Strong Destabilization of Stable Modes with a Half-Frequency Associated with Chirping Geodesic Acoustic Modes in the Large Helical Device

T. Ido (1) , K. Itoh (1) , M. Osakabe (2, 1) , Maxime Lesur (3, 4) , A. Shimizu (1) , K. Ogawa (1, 2) , K. Toi (1) , M. Nishiura (5) , S. Kato (1) , M. Sasaki (3) , K. Ida (1, 2) , S. Inagaki (3) , S. -I. Itoh (3) , Lhd Experiment Grp (1)

1 NIFS - National Institute for Fusion Science
2 SOKENDAI - Graduate University for Advanced Studies [Hayama]
3 Research Institute for Applied Mechanics, Kyushu University
4 IJL - Institut Jean Lamour
5 Graduate School of Frontier Sciences [Kashiwa]

Abrupt and strong excitation of a mode has been observed when the frequency of a chirping energetic-particle driven geodesic acoustic mode (EGAM) reaches twice the geodesic acoustic mode (GAM) frequency. The frequency of the secondary mode is the GAM frequency, which is a half-frequency of the primary EGAM. Based on the analysis of spatial structures, the secondary mode is identified as a GAM. The phase relation between the secondary mode and the primary EGAM is locked, and the evolution of the growth rate of the secondary mode indicates nonlinear excitation. The results suggest that the primary mode ( EGAM) contributes to nonlinear destabilization of a subcritical mode.

Domaines: Physique des plasmas [physics.plasm-ph]

T. Ido, K. Itoh, M. Osakabe, Maxime Lesur, A. Shimizu, et al.. Strong Destabilization of Stable Modes with a Half-Frequency Associated with Chirping Geodesic Acoustic Modes in the Large Helical Device. Physical Review Letters, 2016, 116 (1), ⟨10.1103/PhysRevLett.116.015002⟩. ⟨hal-01285753⟩