| Ion density fluctuation measurements are performed in a low density, linearly magnetized discharge. Using Laser Induced Fluorescence (LIF) and a two-point correlation technique, these fluctuations are resolved in space, time, and ion velocity. The experimental implementation of the two-point correlation technique is achieved by means of spatially resolved volumes using two independently moveable periscopes. Subsequently, by computing cross-power spectra between the two LIF signals (collected using the periscopes), the ion density fluctuations reveal two components with distinct correlation lengths. A long wavelength component, which corresponds to the drift wave parallel wavelength (associated with a long correlation length) is readily observed. In addition, a short wavelength component with a short correlation length consistent with the ion mean free path is detected. The latter wavelength is a component of the ion density fluctuation that propagates close to ion parallel velocities. This newly identified velocity dependent component is referred to as the kinetic component and is found to be associated with a broad drift wave spectrum (deltao/o ∼ 10%). As the collisionality in the discharge is increased, we observe a threshold that is marked by a narrowing of the drift wave spectrum. At this threshold, the kinetic component vanishes. Through correlations between a Langmuir probe and LIF, we also present velocity dependent measurements of the fluctuation induced transport rate near the drift frequency. These measurements are found to significantly depart from the classical prediction of the drift wave transport rate. |
