Experimental and numerical investigation of time evolution of discharge current and optical emission in helium–nitrogen cryoplasmas

Abstract

Cryoplasmas represent a class of non-equilibrium plasmas whose gas temperature can be controlled below room temperature. However, so far, the influence of the plasma gas temperature on the plasma chemical reactions has not yet been examined in detail. Here we investigated the time-dependent reaction dynamics related to optical emission in helium-nitrogen cryoplasmas. We acquired voltage and discharge current waveforms, optical emission spectra, and observed a temporal change of the emission intensity in helium-nitrogen cryoplasmas at two experimental conditions (temperatures of temperature detector: 5 K and plasma gas temperature: 28 K (condition A), 40 K and 54 K (condition B)). Two time-dependent phenomena were observed: the first was a longer duration of the discharge current compared to that of helium emission at both conditions A and B, and the second was nitrogen ion emission delayed by about 8 µs with respect to the emissions of atomic helium and helium dimers at 40 K. The experimental observations could be reproduced qualitatively by a global reaction model, which took into account the effect of the plasma gas temperature on the reaction rate constants and the diffusion coefficients. The simulations suggested that the reactions related to metastable helium atom were the key reactions, and that the long lifetimes of metastable helium atoms at cryogenic gas temperatures are crucial for the appearance of the time-dependent phenomena. These results imply that the plasma gas temperature is one of the key parameters in non-equilibrium plasma chemistry.

Laser absorption spectroscopy diagnostics of helium metastable atoms generated in dielectric barrier discharge cryoplasmas

“…The method of numerical calculation is introduced in Ref. 17 and we modified the calculation conditions considering the DBD possessing a slightly different geometry including polyimide films 29) sandwiched between the electrodes. The electric input powers were measured at 22.3 mW for T ga = 300 K, 24.0 mW for T ga = 100 K, and 15.4 mW for T ga = 14 K. When we assume that all electric power contributes exclusively to the increase in gas temperature (i.e., no other pathways to consume the power are possible), we can calculate the maximum of T gp (T gp_max ), where the power transfer to gas temperature can be assumed to be overestimated.…”

“…We consider the gases of He, N 2 , O 2 , and water vapor (H 2 O), taking into account the following five reactions related to He m quenching in the discussion part based on our previous study. 17) Three-body reaction with ambient He atoms R1 ðRef: 31Þ: He m þ 2He ! He 2 þ þ He þ e À ½k R1 ¼ ð8:7  10 À36 Þ Â T g  expðÀ750=T g Þ þ ð0:41  10 À36 Þ Â T g  expðÀ200=T g Þ cm 6 Ás À1…”

“…We consider the gases of He, N 2 , O 2 , and water vapor (H 2 O), taking into account the following five reactions related to He m quenching in the discussion part based on our previous study. 17) Three-body reaction with ambient He atoms…”

“…16) Recently, we have found that the total chemical reaction system in cryoplasmas also varies dynamically following a change in gas temperature due to changes in reaction rate coefficients, transport parameters, and gas compositions. 17) This chemical feature of cryoplasmas could be confirmed by the optical emission spectroscopy (OES) diagnostics and numerical simulation of the reaction system using a zerodimensional global model where the effect of gas temperature on the rate coefficients is taken into account. The zero-dimensional model led us to predict that the key of the dynamic change of the chemical reaction system in the He=N 2 cryoplasma is quenching of He metastable (He m ) atoms.…”

“…Details of the cryoplasma chamber system have been introduced elsewhere. 9,11,12,17,24) The DBD electrode was a parallel-plate configuration made of stainless steel electrodes with dimensions of 10 × 5 mm 2 and polyimide sheets with a thickness of ∼0.125 mm placed on the surfaces of both electrodes, thereby acting as dielectrics. The discharge gas gap between the polyimide sheets was 0.5 mm.…”