Abstract

The effects of neutral gas heating along the direction of the gas flow inside the discharge channel of a parallel plate micro atmospheric pressure plasma jet, the COST-jet, on the spatio-temporal dynamics of energetic electrons are investigated by experiments and simulations. The plasma source is driven by a single frequency sinusoidal voltage waveform at 13.56 MHz in helium with an admixture (0.05–0.2%) of nitrogen. Optical emission spectroscopy measurements are applied to determine the spatio-temporally resolved electron impact excitation dynamics from the ground state into the He I (3s) \(^3\) S \(_1\) state and the rotational temperature of nitrogen molecules at different positions along the direction of the gas flow inside the 30 mm long discharge channel. The gas temperature, which is assumed to be equal to the N \(_2\) rotational temperature, is found to increase along the discharge channel. This effect is attenuated as the nitrogen concentration is increased in the gas mixture, leading to an eventually constant temperature profile. The experimental data also reveal a plasma operating mode transition along the discharge channel from the \(\Omega\) - to the Penning-mode and show good agreement with the results of 1d3v kinetic simulations, which spatially resolve the inter-electrode space and use the gas temperature as an input value. The simulations demonstrate that the increase of the gas temperature leads to the observed mode transition. The results suggest the possibility of using the nitrogen admixture and the feed gas temperature as additional control parameters, (i) to tailor the plasma operating mode along the direction of the gas flow so that the production of specific radicals is optimized; and (ii) to control the final gas temperature of the effluent. The latter could be particularly interesting for biological applications, where the upper gas temperature limit is dictated by the rather low thermal damage threshold of the treated material.

中文翻译：

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电容耦合大气压微等离子体射流中沿放电通道的气体加热引起的模式转变

摘要

通过实验和模拟研究了平行板微大气压等离子体射流（COST-jet）放电通道内中性气体沿气流方向加热对高能电子时空动力学的影响。等离子体源由混合有 (0.05–0.2%) 氮气的氦气中 13.56 MHz 的单频正弦电压波形驱动。应用光学发射光谱测量来确定从基态到 He I (3s) \(^3\) S \(_1\)态的时空分辨电子碰撞激发动力学以及氮分子在不同温度下的旋转温度。沿30毫米长放电通道内气流方向的位置。假设气体温度等于 N \(_2\)旋转温度，发现气体温度沿着放电通道增加。随着气体混合物中氮气浓度的增加，这种效应减弱，最终形成恒定的温度曲线。实验数据还揭示了等离子体工作模式沿着放电通道从\(\Omega\)到潘宁模式的转变，并与 1d3v 动力学模拟的结果显示出良好的一致性，该模拟在空间上解析了电极间空间并使用气体温度作为输入值。模拟表明，气体温度的升高导致观察到的模式转变。结果表明可以使用氮气混合物和进料气体温度作为额外的控制参数，(i)沿着气流方向定制等离子体操作模式，从而优化特定自由基的产生；(ii) 控制流出物的最终气体温度。后者对于生物应用可能特别有趣，其中气体温度上限由处理材料的相当低的热损伤阈值决定。