## 3 Experimental Setup
### Emission Spectroscopy  
The measurement of the spectral broadening of hydrogen lines as well as molecular spectra requires high spectral resolution. Thus, a similar experimental setup based on a Czerny-Turner spectrograph of type THR1000 made by Jobin-Yvon (now Horiba), Oberursel, Germany, with a focal length of 1000 mm and a diffraction grating of 1200 L/mm is used for both. The horizontally placed effluent plasma jet is projected through a 100 mm planoconvex-fused silica lens onto the vertically oriented entrance slit, as shown in Figure 3. By mounting the plasma generator perpendicularly with respect to the entrance slit, each acquired image contains the spatially resolved intensity distribution in the radial direction. Additionally, the generator is mounted on a micrometer table to vary the axial distance \(d\) from the nozzle exit. Hence, the spectra are measured in 1 mm steps starting from the nozzle exit. Lastly, an intensified charged-coupled device camera of type 4 Picos from  
Figure 2: Typical voltage and current traces recorded prior to the power supply at an operating frequency of 60 kHz and a power setting of 100%. The trigger timeframe at which the experimental setup was triggered is marked in the current plot. Stanford Computer Optics, Berkeley, CA, USA, is placed directly in the imaging plane of the spectrometer. The spectral response of the image intensifier of the camera, a Gen II photo-cathode of type S20 with a single-stage microchannel plate and a P43 phosphor screen, is well suited for the measurement of nitrogen molecular bands as well as the hydrogen beta line [62]. The instrumental profile is estimated by homogeneous illumination of the 100 \(\upmu\)m wide entrance slit with a krypton spectral calibration lamp of type 6031 produced by Oriel Instruments, Stratford, CT, USA. For the spectral line at \(\lambda_{0}\) = 450.24 nm, the resulting instrumental function depicted in Figure 4 has a full width at half maximum (short FWHM) of 0.1 nm. The spatial resolution is determined to be 0.07 mm/px. Additionally, an absolute calibration of the entire experimental setup is performed using a spectral radiance calibration standard, which is a frosted 50 W quartz-halogen lamp in an integrating sphere of type ISS-8P-HP-V01 made by Gigahertz-Optik, Turkenfeld, Germany. The spectroscopic setup is triggered to achieve temporal resolution, with the image intensifier being synchronized to the discharge current. In order to achieve this, the working voltage is stepped down through a resistive voltage divider and triggers a DG645 pulse-delay generator made by Stanford Research Systems, Sunnyvale, CA, USA, which is used to adjust the delay. Accordingly, the microchannel plate of the camera is triggered with the pulse frequency of the plasma generator, utilizing the on-chip integration mode to achieve a better signal-to-noise ratio. The gain voltage is fixed at 800 V, while the exposure time of the microchannel plate was varied from 40 ns to 2 us, depending on the distance from the nozzle outlet. Since the plasma system operates without a control loop, which implies fluctuations in plasma and thus in the tapped electrical signals, no exact trigger time points can be defined. Accordingly, the width of the timeframes in Figure 2 illustrates  
Figure 4: The instrumental profile of the spectroscopic setup determined by a Gaussian fit of a krypton emission line at \(\lambda_{0}\) = 450.24 nm. The FWHM is 0.1 nm. Figure 3: Schematic diagram of the spectroscopic experimental setup. The plasma generator PG31 is positioned perpendicularly to the vertically oriented entrance slit of the spectrograph on a micrometer table that allows the axial distance \(d\) from the nozzle outlet to be varied. the approximate jitter of the trigger time point. In the following, only the spectra emitted during the current maximum are evaluated.