Nuclear fusion has the potential to replace fossil energy sources and cover the energy needs of the world’s population. It is the process that powers the Sun and stars. The energy is produced by fusing together two hydrogen isotopes, deuterium, extracted from water, and tritium, extracted from the Earth’s crust. To replicate this process on Earth, however, extremely high temperatures are required to completely ionise the fusing atoms. This takes place in magnetically confined fusion devices (tokamaks and stellarators) which harness the fusion energy as heat and convert it into steam and then electricity through turbines and generators. However, the entire process is characterised by magnetohydrodynamic instabilities known as edge localised modes (ELMs) which result in heat and particle loss and limit the lifetime of these fusion reactors.
Investigating the ELM phenomena
With the support of the Marie Skłodowska-Curie programme, scientists of the EU-funded FIREFELM project studied the ELM phenomenon by combining high-resolution measurements with state-of-the-art numerical tools. “ELMs expel particles and energy from the plasma similar to solar flares at the edge of the Sun. Understanding and controlling or even suppressing ELMs is central to the successful realisation of fusion,″ explains the research fellow Eleonora Viezzer. FIREFELM researchers modelled the transport channels within a tokamak, shedding light on the dynamic behaviour of the transport coefficients during the ELM cycle. They discovered that ion and electron heat transport recover at different time scales, with the electrons recovering slower. This indicated that the depletion of energy caused by the ELM delays the electron temperature gradient recovery. Results also suggested that the core plasma may dictate the local dynamics of the electron temperature gradient recovery during the ELM cycle. Additionally, scientists identified a resonant mechanism between the beam ion orbits and the parallel electric fields that could be associated with the ELM. For the first time, they were able to observe acceleration of beam ions on the ASDEX Upgrade tokamak. Through numerical simulations of fast-ions (particles with suprathermal energy) and an analytical model, researchers qualitatively reproduced the experimental observations.
Impact and future prospects of the FIREFELM work
Fusion can provide a clean, secure and viable energy source with no carbon emissions. Deuterium is available in our oceans and tritium can be retained from lithium, which is available in our Earth’s crust, rendering fusion a virtually unlimited energy source. A large international effort ITER is underway to advance the process of fusion and translate existing tokamak machines into the fusion power plants of the future. With ELM posing a serious obstacle for steady-state operation of these future fusion devices, the work of the FIREFELM project is paramount for mitigating these phenomena. “Identifying the dominant transport mechanisms will help us better understand the ELM cycle and develop high confinement regimes without ELMs,″ continues Viezzer. FIREFELM findings advance our understanding of the observed particle acceleration and transport in the solar corona, and help identify similarities between tokamak and astrophysical plasmas. Viezzer is continuing this line of research towards the development of ELM control techniques through the 3D-FIREFLUC ERC starting grant.
FIREFELM, fusion, edge localised mode (ELM), tokamak, deuterium, tritium