2 Experimental arrangement

The experimental set-up (shown in Fig. 1) and the methods have already been described in Gigosos et al. (1994), Aparicio et al. (1997, 1998). Here we summarize the specific details corresponding to the present experiment.

Figure 1: Experimental set-up

The source of plasma consists of a cylindrical tube of Pyrex glass with 175 mm in length and 19 mm in interior diameter. The lamp has been designed to avoid sputtering as much as possible. The plasmas were created by discharging a capacitor bank of 20 $\mu$F charged up to 7.5 kV. During the whole experiment the lamp was working with a continuous flow of a mixture of helium and nitrogen, at a rate of 10 cm³/min and 5 cm³/min respectively and a global pressure of 1340 Pa. In these conditions, the NII emission lasts around 200 $\mu$s. The gas was pre-ionised in order to obtain the best discharge reliability. Spectroscopic and interferometric end-on measurements have been made simultaneously through the plasma life, and have been taken 2 mm off the lamp axis, and from symmetrical positions referred to it. The high axial homogeneity and the very good cylindrical symmetry of electron density and temperature in this lamp allows this (del Val et al. 1998).

According to Fig. 1, the lamp is placed in one of the arms of a Twyman-Green interferometer simultaneously illuminated with an argon ion laser (488.0 nm) and by an He-Ne laser (632.8 nm). The spectroscopic beam is directed (3 mm pinholes S1, S2, separated 1.5 m) and focused (cylindrical lens L with 150 mm focal length) onto the entrance slit of a Jobin-Yvon spectrometer (1.5 m focal length, 1200 lines/mm holographic grating), equipped with an optical multichannel analyser (OMA). This OMA has a detector array which is divided into 512 channels (EG&G 1455R-512-HQ). The dispersion was 12.59 pm/channel at 589.0 nm in first order of diffraction. The spectrometer was very carefully calibrated in wavelength as well as in intensity (Aparicio et al. 1997). Time exposures for the spectra were 5 $\mu$s. Mirror M 3, placed behind the plasma column, was used to measure the optical depth and to detect possible self-absorption effects on each line profile. This can be done by comparing the spectra taken with and without the light reflected by this mirror.