Observation of fast stochastic ion heating by drift waves

With use of laser-induced fluorescence in the Caltech Encore tokamak, anomalously fast (40×{}classical) ion heating has been observed and found to correlate with the presence of large-amplitude drift-Alfv\'en waves. Using numerical simulations we demonstrate that the heating is stochastic and occurs when ion displacement due to polarization drift becomes comparable to the perpendicular wavelength, i.e., when k_⊥²{φ} ̃ \~{}{}/ωci{B}₀${}1. Stochastic heating may also be the cause of the anomalously high ion temperatures observed in reverse-field pinches.