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Description
The high heat fluxes in magnetic fusion devices pose an immediate threat to their plasma facing components (PFCs). These fluxes can be mitigated by injecting low to medium Z impurities [1]. These impurities stimulate radiation emission in the edge of the plasma, which dissipates power volumetrically and reduce the heat fluxes to the PFCs. Efficient control of the radiated power ($P_{rad}$) requires accurate representation of its magnitude and dynamics, as well as reliable real-time diagnostics [2]. Especially in 3D devices, such as the Wendelstein 7-X stellarator (W7-X), determining a robust $P_{rad}$ proxy is a challenge due to its asymmetries observed in the past [3]. This contribution reports on the system identification experiments performed on W7-X and on observed asymmetries
of the $P_{rad}$ proxies calculated from different bolometer systems for different actuator (valve) locations.
W7-X utilizes piezo-electric valves [4] to regulate the amount of injected gaseous impurities. Furthermore, a wide angle bolometer camera [3] offers a real-time-capable $P_{rad}$ estimation and streaming to the control system, with the rest of the bolometer systems [3][5][6] providing radiation data from different cross-sections.
Asymmetries are observed in locations with and without plasma-wall-interaction (PWI) which reduce with increasing densities and with decreasing input power. The asymmetries while seeding drop when the plasma transitions to detachment. Additionally, asymmetries between stellarator equivalent locations show a configuration dependence. These observations allow to calculate a ๐๐๐๐ proxy considering the toroidal radiation distribution. This will be addressed in future work.
Despite the asymmetric $P_{rad}$ behaviour, system identification experiments provided the data for designing controllers for different seeding scenarios. Nitrogen and neon actuators are utilized with the controllers for performing $P_{rad}$ scans, enabling studying plasma parameters at different radiation levels. In addition, stable detachment was feasible for a variety of magnetic configurations and during high power scenarios.
[1] A. W. Leonard, Plasma Phys. Control. Fusion 60 044001 (2018)
[2] R. Pintelon et al, System Identification: A Frequency Domain Approach (IEEE Press, 2001)
[3] G. Partesotti et.al, Rev. Sci. Instrum. 95, 103503 (2024); doi: 10.1063/5.02
[4] M. Griener et.al, Rev. Sci. Instrum. 88, 033509 (2017), https://doi.org/10.1063/1.4978629
[5] D. Zhang et al, Rev. Sci. Instrum. 81, 4 (2010) https://doi.org/10.1063/1.3483194
[6] G. Partesotti Rev. Sci. Instrum. 96, 063503 (2025) https://doi.org/10.1063/5.0261413