Public defence in Engineering Physics, MPhys James Simpson

Title of the thesis: The impact of separatrix conditions on pedestal-SOL coupling: an investigation with integrated transport solvers

Doctoral student: James Simpson
Opponent: Dr. Clarisse Bourdelle, CEA Cadarache, Saint-Paul-lez-Durance, France
Kustos: Prof. Mathias Groth, Aalto University School of Science, Department of Applied Physics

This work explores the interaction between confined and unconfined plasma in Tokamak devices, which are used to create nuclear fusion on Earth with the goal of generating electricity.

A Tokamak employs large magnetic fields to confine the plasma, the same substance that makes up the Sun. Creating a plasma is essential for achieving the high temperatures (approximately ten times hotter than the Sun's core) and densities required for nuclear fusion. However, confinement only lasts a short time before the plasma begins to leak out through an exhaust system known as the divertor. As the plasma transitions from the confined to the unconfined state (moving toward the divertor), it passes through a region called the separatrix - a key focus of this thesis.

In this thesis, we investigate how the plasma in the confined and unconfined areas influence one another, particularly the temperature and density, with the separatrix mediating this interaction. Experimental evidence has shown that these two regions are closely linked, with changes in one area significantly affecting the other. We use multiple computer simulation codes to model the evolution of the confined and unconfined regions self-consistently and to simulate the separatrix under various plasma conditions. However, our simulations fail to reproduce experimental results, indicating that the physics in our computer simulations may be incomplete. We also consider simplified analytic models to simulate the separatrix more quickly than computer simulations. These models demonstrate that it is possible to predict the temperature of the separatrix and, under certain strict assumptions, estimate the density of the confined plasma near the separatrix.

Understanding the interaction at the separatrix is crucial for ultimately building a Tokamak power plant, as it will significantly impact both the plant's lifespan and the amount of electricity it can produce.

Keywords: Tokamak, Plasma Physics, SOL, pedestal, core-edge integration

Thesis available for public display 10 days prior to the defence at: https://aaltodoc.aalto.fi/doc_public/eonly/riiputus/

Doctoral theses at the School of Science: https://aaltodoc.aalto.fi/handle/123456789/52

Contact information: james.simpson@ukaea.uk