Spectroscopic Characterization of Nitrogen DC Pulsed Discharge

The densities of most emitting species and gas temperature (Tg) are investigated in a nitrogen DC pulsed discharge. The discharge current and gas pressure are varied from 70 to 150 mA and 133 to 470 Pa, respectively. Tg is determined through rotational temperatures of first positive (1) and second positive (2) emission systems. These temperatures are deduced by comparison of simulated and measured spectra. The Tg values range from 450 to 950 K and increase linearly with increasing the two discharge parameters studying. The densities of N2(BΠg), N2(CΠu) and N2(BΣ + u ) species increase with increasing discharge current. This behaviour is coherent with a production process by direct impact excitation form the ground state. The densities of emitting neutral species decrease with increasing pressure due to fact that the collisional quenching starts to play a significant role at higher pressure. An opposite trend for ionic species is observed. This fact seems to prove that other production mechanisms are involved. Introduction The study of N2 discharges is currently receiving much attention, both experimental and theoretical, due to its great impact in applications related to surface treatments such as steel surface nitriding [Duez et al., 2000] and plasma sources of N atoms [De Souza et al, 1999]. All the applications require high precision of plasma conditions setting. Most particularly, the knowledge of the gas temperature is very important for understanding the physical-chemical phenomena in the discharge. In non-equilibrium plasmas, such as moderated pressure plasmas, the gas temperature (i.e. the kinetic temperature of heavy particles) can be determined via the rotational temperatures of emitting species. Generally the rotational analyse of molecular emissions proves that the rotationaltranslational relaxation is sufficiently fast to equilibrate the gas and rotational temperatures. In this context, we have investigated a nitrogen DC pulsed discharge through the densities of emitting species and gas temperature. This latter parameter is determinate through rotational temperatures of first positive (1) and second positive (2) systems. The effects of nitrogen pressure and current discharge were studied. Experimental set-up A scheme of experimental set-up is shown in Figure 1. The DC discharge is maintained in a Simax type tube (27 mm internal diameter, length 54 cm) fitted with one quartz window. A flow of gas of N2 is from 5.6 sccm to 16.7 sccm for pressures from 133 Pa to 470 Pa in order to allow the gas to refill completely before the next pulse during all experiments. Purity of nitrogen is 99,990%. The gas is pumped by an oil rotary pump. The nitrogen pressure, denoted PN2, is measured by a capacitive gauge (MKS Baratron). The pressure is set by a throttle valve placed at the outlet of the discharge tube. The nitrogen plasma is created between two internal electrodes. The distance between the electrodes is 16 cm. The discharge pulse is initiated by a DC voltage source providing up to 3 kV, 300 mA (Statron) using a fast switch. The values of the discharge current (denoted I) are varied from 70 to 150 mA. The discharge pulse width was 100 ms. The repetition frequency of the pulsed discharge is 1 Hz. The plasma emission is observed along the entire 12 cm length of the positive column in the axial direction by Andor Mechelle-5000 spectrometer coupled with Andor IStar intensified camera through optic fiber in wavelength range from 200 to 950 nm. 56 WDS'08 Proceedings of Contributed Papers, Part II, 56–61, 2008. ISBN 978-80-7378-066-1 © MATFYZPRESS