# A Simple and Compact Laser Scattering Setup for Characterization of a Pulsed Low-Current Discharge
## 3 Experimental Setup  
The implementation of the proposed method, similarly to other scattering techniques, requires the usage of a high power light source [38]. In this work, a Nd:YAG pulse laser of type Surelite SL II-10 made by Continuum, Santa Clara, CA, USA, is used. Coupled with  
Figure 2: Comparison of intensities of the Rayleigh \(M_{RJS,\varepsilon}(\lambda_{0})\) and Thomson \(M_{THS,\varepsilon}(\lambda_{0})\) signals measured for a pulsed, low-current nitrogen discharge and the calculated Raman \(M_{RaS,\varepsilon}(\lambda_{0})\) spectra. The heavy particle temperature needed for the calculation of \(M_{RaS,\varepsilon}(\lambda_{0})\) is determined from the experimentally measured \(M_{RJS,\varepsilon}(\lambda_{0})\) and \(M_{THS,\varepsilon}(\lambda_{0})\) signals and amounts to \(T_{h}=3500\) K.  
a frequency doubler of type SSP-2, also produced by Continuum, pulses with a central wavelength of \(\lambda_{0}=532\) nm and a width of 4-6 ns are generated [35]. As schematically presented in Figure 3, the initial laser beam is diverted by 180\({}^{\circ}\) using two mirrors with a high laser damage threshold and a diameter of 50 mm and focused on the scattering volume using a planoconvex lens with a focal length of 500 mm and a diameter of 50 mm. This allows for a compact setup. To ensure the recording of the highest possible intensity of the scattered signal, the laser beam is polarized vertically and is perpendicular to the propagation direction of the scattered wave, while the scattering plane is chosen to be horizontal and also perpendicular to the beam polarization. Such \"square\" setups are recommended for plasmas with low electron densities [43] and are most commonly used in experiments [21; 26; 34]. A Fastie-Ebert spectrograph made by Garching Instrumente GmbH, Munich, Germany, with a focal length of 250 mm is chosen for the detection apparatus, coupled with an ICCD camera of type 4 Picos from Stanford Computer Optics, Berkeley, CA, USA, positioned in the image plane of the spectrometer. As shown in Figure 3, the scattered beam is projected onto the horizontally oriented entrance slit by a planoconvex lens with a focal length of 100 mm coupled with a diaphragm to control the solid angle of the beam. The moderate focal length of the spectrograph combined with a diffraction grating of 1500 L/mm operated in the first diffraction order yields an aperture of f/4. 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 optically coupled via an internal, distortion-free f/0.8 lens to a CCD chip with a resolution of \(768\times 576\) px, offers a quantum efficiency of about 11% at a wavelength of 532 nm [44]. Moreover, the ICCD camera is triggered externally so that it can be synchronized with the laser pulse and the discharge pulses. Additionally, the PG31 plasma generator is mounted on a motorized XYZ positioning unit, which simplifies adjustment and allows reproducible positioning during the experiments. The setup depicted in Figure 3 is operated in sampling mode, with the laser pulse and camera triggered at maximum amplitude of the current pulse. To synchronize the laser pulse frequency with the operating frequency of the power supply, a slightly modified triggering circuit from [45] was used. The input of the pulse-delay generator DG645 from Stanford Research Systems, Sunnyvale, CA, USA, was set to block the incoming trigger edges for a period of 100 ms, thus ensuring the required laser pulse frequency of 10 Hz. The gain voltage of the ICCD was set to 800 V, resulting in a good signal-to-noise ratio without saturation of the most intense Rayleigh signal. Most of the stray light is caused by backward reflection of the laser along its direction of incidence. This can be reduced significantly by  
Figure 3: Schematic diagram of the experimental setup for laser scattering (not to scale). The entrance slit of the spectrograph is oriented horizontally.