• Promoters: Prof. Geert Verdoolaege, Prof. Kristel Crombé
  • Supervisor: Dr. Yangyang Zhang
  • Study programs: Master of Science in Engineering Physics, Master of Science in Physics and Astronomy, Master of Science in Teaching in Science and Technology (Physics and Astronomy), European Master of Science in Nuclear Fusion and Engineering Physics
  • Location: Technicum, at home

Problem setting

TOMAS is an experimental device primarily dedicated to developing and testing various wall conditioning techniques and studying plasma-wall interactions. To support experimental analysis, several diagnostic systems have been implemented, including single- and triple-pin Langmuir probes.

A single Langmuir probe applies a voltage sweep and measures the collected current. The resulting current-voltage (I-V) curve depends on the electron density (ne) and electron temperature (Te) of the plasma, allowing these parameters to be inferred. The triple probe serves a similar purpose but features a more advanced design, eliminating the need for a voltage sweep and enabling faster and more straightforward analysis.

Additionally, a recently installed interferometer provides line-integrated measurements of electron density, while electron density estimates can also be derived from camera image intensities. Since these diagnostic systems offer complementary insights into ne and Te, a joint analysis is desirable.

A powerful approach for combining diagnostic data is Bayesian probabilistic inference, commonly referred to as Integrated Data Analysis (IDA) in fusion research. IDA systematically merges data from different diagnostics, allowing for robust error propagation studies and reducing uncertainties in inferred plasma parameters. In a device like TOMAS, where diagnostic resources are limited, applying IDA is especially valuable for improving the reliability of experimental results.

I-V curve

Example of an I-V curve for a single Langmuir probe.

Schematic of a Langmuir probe diagnostic

Schematic of a triple Langmuir probe.

Objectives

The objective of this thesis is to develop an IDA framework for the probe, camera, and interferometer diagnostics at TOMAS using Bayesian probabilistic methods. This will enable the determination of spatially resolved electron densities and temperatures.

First, the models of the probe diagnostics will need to be determined for calculating the raw measurements from assumed ne and Te data, the so-called forward models. Next, the main sources of uncertainty entering the forward models will need to be assessed. This will be done in close collaboration with the scientists responsible for the diagnostics at TOMAS. Using the Bayesian formalism, the probability distribution of the spatially resolved electron density and temperature can then be obtained. Finally, specialized computational techniques will be used to sample from this distribution, inferring ne, Te and their uncertainty (error bars). These results will be compared with those obtained from the individual diagnostics.

This research will contribute to a more accurate and comprehensive understanding of plasma behavior in TOMAS, enhancing the reliability of diagnostic data interpretation.