Dispersion relations of density fluctuations observed by heavy ion beam probe in the TEXT tokamak

Wave numbers as functions of frequency for density fluctuations in the core of the TEXT tokamak are measured in Heavy Ion Beam Probe experiments by analyzing the relative phases of signals originating from nearby points in the plasma. The adjacent points are typically 2 cm apart, with their relative orientation (delta(r), delta(theta)) depending on position (r,theta). for angular frequencies omega is less than or equal to 10(exp 6)/s the signals are quite coherent, leading to reasonably well-defined 'dispersion relations.' These do not correspond to known modes of the drift wave type, i.e., ballooning or slab-like electron drift waves or ion temperature gradient modes. The effect of finite sample volume size does not significantly alter this conclusion.