Visible light effects in plasma plume ignition

The breakdown delay time of a closed plasma plume excited by a high-voltage pulse is investigated. The visible monochromatic light of 404, 532, and 662 nm wavelength and narrow-waveband light at a central wavelength of 400, 430, 450, 470, 500, 530, 570, 610, and 630 nm are used to pre-ionize the gas. It is found that the breakdown delay time decreases when the visible light illuminates the discharge tube. The light is most effective when it is applied at the position near the high-voltage electrode. The effect of visible light is found to inversely relate to the wavelength, manifested by the longer breakdown delay times for longer wavelengths. With increasing the frequency and the pulse width of the voltage, the visible light shortens the delay time more effectively. These observations can be explained by the visible light-enhanced generation of free electrons before the ignition. The proposed mechanisms of free-electron generation are the optically stimulated exoelectron emission from the inner surface of the discharge tube wall and the vibrational excitation of nitrogen molecules. The effects of visible light weaken with the addition of oxygen as a result of electron affinity to oxygen.