitaka and LESUR, M. and OSAKABE,  Masaki and SHIMIZU,  Akihiro and OGAWA,  Kunihoro and NISHIURA,  Masaki and YAMADA,  Ichihiro and YASUHARA,  Ryo and Kosuga, Yusuke and Sasaki, Makoto and IDA,  Katsumi and INAGAKI,  Shigeru and ITOH,  Sanae -I. and INOUE, Sanae and the, LHD Experiment Group},
 issue = {7},
 journal = {Nuclear Fusion},
 month = {Apr},
 note = {The abrupt and strong excitation of the geodesic acoustic mode (GAM) has been found in the large helical device (LHD), when the frequency of a chirping energetic particle-driven GAM (EGAM) approaches twice that of the GAM frequency. The temporal evolution of the phase relation between the abrupt GAM and the chirping EGAM is common in all events. The result indicates a coupling between the GAM and the EGAM. In addition, the nonlinear evolution of the growth rate of the GAM is observed, and there is a threshold in the amplitude of the GAM for the appearance of nonlinear behavior. A threshold in the amplitude of the EGAM for the abrupt excitation of the GAM is also observed. According to one theory (Lesur et al 2016 Phys. Rev. Lett. 116 015003, Itoh et al 2016 Plasma Phys. Rep. 42 418) the observed abrupt phenomenon can be interpreted as the excitation of the subcritical instability of the GAM. The excitation of a subcritical instability requires a trigger and a seed with sufficient amplitude. The observed threshold in the amplitude of the GAM seems to correspond with the threshold in the seed, and the threshold in the amplitude of the EGAM seems to correspond with the threshold in the magnitude of the trigger. Thus, the observed threshold supports the interpretation that the abrupt phenomenon is the excitation of a subcritical instability of the GAM.},
 title = {Observation of subcritical geodesic acoustic mode excitation in the large helical device},
 volume = {57},
 year = {2017}
}