First experimental observation of zonal flows in the optimized stellarator Wendelstein 7-X

Abstract

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. As well, indications are found of local turbulence being modulated by the ZF. Measurements show remarkable agreement with the ZF predicted by nonlinear 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.