2 Experiment

The modified version of the linear low pressure pulsed arc has been used as a plasma source (Djenize et al. 1991; Djenize et al. 1998; Milosavljevic & Djenize 1998). A pulsed discharge driven in a quartz discharge tube of various inner diameter: 5 mm and 25 mm has an effective plasma length from 6.2 cm to 14 cm.Various dimensions of the discharge tube enable possibility of the electron temperature variation in wide range. The tube has end-on quartz windows. On the opposite side of the electrodes (Fig. 1 in Djenize et al. 1998) the glass tube was expanded in order to reduce erosion of the glass wall and also sputtering of the electrode material onto the quartz windows. The working gas was nitrogen and oxygen mixture (83% N₂ + 17% O₂) at 70 Pa filling pressure (experiment A) and helium, nitrogen and oxygen mixture (90% He + 8% N₂ + 2% O₂) at 267 Pa filling pressure in flowing regime (experiment B). These gas mixtures are present in many star plasmas. Spectroscopic observation of isolated spectral lines were made end-on along the axis of the discharge tube. A capacitors of 14 and 8 $\mu$F was charged up to 4.2 kV and 4.5 kV, respectively. The line profiles were recorded by a shot- by-shot technique using a photomultiplier (EMI 9789QB) and a grating spectrograph (Zeiss PGS-2; reciprocal linear dispersion 0.73 nm/mm in the first order and 0.008 nm instrumental FWHM) system following the procedure described by Milosavljevic & Djenize (1998). The photomultiplier signal was digitized using oscilloscope, interfaced to a computer. A sample output, as example, is shown in Figs. 1.

  

Figure 1: a,b) Recorded spectrum with the investigated spectral lines of 3s-3p a) Exp. B2 (T = 35 000 K , $N = 1.3 10^{23}\ \rm m^{-3}$) and 3p-3d b) Exp. A (T = 54 000 K, $N=2.8 10^{23}\ \rm m^{-3}$) transitions

Plasma reproducibility was monitored by the N III line radiation and, also, by the discharge current (it was found to be within 6%). The measured profiles were of the Voigt type due to the convolution of the Lorentzian Stark and Gaussian profiles caused by Doppler and instrumental broadening. For electron density and temperature obtained in our experiment the Lorentzian fraction in the Voigt profile was dominant. Van der Waals and resonance broadening were estimated to be smaller by more than an order of magnitude in comparison to Stark, Doppler and instrumental broadening. A standard deconvolution procedure (Davies & Vaughan 1963) was used. The deconvolution procedure was computerized using the least square algorithm. The Stark widths were measured with $\pm$12% error. Great care was taken to minimize the influence of self-absorption on Stark width determinations. The opacity was checked by measuring relative line intensity ratios within multiplet No. 1 and No. 2. The values obtained were compared with calculated ratios of the products of the spontaneous emission probabilities and the corresponding statistical weights of the upper levels of the lines. The necessary atomic data were taken from Glenzer et al. (1994b). It turns out that these ratios differed by less than $\pm$8% which testifies the absence of self-absorption. The Stark shifts were measured relative to the unshifted spectral lines emitted by the same plasma using the method described by Puric & Konjevic (1972). Stark shift data are determined with $\pm$0.0015 nm errors at a given N and T. The plasma parameters were determined using standard diagnostics methods (Rompe & Steenbeck 1967). The electron temperature was determined from the ratios of the relative intensities of the 348.49 nm N IV to 393.85 nm N III and the previous N III to 399.50 nm N II spectral lines, assuming the existence of LTE, with an estimated error of $\pm$10% (experiment A) and from the ratios of the relative intensities of the investigated four N III spectral lines to 463.05 nm and 464.31 nm N II spectral lines with an estimated error of $\pm$9% (experiment B). In the experiment B the electron temperature was, also, determined from the ratio of the relative intensities of the He II P$\alpha$ (468.6 nm) to 587.6 nm He I spectral lines. All the necessary atomic parameters were taken from Glenzer et al. (1994b) and Wiese et al. (1966). The electron density (N) decay was measured using a well known single wavelength He-Ne laser interferometer for the 632.8 nm transition with an estimated error of $\pm$7%.