Public defence, Engineering Physics, MSc Evgeniia Ponomareva

Title of the thesis: Electronic effects in radiation damage of nuclear materials under light ion impact

Thesis defender: Evgeniia Ponomareva 
Opponent: Senior Research Engineer Christophe Domain, EDF, France
Custos: Assistant Professor Andrea Sand, Aalto University School of Science

Modern societies need low-carbon energy sources that can meet growing demand while substantially reducing environmental impact. Nuclear fusion is one possible route toward this goal. In a fusion reactor, hydrogen isotopes are heated above 100 million degrees and confined by powerful magnetic fields. Particles escaping from the resulting plasma continuously bombard the surrounding materials: hydrogen isotopes and fusion-born helium ions can penetrate, damage, and erode reactor walls. Understanding and predicting these ion-solid interactions is therefore essential for developing materials that can withstand the reactor environment. 

This thesis explores how low-energy hydrogen and helium ions interact with fusion-relevant materials and how these interactions should be represented in simulations. When an ion hits a material, it loses energy through collisions with atoms and through electronic excitations. For light ions, electronic energy losses are significant even at the low energies of plasma-wall interactions. Atomistic simulations, the workhorse of radiation damage prediction, commonly neglect this contribution or assume that it depends only on the ion velocity. In a crystal, however, ions can travel along open channels between rows of atoms, where they encounter fewer electrons than when passing close to an atom, so the electronic energy loss depends on the ion's path. This dependence can change how deep ions penetrate, whether they bounce back, and how much material they knock out of the surface. 

The thesis combines quantum-mechanical calculations, atomistic simulations, and experimental data to follow this problem across length and time scales. Electronic energy losses were calculated for hydrogen and helium in tungsten and iron along different paths through the crystal. These results were incorporated into models of path-dependent energy loss, validated against experiments, and used to simulate ion impacts on metal surfaces. 

The main finding is that the importance of path dependence varies between materials. For deuterium in tungsten, including it significantly changes the predicted erosion compared with a simple velocity-dependent friction, while for deuterium in iron the effect is much smaller. These results contribute to a more informed choice of models for simulating plasma-wall interactions. 

The methods and insights developed here can also support ion irradiation modeling in other fields, including space technology, medicine, and material processing.

Keywords: electronic stopping, radiation damage, fusion energy, multiscale modeling

Thesis available for public display 7 days prior to the defence at Aalto University's public display page.