Excitation of Alfvénic Modes via Electromagnetic Turbulence in Wendelstein 7-X.

In this Letter, we demonstrate by comparison of computer simulations and experimental results that excitation of turbulence-driven electromagnetic secondary instabilities can serve as a paradigm to explain the observed activity of otherwise stable modes in fusion devices. In particular, we discuss the excitation of Alfvén eigenmodes in electron cyclotron heated plasmas in the Wendelstein 7-X (W7-X) stellarator. In more than 700 experimental programs of W7-X Operational Phase 1.2b (from which 25 were selected and discussed in this paper), poloidal magnetic fluctuation measurements with a clear Alfvénic nature are seen to correlate with turbulence density fluctuations. In response, we perform nonlinear, global, gyrokinetic, electromagnetic simulations using profiles consistent with the experimental conditions. The results agree remarkably well with the measurements and reveal excitation of zonal flow activity and long wavelength Alfvénic modes by ion temperature gradient turbulence. Integer ratios between the primary and secondary growth rates suggest an excitation mechanism similar to the forced-driven excitation of zonal flow by Alfvén eigenmodes.