Density of hybrid plasma generated by microwave and laser radiation in the Ar:H2:CH4 mixture

The atmospheric-pressure hybrid plasma in the Ar:H2:CH4 mixture, maintained by microwave radiation (2.47 GHz) and CO2 laser radiation (10.6 μm) in the chamber of an experimental plasma-chemical reactor designed to study the synthesis of diamond-like coatings was studied. The electron number density was determined from the Stark broadening of the Hα line shape of atomic hydrogen. It was shown that when laser radiation is focused in the region of a microwave plasma bunch, the Hα line shape in the hybrid plasma spectra has broad wings and is described by a Lorentz function with a two-contour approximation. The complex structure of the Hα line profile of the hybrid plasma indicates its spatiotemporal inhomogeneity. The electron number density corresponding to the contour with a smaller half-width exceeds the electron number density in microwave plasma and lies in the range of (4-8)E15 cm-3, and the electron number density measured by the contour with a larger half-width is (1.5-2)E17 cm-3.

💡 Research Summary

This study investigates the electron number density in a novel hybrid plasma generated at atmospheric pressure within an Ar:H2:CH4 gas mixture, combining continuous microwave radiation and pulsed CO2 laser irradiation. The research was conducted in a plasma-chemical reactor designed for diamond-like coating synthesis, aiming to overcome the limitations of individual plasma sources: the low charged-particle density of microwave plasma and the small volume/high gas temperature of laser plasma.

The experimental setup involved a cylindrical microwave resonator chamber. A microwave discharge (2.47 GHz, 880 W) was first ignited, creating a plasma bunch. Subsequently, pulsed CO2 laser radiation (10.6 μm) with variable repetition rates (3-60 kHz) and average power (140-670 W) was focused into the center of this microwave plasma. The emission from the resulting hybrid plasma was collected and analyzed using a high-resolution spectrometer.

The primary diagnostic method was optical emission spectroscopy based on the Stark broadening of the atomic hydrogen Hα line at 656.3 nm. The Hβ line, often preferred for density measurements, was unsuitable due to overlap with Ar+ ion lines in the hybrid plasma spectrum. The instrumental and Doppler broadening contributions were carefully characterized and found to be negligible (∆λ < 0.03 nm) compared to the observed significant broadening in the hybrid plasma.

The key finding lies in the stark difference between the spectral line shapes. The Hα line profile from the microwave-only plasma was well-fitted by a Voigt function, yielding an electron density of approximately 1.5×10^15 cm⁻³. In contrast, the Hα line from the hybrid plasma exhibited much broader wings and could not be fitted by a single Lorentz or Voigt function. Instead, it required a two-contour approximation using two Lorentzian functions. This complex structure is interpreted as direct evidence of strong spatiotemporal inhomogeneity within the hybrid plasma, arising from the rapid dynamics induced by the laser pulses.

Deconvolving the two Lorentzian components allowed the extraction of two distinct electron density ranges. The narrow central contour corresponded to an electron density in the range of (4-8)×10^15 cm⁻³. This is suggested to represent the peripheral regions of the hybrid plasma or later times in its evolution after the laser pulse, where the density approaches but still exceeds that of the undisturbed microwave plasma. The broad contour, contributing to the wide wings of the line profile, corresponded to a significantly higher density of ~(1.5-2)×10^17 cm⁻³. This high value is attributed to the dense core of the hybrid plasma formed instantaneously in the laser focus region during and immediately after the laser pulse.

The study concludes that the introduction of pulsed CO2 laser radiation into a steady-state microwave plasma successfully generates a hybrid plasma with electron densities orders of magnitude higher than those achievable with microwave plasma alone. The two-component density structure visually captured in the Hα line profile confirms the expected spatial and temporal non-uniformity, linking the laser’s localized and pulsed energy delivery to the creation of a high-density zone within a larger, more stable plasma volume. This work provides crucial experimental data supporting the potential of hybrid plasma sources for enhancing the growth rates in plasma-enhanced chemical vapor deposition processes, such as the synthesis of diamond-like carbon films.