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 io n dynamics are stochastic, the time-dependent ion-fluid velocity field in a coherent, drift-Alfven wave has been measured in Caltech's Encor e tokamak using plasma planar laser induced fluorescence. Many measure d parameters of the wave are well described by the drift approximation above a threshold for stochastic ion dynamics, including the flow pat tern in the single measured component of the ion-fluid velocity field. Surprisingly, the theory fails at a fundamental level by predicting f low speeds 10 times larger than those measured (\mu(max)\ almost-equal -to 10(5) cm/s).