  In this work, we present the first experimental evidence of the presence of
zonal flow (ZF) structures in the optimized stellarator Wendelstein 7-X. Using
an assortment of diagnostics, flux surface-uniform, electrostatic flow
oscillations have been measured, showing a radial scale in the range of tens of
ion gyroradii. Such measurements show remarkable agreement with the ZF
predicted by local and global non-linear gyrokinetic simulations. These results
represent the first direct measurement of ZF in a large stellarator, suitable
for the validation of models in reactor relevant conditions.
