Laser-induced fluorescence measurements of ion fluctuations in electron and ion presheaths

Hood, R.; Baalrud, S. D.; Merlino, R. L.; Skiff, F.

doi:10.1063/1.5142014

Laser-induced fluorescence measurements of ion fluctuations in electron and ion presheaths

Journal Article · Tue May 12 00:00:00 EDT 2020 · Physics of Plasmas

DOI:https://doi.org/10.1063/1.5142014· OSTI ID:1803000

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Univ. of Iowa, Iowa City, Iowa (United States). Dept. of Physics and Astronomy; University of Iowa
Univ. of Iowa, Iowa City, Iowa (United States). Dept. of Physics and Astronomy

Electrostatic flow-driven instabilities have been predicted in the presheath region of low-temperature plasmas for both ion and electron rich sheaths. The ion-acoustic fluctuations that arise from these instabilities can influence transport through an enhanced collision rate. In this work, ion fluctuation spectra were measured using laser-induced fluorescence (LIF) in the presheath region for both sheath polarities. Furthermore, the non-invasive nature of LIF compared to that of probes is particularly valuable for these measurements because the sheath and presheath around a probe may generate their own flow-driven instabilities, which obscure the measurement. Measurements were made using a recently implemented field programmable gate array-based system able to measure ion fluctuation spectra up to 1 MHz using a two-point correlation function technique. Narrow bandwidth ion fluctuations were observed near 0:46f_pi (460 kHz) for both sheath polarities, where f_pi is the ion plasma frequency. The observed fluctuations were significantly stronger in the electron sheath case and were observed over a larger volume of plasma. Fluctuations were measured in the ion sheath case at locations far enough from the sheath that linear theory predicts stability, suggesting that ion-acoustic instabilities generated in the relatively small unstable region near the boundary reflect from the sheath and perturb a much larger volume of the plasma. This is expected to affect probes with both ion and electron rich sheaths, meaning any dc biased probe may effectively act as an ion-acoustic wave antenna. These measurements are consistent with the recent theory and particle-in-cell results.

View Accepted Manuscript (DOE)

Research Organization:: Univ. of Iowa, Iowa City, IA (United States)

Sponsoring Organization:: USDOE Office of Science (SC), Fusion Energy Sciences (FES)

Grant/Contract Number:: SC0016473

OSTI ID:: 1803000

Alternate ID(s):: OSTI ID: 1618161

Journal Information:: Physics of Plasmas, Journal Name: Physics of Plasmas Journal Issue: 5 Vol. 27; ISSN 1070-664X

Publisher:: American Institute of Physics (AIP)Copyright Statement

Country of Publication:: United States

Language:: English

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