Parametric study of fast-ion-driven modes in Wendelstein 7-X

In the 2018 experimental campaign, fast ions in the stellarator Wendelstein 7-X will be generated by neutral beam injection. Later operation phases will also include ion cyclotron resonance heating. The fast ions may excite instabilities in the plasma which can lead to enhanced fast-ion transport and can, in severe cases, cause damage to plasma-facing components. We present a numerical study of fast-ion-driven Alfvén eigenmodes in a Wendelstein 7-X high-mirror equilibrium. Realistic fast-ion parameters are obtained using the ASCOT code. To model the instabilities, we use the CKA-EUTERPE code package. This model is perturbative, since a fixed mode structure - computed by the ideal-MHD code CKA - is used throughout the calculation. The non-linear gyro-kinetic code EUTERPE computes the power transfer from the fast particles to the mode which defines the growth rate of the instability. We show that having a fast-ion collision operator present in the simulations is required to accurately predict the non-linear saturation level of the mode. The scaling of the saturated amplitude with respect to fast-ion drag and the pitch-angle collision frequency is investigated and found to vary for different Alfvén eigenmodes. Furthermore, we study the impact of several other actuators that might be of experimental relevance for finding operation windows that show Alfvén-eigenmode activity. Examples are the effects of a radial electric field and the composition of the background plasma (hydrogen versus helium). While growth rates are found to be reduced in helium plasmas, including a radial electric field, typically present in Wendelstein 7-X, seems to have little influence on the modes.