Control of Alfvénic ITG-driven mode Activity via Steep Density Profiles

This study investigates the impact of plasma configuration on shear Alfvén wave activity at the Wendelstein 7-X (W7-X) stellarator, particularly for the so-called small plasmas. Small plasmas, which are detached from plasma-facing components, are characterised by a reduced effective radius (from reff ≃0.5 m to reff < 0.4m), higher central density, and steeper density profiles [1]. We analyze steep density profiles associated with reduced broadband Alfvénic activity in two distinct experimental scenarios. First, following frozen hydrogen pellet injection, and second, in small plasma scenarios. In both cases, a reduction of broadband Alfvénic activity is observed in Mirnov coil signals. Post-pellet suppression correlates with reduced ion-temperature-gradient (ITG) turbulence. This reduction in ITG turbulence could weaken the ITG drive previously identified for Alfvén eigenmodes (AE) in W7-X [2, 3], and is consistent with the AE suppression observed in the post-pellet phase. In small plasmas, a reduction in Alfvénic activity is seen, despite no clear indication of reduced turbulence [1]. Unlike the post-pellet phase, the number of AE-frequencies remains significant, but their spectral range narrows. This sharpening of the AE spectrum suggests enhanced damping of global modes due to their interaction with the bent continuum [4], caused by the steep density profiles and modified Alfvén gap structure. These findings highlight mechanisms for AE suppression at W7-X by using steep density profiles.