Measurement of helium metastables in micro plasma jet

Summary form only given. Recently, interest in plasma micro-jet at atmospheric pressure has increased due to their unique capabilities and novel applications, such as biomedicine[Laroussi M. and Lu, X., 2005] thin film disposition[Rabbaland, V. et al., 2009] and chemical decontamination[Laroussi M. and Akan T., 2007]. In this work we report on the measurement, by laser absorption spectroscopy, of the distribution of the helium metastable state densities inside both the main discharge and the plasma jet propagating in free atmosphere.The discharge consists of concentric tubular electrodes separated by a dielectric cylindrical structure. The device is made of a dielectric tube with an inner thickness of about 1 mm. A grounded electrode is wrapped around the external side of the dielectric, while a high voltage electrode is glued inside the tube. Pure helium is flowing through the inner electrode at a flow rate in the range 500-1000 cm3/min. High voltage pulses (3-6 kV) are applied between the electrodes at a repetition rate frequency of 20 kHz. A laser diode emitting at 1083 nm is used as the probing light source. The laser beam is focused by a lens of f = 5 cm giving a spatial resolution of about 100 μm and a detection sensitivity of 109 per cm3. Results show that the density of helium metastables can be as high as 1012 cm-3 and is distributed according to a shape of a ring near the discharge. The ring shape collapses into a disc shape as a function of distance. This effect is due to the decrease of the radius of helium channel as a function of the distance and the diffusion of air in this channel. Moreover, it is shown that the lifetime of helium metastables decreases as a function of the distance. This lifetime near the device is about 5000 ns whereas far from the device it is reduced to 100 ns due to the quenching of the helium metastable states by air molecules whose density increases as a function of the propagation distance.