H. Trimino Mora

The Thomson scattering diagnostic at Wendelstein 7-X and its performance in the first operation phase

The optimized stellarator Wendelstein 7-X started operation in December 2015 with a 10 week limiter campaign. Divertor experiments will begin in the second half of 2017. The W7-X Thomson scattering system is an essential diagnostic for electron density and temperature profiles. In this paper the Thomson scattering diagnostic is described in detail, including its design, calibration, data evaluation and first experimental results. Plans for further development are also presented. The W7-X Thomson system is a Nd:YAG setup with up to five lasers, two sets of light collection lenses viewing the entire plasma cross-section, fiber bundles and filter based polychromators. To reduce hardware costs, two or three scattering volumes are measured with a single polychromator. The relative spectral calibration is carried out with the aid of a broadband supercontinuum light source. The absolute calibration is performed by observing Raman scattering in nitrogen. The electron temperatures and densities are recovered by Bayesian modelling. In the first campaign, the diagnostic was equipped for 10 scattering volumes. It provided temperature profiles comparable to those measured using an electron cyclotron emission diagnostic and line integrated densities within 10% of those from a dispersion interferometer.

Acceleration of Bayesian model based data analysis

A general solution for dealing with parameter estimation in a rigorous way is Bayesian data analysis. This analysis allows estimation of specific parameters and their uncertainties for non-linear inverse problems in a strictly mathematical way. Though advantageous, it is computationally intensive with long processing times and therefore, with exceptions such as the Kalman filter for linear systems with Gaussian noise, its full analysis is not commonly used for real-time applications. For non-linear problems, there are many suboptimal Bayesian online algorithms using different approaches which all introduce some kind of approximation. Nevertheless, an approach avoiding approximations to improve inference is desired. An acceleration of Bayesian analysis for inverse problems using reconfigurable hardware or multi-core processing is necessary and could prove valuable for scientific inference. This paper describes the acceleration of the Dispersion Interferometer (DI) for the Wendelstein 7-X (W7-X) magnetic confinement device as a proof of principle of the Bayesian model based data analysis.