# A Simple and Compact Laser Scattering Setup for Characterization of a Pulsed Low-Current Discharge
## 3 Experimental Setup
temporal discrimination if the reflection point is suitably far away from the scattering point, i.e., by selecting a sufficiently short exposure time of the detection apparatus [21, 46, 47]. Hence, considering the laboratory conditions and the jitter due to external triggering (see Section 3.1), an exposure time of 15 ns was set for the experiments. Both camera settings were kept constant for all following experiments. A spectral resolution of 0.055 nm/px was obtained for the detection apparatus with a neon spectral calibration lamp of type 6032 made by Oriel Instruments, Stratford, CT, USA, with the entrance slit width of the spectrograph set to 150 um. The full width at half maximum of the instrumental profile for the above settings amounts hereby to 0.22 nm. Furthermore, the spatial resolution of the setup amounts to 0.041 mm/px. Taking into account the length of the focal point of the laser beam, i.e., the length of the measurement volume, the region of interest in the acquired images is expected to be 50 px or less wide. Because the region of interest is small compared to the width of the whole image, several aberrations exhibited by the simple Fastie-Ebert design as well as the curved focal field [48] are neglected. Although precautions are taken to minimize stray light, it cannot be completely eliminated by temporal discrimination because the incident laser light is also scattered by objects or surfaces in direct vicinity of the measurement volume, such as the metallic surface of the nozzle tip. However, this stray light can be estimated by comparing the measured scattering intensity on nitrogen with an additional measurement of scattering on helium [22, 26, 49]. Since the ground state of helium has one of the smallest scattering cross sections [21, 50], an assumption is thus made under atmospheric conditions, that the detected intensity is only due to reflections from the surrounding objects. Thereafter, the measurement procedure consists of three steps. For better understanding, the average images resulting from each step are shown in Figure 4. First, since the discharge is operated at atmospheric pressure, the stray light is evaluated by measurement of the scattered intensity on helium flowing through the generator (Figure 4a). The discharge is switched off in this step. In the second step, a cold gas measurement is performed to obtain a calibration profile of the scattering setup (Figure 4b). During this step, pure nitrogen flows through the generator without the plasma being ignited. Accordingly, the acquired images, at least 1500 for each measurement, contain a Rayleigh scattering signal as well as the stray light from the surfaces surrounding the measurement volume. Finally, a measurement is performed with the discharge switched on (Figure 4c). The signal registered in this step consists of a Rayleigh scattering signal, a Thomson scattering signal and the stray light. All of the above measurements need to be pre-processed before evaluation of the plasma parameters. The invalid images, mainly due to Mie scatterers caused by dust and other particles in ambient air, are sorted out, and a mean image is calculated from the valid images. Before the evaluation, which is done pixel line by pixel line, the background intensity is subtracted from each selected pixel line of the respective average image to correct for any differences in the dark current of the ICCD camera. Thereafter, the selected pixel line of the average stray light image is subtracted from the corresponding pixel line of the other images. As a result, the pure Rayleigh scattered signal is obtained from the average cold gas image and a Thomson and Rayleigh scattered signal from the average plasma image. These two signals are subsequently used to determine the plasma properties, as theoretically described in Section 2. A sample Thomson scattering signal extracted for a single pixel line is presented in Figure 4d.