Onsager's Regression Hypothesis Applied to Driftwave Fluctuations

Summary form only given. We analyze time correlation functions of drift wave fluctuations in the ion distribution function between spatial points separated along a fixed magnetic field in a plasma cylinder. The measurements are resolved in the component of ion velocity along the magnetic field. Even though Onsager's hypothesis was originally proposed for nearly-thermal fluctuations we observe that the time correlation function does resemble the linear response function even under circumstances where the plasma is unstable and highly non-thermal. In addition to a surprising level of agreement with Onsager's hypothesis of the drift wave fluctuations we also observe a component of the fluctuations that does not appear to be explained by the drift waves and which, in contrast to the drift wave component, lacks a number of symmetries (isotropy along the magnetic field and isotropy with respect to ion velocity direction along the magnetic field). We explore possible explanations for the second component.