Journal of Physics D: Applied Physics PAPER Related content - Photoionization capable, extreme and Time-discretized extreme and vacuum ultraviolet vacuum ultraviolet emission in developing low temperature plasmas in air J Stephens, A Fierro, S Beeson et al. spectroscopy of spark discharges in air, N2 and O2 - A nanosecond surface dielectric barrier discharge at elevated pressures: time- To cite this article: D Trienekens et al 2016 J. Phys. D: Appl. Phys. 49 035201 resolved electric field and efficiency of initiation of combustion I N Kosarev, V I Khorunzhenko, E I Mintoussov et al. - Optical emission spectroscopy study in the View the article online for updates and enhancements. VUV–VIS regimes of a developing low- temperature plasma in nitrogen gas A Fierro, G Laity and A Neuber Recent citations - Guided ionization waves: The physics of repeatability XinPei Lu and Kostya (Ken) Ostrikov - Practical considerations for modeling streamer discharges in air with radiation transport J Stephens et al - Photoionization capable, extreme and vacuum ultraviolet emission in developing low temperature plasmas in air J Stephens et al This content was downloaded from IP address 129.118.3.25 on 05/09/2018 at 18:26 IOP Journal of Physics D: Applied Physics Journal of Physics D: Applied Physics J. Phys. D: Appl. Phys. J. Phys. D: Appl. Phys. 49 (2016) 035201 (5pp) doi:10.1088/0022-3727/49/3/035201 49 Time-discretized extreme and vacuum 2016 ultraviolet spectroscopy of spark © 2016 IOP Publishing Ltd discharges in air, N2 and O2 JPAPBE D Trienekens1,2, J Stephens2, A Fierro2, J Dickens2 and A Neuber2 035201 1 Elementary Processes in Gas Discharges, Eindhoven University of Technology, Eindhoven, The Netherlands 2 Texas Tech University Center for Pulsed Power and Power Electronics, Lubbock, TX 79407, USA D Trienekens et al E-mail: [email protected] Received 28 July 2015, revised 21 October 2015 Accepted for publication 2 November 2015 Published 9 December 2015 Printed in the UK Abstract JPD In this paper we present time-discretized spectra of spark discharges in air, N2 and O2. In previous work, a system for temporally resolved spectral analysis of extreme ultraviolet (EUV) and vacuum ultraviolet (VUV) emission from spark discharges was presented, along 10.1088/0022-3727/49/3/035201 with some initial results. As was noted in this paper, statistical variances and the lacking of an apparatus sensitivity profile limited the usability of the data obtained. We have investigated the cause of these variances and improved the setup to reduce their effect. We also investigated Paper the apparatus sensitivity profile to correct the intensity of measured lines. Newly obtained spectra in dry air, N2 and O2 are presented. Air and N2 show high emission in the vicinity of 100 nm, where direct photoionization of molecular oxygen is possible, in the first 250 ns of 0022-3727 the discharge. We conclude this emission originates from nitrogen, which has several intense molecular transitions in this region. This finding is confirmed by our experimental results which show the emission in this region is much lower in oxygen. 3 Keywords: VUV spectroscopy, high voltage breakdown, photoionization (Some figures may appear in colour only in the online journal) I. Introduction (referred to as a puff valve), a pressure gradient is introduced in a discharge chamber, such that a spark discharge takes Photoionization plays an important role in the early stages place in near atmospheric conditions, whereas the rest of the of streamer and spark discharges, as becomes evident from system remains under vacuum. The latter allows for the prop- experiments on discharges in N2/O2 mixtures [1–3]. Discharge agation of EUV/VUV radiation generated by the discharge. models incorporating photoionization generally assume Removing the transparent dielectric barrier separating the the necessary photoionization capable emission originates discharge chamber from the spectrograph (as was used previ- from molecular bands of nitrogen in the vacuum ultraviolet ously in [13, 14]) enables studying EUV radiation down to (98–102.5 nm) [4–9]. This assumption is based on spectra 60 nm, which would otherwise be absorbed by the dielectric obtained by measuring the spectra resulting from electron barrier. impact in N2 [10, 11]. However, experimental proof that this In [12], it was noted that statistical variances significantly emission also takes place in the early stages of streamer and affected the measured spectra, leading to physically unresolv- spark discharges is lacking. able negative values for background corrected intensities. In a previous paper [12], a system was presented that is Also, it was noted that spectral lines often appeared to be very capable of performing spectroscopic analysis of spark dis- broad. Although