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Helium metastable species generation in atmospheric pressure RF plasma jets driven by tailored voltage waveforms in mixtures of He and N2

Spatially resolved tunable diode-laser absorption measurements of the absolute densities of He-I (23S1) metastables in a micro atmospheric pressure plasma jet operated in He/N_2 and driven by ‘peaks’- and ‘valleys’-type tailored voltage waveforms are presented. The measurements are performed at different nitrogen admixture concentrations and peak-to-peak voltages with waveforms that consist of up to four consecutive harmonics of the fundamental frequency of 13.56 MHz. Comparisons of the measured metastable densities with those obtained from particle-in-cell/Monte Carlo collision simulations show a good quantitative agreement. The density of helium metastables is found to be significantly enhanced by increasing the number of consecutive driving harmonics. Their generation can be further optimized by tuning the peak-to-peak voltage amplitude and the concentration of the reactive gas admixture. These findings are understood based on detailed fundamental insights into the spatio-temporal electron dynamics gained from the simulations, which show that voltage waveform tailoring allows to control the electron energy distribution function to optimize the metastable generation. A high degree of correlation between the metastable creation rate and the electron impact excitation rate from the helium ground state into the He-I ((3s)3S1) level is observed for some conditions which may facilitate an estimation of the metastable densities.

FieldValue
Publisher
Authors
Release Date
2020-09-15
Resources
Identifier
ed0c0b96-51e1-4927-aaa8-db3b57318ac9
Permanent Identifier (DOI)
Permanent Identifier (URI)
Plasma Source Name
Plasma Source Application
Plasma Source Specification
Plasma Source Properties
f_0=13.56MHz, consist of two identical co-planar electrodes made from stainless steel. The electrode gap is 1 mm. The electrodes are enclosed by two quartz plates confining the plasma volume to 1x1x30 mm^2
Plasma Medium Name
Plasma Medium Properties
The discharge is operated in helium (5.0 purity) with nitrogen (5.0 purity) admixture. The flow rate of helium is fixed at 1000 sccm while the nitrogen flow rate is varied from 0.5 to 4 sccm. This flow rate range of N_2 corresponds to reactive gas concentrations between 0.05% and 0.4% in the He buffer gas. The gas pressure inside the jet is estimated to be 1.02(±0.02)×10^5 Pa. The temperature is roughly room temperature.
Plasma Diagnostic Name
Language
English (United States)
License
Public Access Level
Public
Funding Agency
Project
Subproject
Contact Name
Ihor Korolov
Contact Email

Data and Resources