Zero-frequency zonal modes and predator-prey behavior driven by Alfvén eigenmodes in the JET tokamak: measurement and gyrokinetic simulations

A seminar by Juan Ruiz Ruiz

Energetic particles will be ubiquitous in next-generation burning plasmas, most prominently in the form of 3.5 MeV alpha particles issued from the DT reaction. Understanding how they will impact the overall plasma confinement is therefore of dire importance for predicting fusion performance. For decades we have learned that energetic particles can linearly destabilize certain types of Alfvén modes, which drive large transport of the energetic particles. More recently, theory and numerical simulations have suggested that Alfvén modes could have a beneficial effect on the thermal plasma by generating a stationary zonal perturbation, which could tear apart the turbulent eddies and result in improved confinement. This zero-frequency zonal perturbation, however, has remained without experimental confirmation until now. In this work, we report the first experimental detection of a zero-frequency fluctuation that is generated by Alfvén modes in a magnetically confined plasma. Experiments in the JET tokamak have shown that Alfvén modes destabilized by energetic particles exhibit three-wave coupling interactions with a zero-frequency fluctuation, and its presence is correlated with (otherwise unexplained) increased ion temperature and improved confinement. The Doppler-backscattered spectrogram reveals an intricate predator-prey behavior between the Alfvénic modes, which manifest themselves in periodic bursts, and the turbulence fluctuations, which are reduced by an order of magnitude during the Alfvénic bursts. Nonlinear gyrokinetic simulations confirm the three-wave coupling between zero-frequency zonal modes and Alfvén modes, resulting in reduced turbulence and transport levels. This suggests that the zero-frequency perturbation that we detect is a zonal mode that is responsible for the improvement in confinement observed in the JET experiment. The implications from this work and prospects for next-generation burning plasmas will be discussed.

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