2 Experimental arrangement and measurements performed

Most of the experimental information relative to the plasma source, design and management have already been described by Gigosos et al. (1994), and by del Val et al. (1998). Here we summarize the specific details concerning this experiment. An scheme of the experimental set-up is shown in Fig. 1.

Figure 1: Experimental arrangement.

The source of plasma consists of a cylindrical tube of Pyrex glass 175 mm in length and 19 mm inner diameter. The lamp has been designed to avoid sputtering as much as possible. The plasma was 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 pure krypton at a rate of 0.82 cm³/min and a pressure of $3.3\times 10^{2}$ Pa. In these conditions the KrII emission lasts approximately 150 $\mu$s. The gas was pre-ionized in order to obtain the best discharge reliability and the necessary equal initial conditions for the different pulses to be comparable. Spectroscopic and interferometric end-on measurements were made simultaneously during the plasma life, and were taken 2 mm off the lamp axis, and from symmetric positions relative to it (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 two He-Ne lasers (543.0 nm and 632.8 nm) in order to determine the electron density evolution curve $N_{\rm e}(t)$from the refractivity changes due to free electrons. The spectroscopic beam is directed by two pinholes, 2 mm diameter (D1, D2), separated 1.5 m and focused by a cylindrical lens (L) of 150 mm focal length into the entrance slit of a Jobin-Yvon spectrometer (1.5 m focal length, 1200 lines/mm holographic grating), equipped with an optical multichannel analyzer (O.M.A.). The O.M.A. has a detector array, which is divided into 512 channels (EG&G 1455R-512-HQ).

After a calibration in wavelength, dispersion was measured to be 12.59 pm/channel at 589.0 nm at the first order of diffraction with an uncertainty lower than 1% (Aparicio et al. 1998). A relative intensity calibration of the spectrometer was also very carefully performed. An exhaustive description of the procedures followed can be found in González (1999, 2000). This calibration provides a transmittance function which not only includes the dependence in wavelength of the whole optical system traversed by the spectroscopic beam, but also the different behaviour of the 512 channels of the detector. Its uncertainty has been measured to be lower than 4%.

All measurements were carried out in the first order of diffraction, the same order for which the calibration in wavelength and intensity was performed. Time exposure for the detector was always 5 $\mu$s. Mirror M3, placed behind the plasma column, was used to measure the optical depth and to detect possible self-absorption effects in each line profile. This is detected in any spectral line if the intensities ratio between the spectrum taken without mirror M3 and with it is lower at the peak than at any other part of the profile (González 1999).

As a whole, more than 1000 discharges were performed, corresponding to 8 different spectral intervals. All KrII lines were recorded in 12 different instants of the plasma life, with 10 runs for each instant, five with mirror M3, five without. All measurements were made in the region 450-580 nm. KrII lines were typically registered in the first 150 $\mu$s of plasma life, with the exception of the most intense ones as well as some KrI lines, which were recorded also up to 240 $\mu$s after the discharge. The intensity of the KrI spectral lines increases as the krypton ions recombine. One example of the spectra recorded can be seen in Fig. 2. Concerning the interferometric recordings, 15 interferograms for both laser wavelengths were taken at the end of the experiment, all of them 1 ms long. They have been used to measure $N_{\rm e}(t)$.