# A Simple and Compact Laser Scattering Setup for Characterization of a Pulsed Low-Current Discharge
## 1 Introduction  
The advantages of non-equilibrium plasmas at atmospheric pressure, such as high chemical reactivity [1; 2] and low energy consumption [3; 4], have ensured that such plasmas are now well established industrially and are steadily growing [5; 6]. As a result, according to recent research trends [6; 7; 8], a comprehensive characterization of non-equilibrium plasmas is increasingly important, especially in the environmental and health application fields [9; 10; 11; 2; 2]. Non-equilibrium conditions in a plasma can be achieved in discharges driven by DC power supplies, but are commonly generated by a pulsed low-current high-voltage discharge [1; 5; 6; 9], since the choice of the operating frequency as well as the shape of an excitation pulse makes the separate controllability of electron and heavy particle temperatures possible [1; 13]. This enables a better adjustment of the plasma to the required application. In general, the results of plasma statements depend on the chemistry at the interface between the plasma and the treated object, which is governed by an interplay of the collision rates of energetic electrons and of ions with other particles or neutrals of the surrounding gas [14; 15; 1]. Because electrons can absorb more energy from the external electric field due to their high mobility compared to other plasma constituents, plasma chemistry is defined by electron number density \(n_{e}\) and electron temperature \(T_{e}\)[6; 16]. As a result, if the electron number density is high, the ionization and dissociation reactions are driven by inelastic electron collisions. Nevertheless, these reactions can also be governed by inelastic atom and molecule collisions, depending on the conditions and the plasma carrier gas used [17; 18; 19]. Thus, the knowledge of the heavy particle temperature \(T_{h}\) is, in general, as important as theknowledge of the electron parameters, i.e., \(n_{e}\) and \(T_{e}\)[20]. The estimation of the quantities \(n_{e}\), \(T_{e}\) and \(T_{h}\) allows the assessment of the chemical reactivity of a plasma and thus a better control over the occurring reactions [6]. Laser scattering is one of several diagnostic methods to experimentally determine the plasma parameters, \(n_{e}\), \(T_{e}\) and \(T_{h}\). This diagnostic technique is neither limited by the assumption of equilibrium conditions [21; 22], nor by the plasma composition, as both atomic and molecular plasmas can be investigated [23; 24], nor by the operating pressure range [25; 26]. Laser scattering not only provides a high spatial resolution [27; 28], but also a high temporal resolution when the laser is synchronized with the pulse frequency of the power supply [29]. In order to experimentally estimate the plasma parameters by means of laser scattering, the intensity of the scattered signal, which is proportional to both the concentration of scattering species and to the intensity of the incident laser radiation [30], has to be high enough to be clearly detected by the diagnostic setup. The results also depend on the scattering angle, i.e., the angle between the wave vector of the incident radiation and the wave vector of the scattered radiation, as reported by [PERSON] et al. [31]. Thus, laser scattering has its limitations. According to [26; 27], the main detection-limiting factors are the inherent plasma radiation and the stray light, i.e., the light scattered by the surroundings of the measurement volume and not by the particles inside the volume. Thus, the lowest electron density that can be determined with a diagnostic setup consisting of a laser, a spectrograph and a camera is in the range of \(10^{18}\) m\({}^{-3}\)[27]. Usually, at such low number densities, a mask is placed at the exit of the spectrograph to block the laser wavelength so that the detection sensitivity of the camera can be increased. However, the use of a blocking mask requires an intermediate image plane and thus another optical image on the detector.