Anomalous structure in drift-wave turbulence

We report the first experimental observations of a two-point correlation function in driftwave fluctuations resolved in the ion phase-space. A high optical efficiency two-point scanning LIF system that resolves ion velocity along the fixed magnetic field of the plasma cylinder is used to observe correlations in drift-wave fluctuations. Tests are performed to demonstrate the validity of the measurement despite the effects of photon statistical fluctuations. The plasma is a CW 2 meter length, five centimeter radius singly-ionized Argon gas discharge with n ∼ 10 ⁹ cm ^-3, T _e ∼ 2 eV and T _i ∼ 0.1 eV. The neutral pressure is 2 × 10 ^-4 torr. Driftwaves are observed in the few kHz frequency range with spectra that are typical of low density gas discharges. What is new is the possibility to measure the two-point correlation as a function of ion particle velocity along the magnetic field. What we observe is firstly the contribution of the long-parallel wavelength driftwaves, in agreement with their usual description. The correlation length along the magnetic field is long compared with 20cm. A phase velocity on the order of 106 cm/s is observed for the drift wave. However, the data also have a slow phase velocity component that travels at a velocity near the ion thermal speed (a hundred times slower) and varies with the ion velocity selected by the laser. This component has no conventional explanation, though there have been theories of ion holes/clumps which could be relevant to the experiment. The two components are readily observed through the phase dependence of the complex cross-power. Plots of phase as a function of spatial separation along the magnetic field clearly reveal the two wavelengths. We will present the status of the measurements and discuss possible points of contact with theory.