Measurement of zero-frequency fluctuations generated by coupling between Alfvén modes in the JET tokamak*

We report the first experimental detection of a zero-frequency fluctuation that is pumped by an Alfvén mode in a magnetically-confined plasma. Core-localized bidirectional Alfvén modes of frequency inside the toroidicity-induced gap (and its harmonics) exhibit three-wave coupling interactions with a zero-frequency fluctuation. The observation of the zero-frequency fluctuation is consistent with theoretical and numerical predictions of zonal modes pumped by Alfvén modes, and is correlated with an increase in the deep core ion temperature, temperature gradient, and confinement factor $H_{89,P}$. Despite the energetic particle transport induced by the Alfvén eigenmodes, the generation of a zero-frequency fluctuation that can suppress the turbulence leads to an overall improvement of confinement.