Observation of Helicity-Induced Alfvén Eigenmodes in Large-Helical-Device Plasmas Heated by Neutral-Beam Injection

Abstract

The helicity-induced Alfvén eigenmodes (HAEs) with the toroidal mode number $n=2$ and 3 are observed for the first time in the Large Helical Device plasmas heated by neutral beam injection. The observed mode frequency is about 8 times higher than that of the observed toroidicity-induced Alfvén eigenmodes, and is proportional to the Alfvén velocity. The modes are excited when the ratio of the beam velocity to the Alfvén velocity exceeds about unity. The frequency lies just above the lower bound of the HAE gap in the plasma edge region of $\rho >0.7$ ($\rho$: normalized minor radius).