Wave-particle interactions are ubiquitous in nature and play a fundamental role in astrophysical and fusion plasmas. In solar plasmas, magnetohydrodynamic (MHD) fluctuations are thought to be responsible for the heating of the solar corona and the generation of the solar wind. In magnetically confined fusion (MCF) devices, enhanced particle transport induced by MHD fluctuations can deteriorate the plasma confinement, and also endanger the device integrity. MCF devices are an ideal testbed to verify current models and develop mitigation / protection techniques.
In this project we aim to provide a solid understanding of the interaction between particles and MHD instabilities in the presence of electric fields and plasma currents with the ultimate goal to provide control techniques to tame the MHD induced particle transport in a fusion plasma. To this end, we have developed innovative diagnostic techniques which will soon be exploited scientifically as we achieve the first measurements. Combined with state-of-the-art hybrid MHD codes, a deep insight into the underlying physics mechanism of wave-particle interactions will be gained.
We have successfully developed, commissioned and operated two innovative scintillation imaging diagnostics at the ASDEX Upgrade tokamak. The diagnostics provide, for the first time, unprecedented spatio-temporal measurements of the confined energetic ion population and edge current density and filamentary transport of electron density fluctuations. We were able to decipher important wave-particle interactions between several magnetohydrodynamic fluctuations and the ion species (both thermal and supra-thermal) by combining both experiment and theory. Non-linear hybrid kinetic-MHD simulations of our experiments reveal that the kinetic treatment of energetic ions is key in understanding their effects on the spatio-temporal structure of ELMs in tokamaks. The resonant interaction between the drift orbits of the edge fast-ion population and the ELM electromagnetic perturbations results in a net exchange of energy and momentum between the wave and particles, which ultimately determines the spatio-temporal structure of the ELMs. We revelead that the ion and electron energy transport recover on different timescales, with the electrons recovering on a slower timescale. Combining the measurements with modelling, we were able to identify that the dominant mechanism for the additional energy transport in the electron channel, that could cause the delay in the electron temperature gradient recovery, is due to the depletion of energy caused by the ELM itself.
