| A divergent Electron Cyclotron Resonance (ECR) plasma source has been built at Columbia University to study plasma equilibrium and stability. Using movable Langmuir probes and microwave pick-up probes, plasma fluctuations have been observed in a frequency band below 50 kHz. Two probe correlation measurements have been conducted to study the fluctuations and mode structure as the plasma source operating parameters are adjusted. As the neutral gas pressure increases, we observe three types of plasma fluctuations. We refer to these as the "hot electron relaxation", the "coherent", and the "cold electron relaxation" fluctuations because of the characteristics of the floating potential oscillations. The frequency spectra of the plasma fluctuations depend sensitively on the gas pressure and absorbed microwave power. When operating at intermediate gas pressure, coherent fluctuations exist and the radial, azimuthal and axial structures of these modes have been studied using digital cross-correlation techniques with movable probe pairs. Plasma fluctuations at 3 kHz and 6 kHz are observed to have low azimuthal mode numbers, with m = 1 and m = 2, respectively, The scaling of this frequency with source parameters indicates that the frequency corresponds to the electron diamagnetic drift frequency. By showing that the observed fluctuations are comparable to the predictions of a simple linear model, density gradient driven drift wave instability is considered to be the cause for the plasma fluctuations. |
