Abstract
The microwave plasma jet has the advantage of high plasma density and abundant active particles but fails to produce large-scale microwave plasmas in ambient air which hinders the application of microwave plasma ignition and combustion. We have applied a surface wave resonator (including a Zn-coated iron wire trigger) to produce a large-scale Ar/Zn pulsed microwave plasma jet. The discharge experiment shows that the plasma jet generally presents three discharge modes, namely, filamentous argon discharge (P < 120 W), bright argon plasma filaments covered by Ar/Zn thin plasma layers (120 W ≤ P ≤ 150 W), and bright thick Ar/Zn plasma columns (P ≥ 155 W). The optical emission spectrum indicates that the electron temperature is ∼4000–5000 K, the electron density is on the order of 1015 cm−3, and the plasma has the characteristic of local thermodynamic equilibrium. According to the transient discharge photos and the simulated electric fields, the mechanism of the three discharge modes and their transformations could be attributed to the combined interactions (the mutual resonance enhancement between the surface wave and the plasma jet, the propagation of the ionization wave, and the different particle states in the Ar/Zn pulsed microwave plasma). The results have suggested that the large-scale Ar/Zn pulsed microwave plasma jet can be generated by adding Zn vapor into the Ar microwave plasma jet and the proposed Ar/Zn pulsed microwave plasma jet is suitable for the application of plasma ignition and combustion.
Highlights
The atmospheric plasma jet has attracted much attention due to its advantages of no vacuum system and simple operation process.1–3 Under atmospheric pressure, the plasma jet is normally in non-equilibrium, that is to say, the plasma formed by gas discharge is pushed forward with the gas flow when a strong electric field acts on the working gas and the electron temperature is much higher than the temperature of ions and gas molecules; a non-equilibrium plasma jet is formed
The discharge experiment shows that the plasma jet generally presents three discharge modes, namely, filamentous argon discharge (P < 120 W), bright argon plasma filaments covered by Ar/Zn thin plasma layers (120 W ≤ P ≤ 150 W), and bright thick Ar/Zn plasma columns (P ≥ 155 W)
The optical emission spectrum indicates that the electron temperature is ∼4000–5000 K, the electron density is on the order of 1015 cm−3, and the plasma has the characteristic of local thermodynamic equilibrium
Summary
The atmospheric plasma jet has attracted much attention due to its advantages of no vacuum system and simple operation process. Under atmospheric pressure, the plasma jet is normally in non-equilibrium, that is to say, the plasma formed by gas discharge is pushed forward with the gas flow when a strong electric field acts on the working gas and the electron temperature is much higher than the temperature of ions and gas molecules; a non-equilibrium plasma jet is formed. Lirikenheil et al. have designed a microwave resonator to replace the ignition technology of spark plug discharge, improved the ignition performance of the direct injection gasoline engine, successfully realized the ignition of the methane–oxygen mixture by a low-power discharge, and developed a new type of space plasma propulsion ignition technology It needs complex control technology and large equipment to form a pulse-modulated microwave source. Whether the large-scale microwave plasma jet could be obtained by using a pulse modulation low-power source to realize large-volume synchronous bulk mode ignition in a cylinder is the top priority of microwave plasma ignition and combustion. The mechanism of the discharge mode and its transformation of the atmospheric pressure Ar/Zn pulsed microwave plasma jet should be studied in detail, in view of the application of microwave plasma ignition and combustion.
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