We activated combustion reactions by superposing a dielectric barrier discharge (DBD) onto a premixed burner flame with CH 4 /O 2 /Ar mixture gas. We observed the shrinkage of the reaction zone by the superposition of DBD. The shrinkage of the reaction zone indicates the enhancement of the burning velocity in the flame. The enhancement of the burning velocity was not caused by thermal effects, since we detected no increase in the gas temperature by the superposition of DBD. On the other hand, we found a unique relationship between the magnitude of the shrinkage of the reaction zone and the optical emission intensity of argon. This experimental result reveals that the enhancement of the burning velocity is determined by the energy and the density of electrons. The superposition of DBD realizes nonequilibrium combustion chemistry in the premixed burner flame, where the energy and the density of electrons are the most important parameters for determining the burning velocity.

We examined the responses of OH(X2Π) and OH(A2Σ+) in a premixed CH4 flame to high-energy electrons produced by a dielectric barrier discharge. The density of OH(X2Π) did not respond to the pulsed production of high-energy electrons; however, we observed a pulsed increase in the density of chemically produced OH(A2Σ+). In addition, we observed that OH(A2Σ+) produced at the same time as high-energy electrons had a lower rotational temperature. We discussed possible key reactions in plasma-assisted combustion on the basis of the experimental observation showing the production of cold OH(A2Σ+).

The objective of this work is to understand the mechanism of plasma-assisted combustion in a steadystate premixed burner flame. We examined the spatiotemporal variation of the density of atomic oxygen in a premixed burner flame with the superposition of dielectric barrier discharge (DBD). We also measured the spatiotemporal variations of the optical emission intensities of Ar and OH. The experimental results reveal that atomic oxygen produced in the preheating zone by electron impact plays a key role in the activation of combustion reactions. This understanding is consistent with that described in our previous paper indicating that the production of "cold OH(A 2 Σ + )" via CHO + O → OH(A 2 Σ + ) + CO has the sensitive response to the pulsed current of DBD [K. Zaima and K. Sasaki, Jpn. J. Appl. Phys. 53, 110309 (2014)].

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We compared the absolute flux of positive ions with the flux of photons in a vacuum ultraviolet (VUV) wavelength range in an electron cyclotron resonance hydrogen plasma. The absolute flux of positive ions was measured using a Langmuir probe. The absolute flux of VUV photons was evaluated on the basis of the branching ratio between the Lyman and Balmer lines emitted from electronic states with the same principal quantum numbers. The absolute intensities of the Balmer lines were obtained by calibrating the sensitivity of the spectroscopic system using a tungsten standard lamp. It has been found that the flux of VUV photons is, at least, on the comparable order of magnitude with the positive ion flux, suggesting the importance of VUV photons in plasma-induced damage in fabrication processes of ultralarge-scale integrated circuits.