*2.2. Optical Setup*

Different methods were applied for the optical analysis. Firstly, high-speed cameras (HSC), the 24bits color Y6 or the 10bits monochrome Y4, both from Integrated Design Tools (IDT), were used to observe the general discharge behavior. Although the available area was reduced to a 2 mm-wide slit, relevant information was obtained regarding gas flow, asymmetry, dust, and droplets. As an example, a special double frame optics (DFO) could be applied between camera and lens that divided the optical path into two ways and produced two images of the same area that could be filtered at different wavelengths.

Secondly, optical emission spectroscopy was carried out by means of an imaging spectrograph with a 0.5 m focal length (Roper Acton SpectraPro SP2500i). The nozzle slit was imaged on the entrance slit of the spectrograph by a focusing mirror to spectrally investigate arc cross sections, i.e., perpendicular to the arc axis. Having the development during arc discharge in the focus, the spectrograph was equipped with a high-speed video camera (Y4 series of IDT). That enabled to record a series of 2D-spectra with typical repetition rates of 100 μs, allowing approximate exposure times of 98 μs (frame rate of 10,000 fps). It should be noted that rather long exposure times were necessary due to limited sensitivity of non-intensified HSC. Alternatively, the HSC was replaced by an ICCD camera (PI-MAX4 from Princeton Instruments) with higher sensitivity, allowing only single images but of shorter exposure times and at lower intensities, e.g., around current zero.

In a compromise between light intensity, spectral resolution, and exposure time, the entrance slit of the spectrograph was set to 50 μm. With gratings of 150 lines per mm for overview and 1800 L/mm for detailed spectra, the spectral range was 150 nm and 10 nm, and a spectral resolution (full width half maximum) of 0.7 nm and <0.1 nm, respectively. The intensity of side-on spectra was calibrated in units of spectral radiance by means of a tungsten strip lamp (OSRAM Wi 17/G) at the arc position. The window transmission of 50–70% was taken into account, mainly resulting from the coating of the quartz plates at the nozzles.

A positioning laser was used to adjust line of sight and check the stability between the shots. However, it was found that the width of 2 mm of the observation slit at the nozzle allowed a stable and free optical path. Usually, no corrections between the shots were needed thanks to the absence of moving parts.
