Acoustic mode driven by fast electrons in TJ-II Electron Cyclotron Resonance plasmas

Intense harmonic oscillations in radiation signals $(\delta I/I\sim 5{\%})$ are commonly observed during Electron Cyclotron Resonance (ECR) heating in TJ-II stellarator plasmas at low line-averaged electron density, $0.15 < \bar{n}_e < 0.6 \times10^{19} \ \text{m}^{-3}$ . The frequency agrees with acoustic modes. The poloidal modal structure is compatible with Geodesic Acoustic Modes (GAM) but an $n \neq 0$ toroidal structure is detected. The modes are found in the proximity of low-order rationals of the rotational transform (${\lower3pt\hbox{$\mathchar'26$}\mkern-7mu\iota} = 1/q = 8/5$ in this work) and are excited by fast-electron populations, but they disappear at the onset of ${\lower3pt\hbox{$\mathchar'26$}\mkern-7mu\iota}$ -islands rotation.