Comparison of probe and narrow-band imaging measurements in a magnetized cylindrical plasma

We analyze similarities and differences between electrostatic probe measurements and high-speed visible light imaging measurements in a low-temperature argon helicon plasma. Electrostatic probes are the usual diagnostics for making fluctuation measurements in low-temperature plasmas, although fast-imaging of visible emission is becoming increasingly common. The two diagnostics are highly complementary, and typical analysis takes fluctuations in both ion saturation current and visible light intensity to be proxies for plasma density fluctuations. The observations presented here indicate that care is needed in applying this approximation. The plasma is produced with a helicon antenna, and we measure fluctuations in ion saturation current, floating potential, and visible emission near wavelengths of 488 nm (argon ions) and 750 nm (argon neutrals). Simultaneous measurements show relatively low correlation values between ion saturation current and ion line emission and an anti-correlation between floating potential and ion line emission. Variations in cross-coherence between ion emission and probe quantities indicate the frequency dependence in the relationship between the measurements. For various frequency bands where probe/imaging coherence is relatively high, fluctuation intensity profiles differ significantly and coherent fluctuations are observed in ion light which are not apparent in the probe measurements. A sensitivity analysis indicates that electron temperature fluctuations, although nearly always neglected when analyzing data from low-temperature plasma experiments, may play a role in the discrepancy between probe and narrow-band imaging measurements.