Laser diagnostics on atmospheric-pressure low-temperature helium pulsed plasmas in room- and cryogenic-temperature environments

In atmospheric-pressure low- temperature plasmas, the control of the plasma gas temperature ($T_{\mathrm{g}})$ by a few kelvin is considered to be crucial for their applications to novel materials processing such as bio-materials. However, there have been only few studies that focused on the influence of $T_{\mathrm{g}}$ on the plasma characteristics. On the other hand, it was reported that helium metastables played a key role in the dependency of chemical reactions on $T_{\mathrm{g}}$ in helium-nitrogen plasmas. In this study, laser diagnostics were carried out in atmospheric-pressure helium pulsed plasmas near or below room temperature, at 340$-$100 K. Parallel electrodes of copper rods (diameter: 2~mm) with a gap distance of 535~$\mu$m were used and pulsed discharges with a pulse width of a few hundred nanoseconds were generated inside a reactor. The density and lifetime of helium metastables were estimated by laser absorption spectroscopy measurements and $T_{\mathrm{g}}$ was evaluated by near-infrared laser heterodyne interferometry measurements. At 300~K, the helium metastable density was 1.5 $\times$ 10$^{13}$ cm$^{-3}$ while the lifetime was 3.1~$\mu$s, and increase in $T_{\mathrm{g}}$ was up to 70~K. Dependency of the density and lifetime of helium metastables on $T_{\mathrm{g}}$ was observed and also discussed.