Description
At Wendelstein-7X (W7-X), the largest stellarator of HELIAS type, the neoclassical transport is reduced, and radial transport is mainly caused by turbulence. Gradients in either electron or ion temperature and electron density are driving fluctuations in the plasma core and edge. Within the OP2-campaigns, several high-frequency modes were observed. They show up as density fluctuations in different diagnostics[1,2]. These modes occupy a broad frequency range and have quasi coherent character. Evidence for $\nabla T_e$ driven trapped electron modes (TEMs) is found e.g. in standard and low-mirror configuration, where the Poloidal Correlation Reflectometry position coincides with a local minimum of the magnetic field strength. The deduced $k_\perp\rho_s\approx 1$ and the velocity of the mode, propagating in electron drift direction, supports a TEM nature[3].
In the last campaign, experiments with neutral beam injection were performed, causing a peaking of the density profile in the plasma core due to beam fuelling and increased density gradients are observed. The radial interval of the increased density gradients corresponds with the observation of another type of high frequency mode, whose frequency is larger than those of the $\nabla T_e$ driven TEMs. These modes are also found in pellet fuelled plasmas, where steep density gradients are achieved. Similarities are found with the observed high frequency modes in a configuration with internal $5/5$-island close to the last closed flux surface are found[4]. The phase velocity of the mode as well as the calculated $\eta_e$-value is in favour for $\nabla n_e$-driven TEMs.
The paper will summarise the known properties of the high-frequency modes and discuss their origin by comparing the experimental results with linear simulations.
[1] A.Krämer-Flecken et al., http://stacks.iop.org/0029-5515/57/i=6/a=066023
[2] J.-P.Bähner et al., this conference
[3] A.Krämer-Flecken et al., https://dx.doi.org/10.1088/1361-6587/ada0db
[4] A.Krämer-Flecken et al., https://lac913.epfl.ch/epsppd3/2023/html/Tu/Tu_MCF07_Kramer-Flecken.pdf