2 Experiment

The modified version of the linear low pressure pulsed arc (Djenize et al. 1990; Milosavljevic & Djenize 1998) has been used as a plasma source. A pulsed discharge driven in a quartz discharge tube of various inner diameters ($\Phi$): 5 mm and 25 mm with an effective plasma length (H) from 6.2 cm to 14 cm. Varying the dimensions of the discharge tube allows variation of the electron temperature over a wide range. The tube has end-on quartz window. On the opposite side of the electrodes 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 helium -nitrogen-oxygen mixture (90% He + 8% N₂ + 2% O₂) at 267 Pa filling pressure in the flowing regime. Spectroscopic observation of isolated spectral line was made end-on along the axis of the discharge tube. A capacitor of 8 $\mu$F was charged up to 4.5 kV (exp. a) and a capacitor of 14 $\mu$F was charged up to 2.6, 3.4 and 4.2 kV, respectively (exp. b). The line profiles were recorded using a shot-by-shot technique with a photomultiplier (EMI 9789 QB) and a grating spectrograph (Zeiss PGS-2, reciprocal linear dispersion 0.73 nm/mm in first order) system. The instrumental FWHM of 0.008 nm was determined by using the narrow spectral lines emitted by the hollow cathode discharge. The recorded profile of these lines are Gaussian in shape within 8% accuracy in the range of the investigated spectral line wavelengths. The exit slit (10 $\mu$m) of the spectrograph with the calibrated photomultiplier was micrometrically traversed along the spectral plane in small wavelength steps (0.0073 nm). The photomultiplier signal was digitized using an oscilloscope, interfaced to a computer. A sample output, as example, is shown in Figs. 1 and 2.

  

Figure 1: Recorded spectrum with the investigated 463.054 nm NII spectral line. (Exp. b3: T = 30000 K , N = 0.751023 $\rm m^{-3}$)

  

Figure 2: Temporal evolution of the investigated spectral line profile during the plasma decay (exp. b3)

A standard deconvolution procedure (Davies & Vaughan 1963) was used. The measured profiles were of the Voigt type due to the convolution of the Lorentzian Stark and the Gaussian profiles from Doppler and instrumental broadening. For the electron densities and temperatures of our experiments the Lorentzian fraction in the Voigt profile was dominant (over 85%). 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. 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 selfabsorption on Stark width determinations. The opacity was checked by measuring relative line intensity ratios within multiplet No. 5 (463.054 nm and 464.309 nm, see Fig. 1).

  

Figure 3: Temporal evolution of the electron temperature (T) at various discharge conditions: a (8 $\mu$F, 4.5 kV, $\Phi$ = 5 mm, H = 6.2 cm), b1 (14 $\mu$F, 4.2 kV, $\Phi$ = 25 mm, H = 14 cm), b2 (14 $\mu$F, 3.4 kV, $\Phi$ = 25 mm, H = 14 cm), b3 (14 $\mu$F, 2.6 kV, $\Phi$ = 25 mm, H = 14 cm)

  

Figure 4: Electron density (N) decay at various discharge conditions. The symbols are same as in Fig. 3

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 Wiese et al. (1966). It turns out that these ratios differed by less than $\pm$9% testifing to the absence of selfabsorption, which could be caused by small partial pressure of the N₂ in the discharge tube. The plasma parameters were determined using standard diagnostic methods (Rompe & Steenbeck 1967). The electron temperature was determined from the ratios of the relative intensities of four NIII spectral lines (409 74 nm; 410 34 nm; 463 42 nm and 464 06 nm) to the investigated NII spectral line with an estimated error of $\pm$10%, assuming the existence of LTE. All the necessary atomic data were taken from Wiese et al. (1966) and Glenzer et al. (1994). The electron temperature decay is presented in Fig. 3. The electron density decay was measured using a well known single wavelength He-Ne laser interferometer (Ashby et al. 1965) for the 632.8 nm transition with an estimated error of $\pm$7%. The electron density decay is presented in Fig. 4.