it was not made clear whether this was due to charges in the EUV/VUV region (60–160 nm in this case). the apparatus profile or due to physical line broadening mech- Using an electrically actuated fast pneumatic gate valve anisms, it is reasonable to assume that the apparatus profile 0022-3727/16/035201+5$33.00 1 © 2016 IOP Publishing Ltd Printed in the UK J. Phys. D: Appl. Phys. 49 (2016) 035201 D Trienekens et al 100 80 60 40 Reflectance (%) 20 0 60 80 100 120 140 160 Wavelength (nm) Figure 1. Grating reflectance (%) as a function of wavelength (nm) for the Princeton #1200 coated grating [16, 17]. is the dominant contributor to measured spectral line widths. Thirdly, the apparatus sensitivity profile was unknown, making it impossible to directly compare intensity of lines from different positions in a given spectrum. In general, earlier results obtained using this system have not yet provided us with a clear view of the temporal devel- opment of the emission spectrum of spark discharges in the EUV/VUV region. Measurements in air and air-relevant gases and a more extensive analysis of the system are therefore needed. II. System analysis To be able to compare line intensities, the obtained spectra have to be corrected using an apparatus sensitivity profile. The fluorescent quantum efficiency of the sodium salicylate scin- tillator used has been assumed constant throughout the mea- surement range, as suggested by literature [15]. The grating reflectance was also taken into account. It was found that the percentage of light that is reflected by the grating varies from 5% to 89% through the measurement range (see figure 1) [16, 17]. The grating reflectance is assumed to be the dominant mechanism influencing the sensitivity of the system, and is used to correct the obtained intensity of the lines. This finding may explain why the spectra presented in [12] show relatively low intensity for lower wavelengths. It was found before that spectral lines obtained by the system are relatively broad, mostly due to the apparatus pro- file. This may lead to an overestimation of some lines, espe- cially in the steep parts of the reflectance curve. Therefore, the reflectance curve is convoluted with a triangular profile, Figure 2. ICCD images of the discharge without the Kel-F spacer. yielding a more realistic sensitivity correction. As can be seen, the discharge would sometimes propagate along the In order to reduce statistical variances in the obtained puff valve cap above ((a) and (c)) or the Kel-F surface beneath the needles (b) initially. Later on, a spark discharge between the needles spectra, we investigated discharge behavior for a large number would form (d). Needle–needle distance is approximately 4 mm. of shots using an Andor DH734-25U-03 ICCD camera. Analysis of the obtained images showed the discharge would for geometry details. The close proximity to the discharge often propagate along the puff valve cap or on the dielec- ruled out the use of a metal entrance slit. Kel-F (PCTFE) tric surface beneath the electrodes in its early stage, as can (polychlorotrifluoroethene (CF2CC(F)n) was used to construct be seen in figure 2. Figures 2(a) and (c) show the discharge the entrance slit due to its opacity and high machinability. In propagating upwards from the electrodes and along the puff both cases, the discharge largely avoids the entrance slit to valve cap. Figure 2(b) shows the discharge propagating along the spectrometer chamber, which is located directly between the Kel-F surface beneath the electrodes, see reference [12] the needles. In a later stage, after ~50 ns, the discharge would 2 J. Phys. D: Appl. Phys. 49 (2016) 035201 D Trienekens et al 0 -0 -2 0 -2 -2 0 -4 -2 0 -6 -2 120 nm Corrected PMT signal (AU) -8 107 nm ) 0 (V -2 0 100 200 300 400 500 600 PMT Time (ns) U 0 -2 Figure 4. Averaged PMT signal for 107 and 120 nm. The signal is 0 corrected by dividing by the grating reflectance percentage. -2 An example of two averaged PMT signals that are cor- 0 rected for grating reflectance is shown in figure 4. -2 In order to reduce the effect of statistical variance, we 0 reject shots where the time difference between the start of the -2 PMT signal and the voltage pulse is larger than 50 ns. Also, shots were the PMT signal stays below a threshold value of 0 1 V are taken as zero to suppress noise. -2 Combined, the improvement made on the setup and the 0 100 200 300 400 500 acquisition of an apparatus sensitivity profile is expected to Time (ns) result in more reliable obtained spectra. Figure 3. Example of PMT signals for air at 130 nm. Shown are III. Experiments 10 shots over which averaging takes place. Average correlation between two shots is 67%. Spectra in the 60–160 nm region were obtained for air, O2 and ultra high purity (uhp) N2.