| A novel discharge scheme to generate atmospheric pressure cold plasma jet at millimeter scale is presented in this paper. The discharge is easy to ignite and sustain via a co-axial columnar Dielectric Barrier Discharge configuration and the resultant plasma jet that can be easily produced with high repeatability is highly stable. The scheme configuration is designed to be apt for multiplication and integration, by which a plasma jet array can be achieved to obtain large scale plasma at atmospheric pressure. The cold plasma jet has been achieved in some working gas flows (such as Ar, He, N2.) at atmospheric pressure with capillary dielectric barrier discharge (DBD).Steady non-equilibrium plasma jets are obtained in a capillary discharge tube(100mm long, 7mm ,4mm in outer and inner diameter) fed with gas flows of Ar, He, N2., driven by a power source with 12KV peak voltage and adjustable frequency in range of 30-50kHz. The spatial profile of the plasma jets have been investigated with respect to discharge conditions of voltage and gas flowing velocity through imaging their spatial optical emission distribution by a video CCD camera. Two different running modes are identified, one of which is the DBD area in flowing working gas and the other is the plasma jet region formed outside the capillary spout. In DBD area, filamentary discharge can be observed in stagnating gas, but a transition to glow-like discharge will occur when gradually speed up gas flow to some critical velocity for certain kind of working gas. The plasma jets will form near the capillary exit when the gas flow is above the critical velocity, which take on an appearance like a conical flame and whose light emission intensity varies a lot at different flowing velocity and for different working gases. The critical velocities of Ar, He, N2 working gases have been identified to be in range of 3-8m/s for formation ofplasmajet by means of a well-designed Pitot enthalpy probe. The jet range has also been measured from the imaging investigation and been verified becoming longer proportionally with higher flowing velocity of working gas. However, the jet range differs very much for different gases. A maximum jet range of 44mm for helium plasma jet is obtained at flowing velocity of 20m/s, because the gas flow becomes torrent when then velocity exceeds 20m/s due to viscosity of helium and therefore the jet range becomes shorter.The emission spectra of the helium jet in visible and near UV band is measured to calculate the jet temperature by an emission spectrometer system. In any spectra of helium jet,there appear some spectral lines from helium atom, together with some spectral bands from nitrogen ion and molecule, because the plasma jet is run under environment of atmosphere air and the nitrogen component of air permeates inevitably into the helium flow. The spectrum from helium atom is used to figure out the excited temperature of helium plasma jet by a so-called Boltzmann Plot scheme. It is evaluated that the excitation temperature of helium jet under varied discharge conditions is in range of 2000K-3000K, which is much lower than the typical temperature for atmospheric arc plasma jet. Meanwhile, the rotation temperature of nitrogen, which is usually equal to the thermodynamic temperature of the plasma jet, is calculated to be 290K-380K by simulating the first negative band of nitrogen. The calculated results of excitation and rotation temperature consistently illustrate the plasma jet is non-thermal in nature. Moreover, the discharge, current and the voltage waveforms have also been recorded by a current probe and high-voltage probe in a storage oscilloscope simultaneously. The tendency of discharge current with voltage and gas flowing velocity is also investigated. In conclusion, the profile of discharge current pulse is in line with that of a typical atmospheric glow discharge, and therefore stable atmospheric glow-Eke discharge has been produced by the presented novel discharge scheme. At least, the most common filament discharge for DBD is suppressed.
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