Speaker
Description
Understanding the basic plasma parameters of temperature and density, as well as their gradients in the scrape-off layer (SOL), is a topic critical for providing information about the performance of a divertor concept. The stellarator Wendelstein 7-X features a novel resonant island divertor with an adjustable rotational transform of ι = 2π (5/6, …, 5/4). In order to study the performance of this divertor concept, an active spectroscopy system on a thermal helium beam [1] was developed and installed on the device [2]. The system consists of four identical Czerny-Turner spectrometers imaging two stellarator-symmmetric upper and lower divertor modules, allowing for tomographic reconstruction of impurity radiation in the island divertor region. The helium beam diagnostic operates using the technique of line ratio spectroscopy, applying a collisional-radiative model (CRM) to relate the observed line radiation to the underlying plasma parameters [3]. Despite successful operation during previous experiments, there have been systematic disagreements observed between the helium beam and other edge diagnostics. In order to investigate the uncertainties of the helium beam diagnostic and the vulnerabilities of the underlying atomic data set, a complete Bayesian treatment has been undertaken with the Minerva Bayesian modeling framework [4]. First, it has been shown through a sensitivity study that the diagnostic method is robust against random measurement errors and systematic calibration errors on the scales achievable with the current diagnostic setup. From this, it is concluded that the majority of the uncertainty in the reconstructed temperature and density arises from systematic uncertainties in the underlying CRM rather than from measurement errors, and that a narrow subset of these processes is disproportionately responsible for plasma profile reconstruction uncertainties [5]. A new R-matrix data set for helium has been calculated and its differences from previous data sets will be discussed, as well as the implications on plasma parameter reconstructions. Finally, corrections for finite magnetic field effects are discussed and the relevant regimes for neglecting these effects are shown.
[1] B. Schweer, et al., J. Nucl. Mater. 196–198 174–8
[2] T. Barbui, et al. Nuclear Fusion 60.10 (2020): 106014
[3] J. M. Muñoz Burgos et al. 2012 Phys. Plasmas 19 012501
[4] Svensson, J., and A. Werner 2007 IEEE, 2007.
[5] Flom, E. et al. Nuclear Mat. & Eng. 2023.
Acknowledgements: This work was funded in part by the U.S. Department of Energy under grant DE-SC00014210 and DE-SC00013911. This work has been carried out within the framework of the EUROfusion Consortium, funded by the European Union via the Euratom Research and Training Programme (Grant Agreement No 101052200 — EUROfusion). Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or the European Commission. Neither the European Union nor the European Commission can be held responsible for them.
| Presenting Author | Erik Flom |
|---|---|
| Presenting Author Email Address | erflom@wisc.edu |
| Presenting Author Affiliation | UW-Madison / IPP-Greifswald |
| Country | USA |
| Presenting Author Gender | Male |
