Measurement of coherent drift-wave ion-fluid velocity field when ion dynamics are stochastic

In order to test the validity of the fluid drift approximation when ion dynamics are stochastic, the time-dependent ion-fluid velocity field in a coherent, drift-Alfv\'en wave has been measured in Caltech's Encore tokamak using plasma planar laser induced fluorescence. Many measured parameters of the wave are well described by the drift approximation above a threshold for stochastic ion dynamics, including the flow pattern in the single measured component of the ion-fluid velocity field. Surprisingly, the theory fails at a fundamental level by predicting flow speeds 10 times larger than those measured ({}uₘₐₓ{}{}10⁵ cm/s).