Description
We present experimental observations of Alfvén eigenmodes in the presence of energetic particles at the Wendelstein 7-X stellarator (W7-X). Typically within the first 300 ms these modes destabilize during the initial phase of plasma discharges under low magnetic field conditions (1.7–1.8 T, as opposed to the standard 2.5–2.6 T). Here plasma heating is exclusively provided by neutral beam injection (NBI). The observed modes exhibit high amplitude and narrow frequency bands, following the expected Alfvénic density scaling 1/sqrt(n), with n denoting the electron plasma density. The sharp spectral signature clearly distinguishes them from the broadband, low-amplitude Alfvén eigenmodes that are known to be driven by ion temperature gradient turbulence in electron cyclotron heated W7-X plasmas. This work provides an experimental characterization of the observed mode activity, including a detailed frequency and mode structure analysis. The results are supported by corresponding theoretical predictions.