The field of magnetically confined fusion physics changes rapidly in these years. From the laboratory size experiments we move towards reactor size machines. Wendelstein 7-X (W7-X) is the most advanced stellarator type fusion experiment, which was built in Greifswald, Germany. The W7-X experiment started in 2015 and provides unique results from the first plasma discharge. The experiment was shut down in 2018 for a major upgrade, which was completed in 2022. The Operational phase 2 has started in 2022 October and provided great results since. In this campaign the W7-X demonstrated steady-state operation at higher power levels, as an energy turnover of 1.3 gigajoule and a new record of eight minute discharge time was achieved.

Although fusion research globally focuses increasingly on technology development, the plasma physics is still key to the design of the new devices. A key question towards the fusion reactors is to understand and experimentally study the particle and heat transport in fusion plasmas.

Wendelstein 7-X is neoclassically optimized and turbulent transport is expected to play a significant role in the regulation of radial heat diffusivity and particle exhaust. Turbulence studies are therefore of key importance. The W7-X stellarator is equipped with state of the art core and edge turbulence diagnostics, namely phase contrast imaging and correlation reflectometry provides core measurements. While beam emission spectroscopy, gas puff imaging and fast visible cameras provide detailed edge turbulence data.

In 2017 a high sensitivity alkali Beam Emission Spectroscopy (BES) diagnostic was installed to W7-X by the Fusion Plasma Physics Laboratory of the Centre for Energy Research. The CER jointly operates the BES diagnostic and is also responsible for the data evaluation. This gives a unique opportunity to participate in the physics program. The diagnostics suffered from various technical difficulties in 2022, but sufficient experimental data is already available from this campaign.

The edge transport is expected to change with different heating scenarios, magnetic configuration and due to transient events. An interesting observation was made in last campaign, when the core turbulence was found to be supressed by core pellet fuelling. In the 2022 campaign similar observations were made with impurity TESPEL pellets and also by modulating the Neutral Beam Injection heating power. The sudden suppression of core turbulence was also shown to change the edge turbulence level. The detailed analysis of this phenomena, as well as turbulence behaviour of various scenarios is the primary aim of this study. Edge or core localized transient events can also play an important role in the transport, instabilities of the divertor islands have already been identified in some scenarios.  In the near future new phenomena can also be observed as the heating power is increased. The predecessor W7-AS stellartor could achieve High Confinement operation (H-mode), and W7-X can also achieve this in the future. The H-mode is a good confinement mode of the fusion plasmas, where a transport barrier is formed in the edge of the plasma. The H-mode is characterized by suppressed edge plasma turbulence.

To characterize edge plasma turbulence and transient events via the analysis of the BES data is the primary aim of this study. These finding will be checked against complementary diagnostics data and simulation results.  A python program package was developed in CER for fluctuation analysis. A part of the work is to develop numerical methods and implement codes to improve the analysis. These codes are then to be tested in simulated data sets and then used in real measurement results. In case of interest the candidate can participate in the operation the diagnostic in W7-X experiments. As the experiments are in Germany strong collaboration is necessary with  W7-X team and other Eurofusion laboratories.