Passive High-speed Imaging of Ion Acoustic Turbulence in a Hollow Cathode

摘要

A passive, non-invasive plasma diagnostic was developed and validated to estimate the dispersion relation of propagating electrostatic modes in a hollow cathode discharge. This new technique used a Silicon photomultiplier (SiPM) array to measure fluctuations in light intensity and relate them to fluctuations in plasma density. The development and validation of the SiPM technique is discussed in the context of non-invasive measurements of plasma waves. This technique is compared to a similar method using ion saturation probes to measure density fluctuations. Ion acoustic waves, believed to be responsible for the anomalous resistivity of the cathode plume, were measured using both ion saturation probes and the SiPM technique. While the raw data and power spectra acquired with the SiPM were different from the probes, the estimated dispersion relations were similar and the phase velocities, were within uncertainty of each other. These measurements were compared to similar measurements found in the literature to confirm that ion acoustic turbulence was measured. The weaker signal to noise in the SiPM signal is shown to most likely be a result of spatial averaging. The results suggest that the SiPM technique may be capable of estimating the dispersion of high-frequency waves in other plasma devices.