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Public defence in Engineering Physics, M.Sc. (Tech) Henri Kumpulainen

Advancements in fusion simulations
Doctoral hat floating above a speaker's podium with a microphone

Title of the doctoral thesis: Validation of tungsten erosion and transport simulations in tokamaks

Doctoral student: Henri Kumpulainen
Opponent: Dr. Thomas Pütterich,Max Planck Institute for Plasma Physics, Germany 
Custos: Prof. Mathias Groth, Aalto University School of Science, Department of Applied Physics

Magnetic-confinement fusion devices, such as tokamaks, are being studied as a potential safe and sustainable emission-free method of energy production. Due to the extremely high temperature required to sustain the fusion reactions, the fusion fuel is confined as a plasma inside a vacuum chamber using a magnetic field. One of the most significant challenges in designing a fusion power plant is extracting the energy produced by fusion without destroying the plasma-facing components in the vacuum chamber. 

Tungsten is a leading candidate as the plasma-facing material due to its high melting point and resilience to erosion by the plasma, among other reasons. On the other hand, eroded tungsten impurities contaminating the core of the fusion plasma may prevent viable power production even at very low concentrations. Therefore, the ability to understand and predict the erosion and transport of tungsten in the plasma is crucial to designing and operating tokamaks. 

This dissertation studies the ability of simulation codes to predict the erosion of tungsten components and the transport of tungsten in the fusion plasma. Several modelling approaches are compared and analysed, and the validity of the models is assessed using measurement data from the JET and ASDEX Upgrade tokamaks. The studied plasmas range from well-diagnosed low-confinement mode to record-performance high-confinement mode plasmas. 

Most of the tungsten erosion occurs on the wall components receiving the highest flux of heat and plasma particles, called the divertor targets. However, all of the studied simulations predict that the tungsten eroded at the divertor targets is almost completely redeposited near the targets without contaminating the core plasma. Instead, the tungsten predicted to reach the core plasma is primarily from tungsten components closer to the core plasma, caused by atoms created in ion-atom charge-exchange collisions. 

This dissertation provides the first demonstrations of the ability of predictive tungsten erosion and transport simulations to forecast the tungsten density in the JET tokamak core plasma within a factor of 2 of the measurements. The level of code-experiment agreement is within the modelling uncertainties. The validation of the studied models supports their applicability to designing new fusion devices and plasma scenarios, and the identified limitations of each model guide the choice of an appropriate model for each purpose.

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

Contact information:

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

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