## 2 Materials and Methods  
### Experimental Setup  
The schematic diagram of the experimental setup is presented in Figure 2. The system consists of a DBD reactor, a gas blending system, a power supply, measurement instruments, and the control interface:  
* The reactor has a coaxial cylindrical geometry made up of quartz. The inner electrode is a stainless-steel foil, and the outer electrode is a metallic mesh (knitted thin wires made of tinned copper steel, usually used for EMI/RFI shielding of cables [17]) wrapped around the quartz tube. The diameter of the outer dielectric is 28 mm, and the diameter of the inner dielectric is 22 mm. The length of the mesh is 60 mm. Detailed dimensions are given in Figure 2.
* The feed-gas stream is composed of NO and N\({}_{2}\) and flows through the DBD reactor. The gas composition and total flow rate are adjustable. Two mass flow controllers (Bronkhorst EL-FLOW Prestige) are used to measure and regulate them. Presented results are for 3 ppm flow, with a NO concentration of 800 ppm.
* The DBD voltage is measured with a 200 MHz digital oscilloscope (LeCroy HDO4024) connected through a 1000:1 voltage probe (Testec TT-SI 9010), and the current is measured using a current probe (LeCroy AP015). The NO and NO\({}_{2}\) concentrations are obtained by the gas analyzer (Testo 350).  
Two different electrical supplies are used for the experimentations: a sinusoidal voltage source and a square current source. The sinusoidal voltage waveforms (Figure 3b) are generated by a function generator connected to an audio amplifier. Since the output voltage  
Figure 2: Experimental setup (**a**) and reactor dimensions (**b**).  
of the amplifier is too low to ignite a discharge, the amplifier output is connected to a high-voltage transformer. Frequency and peak voltage are the degrees of freedom made available by this supply: a maximum frequency of 20 kHz and a maximum voltage of 12 kV can be generated.  
A specific power supply has been also developed to create the plasma and to control the energy injected into the NTP. This one delivers square current waveforms (Figure 3a), which is generated by the power converter presented in [18; 19]. This converter consists of a DC current source connected in cascade with a full-bridge current inverter and a step-up transformer. The converter delivers current pulses, whose shape is controlled by three degrees of freedom: frequency _fsw_, current amplitude \(J\), and pulses duration _\(\Delta tp\)_. (\(\Delta tp=d/(\)fsw\({}^{\circ}\)2), \(d\) being the duty cycle, in the range 5-95%). The maximum output current \(J\) is 1.2 A, and the maximum voltage is 11 kV. The frequency _fsw_ can be varied from 80 kHz to 200 kHz. In order to control the rise of the reactor temperature, this supply is operated in \"Burst mode\" [1]: pulses trains are separated by power injection pauses; the durations of both bursts and pauses are fully controlled.  
Side view images of the discharge are acquired by a digital camera (Logitech C920), with an interval of 20 s and an exposure time of 1/128 s. This camera is installed 40 cm from the axis of reactor and provides images such as the ones displayed in Figure 1. The area of interest is 28 mm high (outer diameter of the reactor) and 60 mm long (length of the metallic mesh). The camera acquires images with 2304 \(\times\) 1536 pixels; once cropped for the area of interest, these values reduce to 437 \(\times\) 248 (the difference in proportions results from a very slight misalignment of the camera with respect to the reactor axis). Despite such a device having an exposure time much longer than the lifetime of the streamers, as will be shown below, very interesting information concerning the distribution of the plasma can be drawn from the analysis of these images.