Laser-induced fluorescence measurement of very slow neutral flows in a dusty plasma experiment
Laser-Induced Fluorescence (LIF) provides the temperature and flow velocity of a target species by direct measurement of its velocity distribution via Doppler shift. A LIF diagnostic has been developed at the Caltech water–ice dusty plasma experiment that uses an ultra-narrow tunable diode laser to pump the λ_(vac) = 696.735 nm argon neutral transition. A photomultiplier detects fluorescence emission at λ_(vac) = 772.633 nm. Signal to noise ratios in excess of 100 are achieved along with a high degree of reproducibility between measurements. A Labview program fully automates data collection throughout the three-dimensional plasma volume by controlling four stepper motors and recording measured data. The argon neutral temperature is measured to be slightly above room temperature. Challenges such as the lack of absolute calibration of diode lasers and wavelength drift due to slight changes in ambient room conditions are overcome to measure bulk neutral flow speeds on the order of 1–2 m/s with resolution on the order of 2/3 of a meter per second. High-speed video shows that introducing an argon flow to a cloud of ice grains causes the cloud of ice grains to move and change shape. Ice grain motion is analyzed and found to be in agreement with neutral LIF flow measurements. Surprisingly, when the flow ceases, the ice grain cloud reverts to its original location and shape.
© 2020 Published under license by AIP Publishing. Submitted: 5 March 2020; Accepted: 22 May 2020; Published Online: 9 June 2020. The authors thank Professor Truell W. Hyde of Baylor University in Waco, TX, for graciously inviting us to use an early version of the LIF diagnostic described here on the PK4 replica experimental setup at CASPER to measure temperature and to attempt (unsuccessfully) to measure flow speeds. We also thank Jorge Carmona Reyes and Kenneth Ulibarri for assistance over 10 days in operating the PK4 replica experimental setup. We would also like to thank Armin Ewert of Bundeswehr University Munich for assistance with some of the early attempts to measure flow velocity. This material was based upon work supported by the NASA/NSF Partnership on the Science of Dusty Plasmas via NSF Award No. 1740655 and via NASA Jet Propulsion Laboratory Award No. 1573433. Data Availability: The data that support the findings of this study are openly available in the CaltechDATA repository at https://doi.org/10.22002/D1.1379.
Published - 5.0006684.pdf
Accepted Version - 2020_Marshall_Bellan_RSI_final_accepted_manuscript.pdf