Time: Wednesday, Sept. 25, 2:00 pm
Venue: Rm. 413, Bld. 11#, Yuquan Campus
Speaker: Prof. Zhiyong Qiu
Title: Two plasmon decay of energetic particle induced geodesic acoustic mode
Abstract:
Energetic particle induced geodesic acoustic mode (EGAM) has draw much attention due to its potential role as an active control of drift wave type turbulences. EGAM is typically characterized by a frequency lower than the local geodesic acoustic mode (GAM) frequency. However, in a Large Helical Device (LHD) experiment [1], it is observed that EGAM frequency can be significantly higher than local GAM frequency; and as the EGAM frequency chirping up, a secondary mode with frequency being half of the primary EGAM is suddenly driven unstable. This experiment is reproduced in a recent MEGA simulation [2].
In this work, we show that, this unusual phenomenon is related to the peculiar discharge of LHD, i.e., the plasma temperature is very high while the density is very low, so the injected neutron beam is not fully slowed down before driving EGAM unstable. The EGAM driven unstable by the positive energy slope can have a frequency significantly higher than local GAM frequency [3]. The excitation of the secondary mode is perfectly explained using two-plasmon decay of the primary unstable EGAM into two linearly stable GAMs [4].
References
T. Ido, K. Itoh, M. Osakabe, et al, Phys. Rev. Lett. 116, 015002 (2016).
H. Wang, Y. Todo, T. Ido and Y. Suzuki, Phys. Rev. Lett. 120, 175001 (2018).
J. Cao, Z. Qiu and F. Zonca, Phys. Plasmas 22, 124505 (2015).
Z. Qiu, L. Chen and F. Zonca, in preparation (2019).
