Preprint.

http://www.diva-portal.org Preprint This is the submitted version of a paper presented at 21st International Conference on Gas Discharges and Their Applications (GD2016). Citation for the original published paper: Pettersson, J., Becerra, M., Franke, S., Gortschakow, S., Bianchetti, R. et al. (2016) Space-Resolved Spectroscopic And Photographic Studies of the Vapor Layer Produced By Arc-Induced Ablation of Polymers. In: (pp. 1-4). Nagoya, Japan N.B. When citing this work, cite the original published paper. Permanent link to this version: http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-218060

SPACE-RESOLVED SPECTROSCOPIC AND PHOTOGRAPHIC STUDIES OF THE VAPOR LAYER PRODUCED BY ARC-INDUCED ABLATION OF POLYMERS J. PETTERSSON 1 *, M. BECERRA 1,2, ST. FRANKE 3, S. GORTSCHAKOW 3 A. KHAKPOUR 3 AND R. BIANCHETTI 4 1 KTH Royal Institute of Technology, School of Electrical Engineering, 100 44, Stockholm, Sweden 2 ABB Corporate Research, 722 26, Västerås, Sweden 3 INP Leibniz-Institute for Plasma Science and Technology, 174 89, Greifswald, Germany 4 ABB Corporate Research, 5405, Dättwil, Switzerland *jonaspe2@kth.se ABSTRACT Utilization of polymers in switching devices is of increasing interest. Therefore, outgassing of polymeric walls (Polyamide PA6 C6H11ON and Polyoxymethylene POM CH2O ) exposed to arc plasmas (fed by 1.9 ka peak AC currents) is investigated. Space-resolved optical emission spectroscopy complemented with high speed photography is used to investigate the layer of vapour produced by arc-induced ablation of polymers in air. It is found that the vapour layer in front of an ablating polymer strongly scatters light from the arc core, hindering the evaluation of the layer temperature and composition. The measured light right in front of the polymer surface is significant. This signal has a similar optical signature as the arc core although considerably attenuated. It is found that the detected spectra contain a fraction of the arc radiation scattered by large ablated fragments released from the polymer surface. This paper focuses on categorizing the scattering from the vapour layer adjacent to the polymer surface and to estimate the ablated particle sizes. 1. INTRODUCTION Arc-induced ablation occurs when arcs are cooled convectively by the injection of matter from an ablating (evaporating) polymer into the arcs [1]. This ablation process is utilized in modern day power switching devices to efficiently control and extinguish arc plasmas during electrical current interruption. To understand and predict the behaviour of this cooling effect, temperature and composition evaluations of the near wall area are of great interest. Well established methods already exist for evaluating the temperature and composition of the arc itself [2]. Unfortunately, not many studies are available describing those properties at the near wall area of an arc ablated polymer. The presented work attempts to investigate the near wall area using optical emission spectroscopy and high-speed photography. The optical signal is used to determine possible scattering effects and to classify the type of scattering by analysis of optical emission spectra from the near wall area and the arc core for polymer walls of different materials exposed to arcinduced ablation. 2. EXPERIMENTAL SETUP 2.1. Test Object The test object consists of two electrodes with 10 mm in diameter, one with a hemispherical copper tip and one with a flat 8 mm in diameter tungsten tip. The two electrodes are mounted vertically, forming a 24 mm air gap in which the arc is ignited, see Fig 1(a). On the top electrode a nozzle is placed to generate a stable, anode arc jet. The nozzle consists of a fiberglass holder which keeps two 4 mm thick polymer walls positioned in parallel, with a distance of 12 mm in between. Moreover, two parallel 0.5 mm thick quartz windows are held in parallel with 12mm distance in between. The quartz windows are mounted Figure 1 Sketch of the electrode configuration. (a), the electrodes with the nozzle mounted, with a clear view of the top electrode through one of the quartz windows and the line of observation indicated with the dashed red line. (b) Detail of the line of observation of the spectrograph and the arc cover used.

perpendicular with respect to the polymer walls to form a square nozzle (Fig 1(a)). The polymer walls reach 5 mm further below the quartz walls, allowing direct observation of the arc core and the wall areas of both polymer surfaces with the spectrograph. In addition, a second configuration is used where the view of the arc core is blocked by a strip of aluminium tape. This blocking strip is used to improve the signal-to-noise ratio of the weak spectra detected near the wall, without over-exposing the spectrograph (Fig 1(b)). The arc is ignited using an exploding wire and an external impulse source, generating voltages of 28 kv. The ignition generates a spark, bridging an air gap between the anode and the exploding wire. Then an AC current is injected by discharging a capacitor bank connected in parallel with the electrodes. The peak current and capacitor voltage used in the presented experiments correspond to 1.9 ka and 2.8 kv, respectively. 2.2. Instrumentation The measurements are performed with an intensified optical spectrograph complemented with two highspeed cameras as shown in Fig 2. A Roper Action SP- 2500i spectrograph is focused along a horizontal line perpendicular to the arc axis (Fig 1(b)). The spectrograph has a focal length of 2250 mm and a slit width of 40 µm. An edge filter is used to cut of wavelengths below 430 nm to avoid second order lines in the ICCD camera images. One colour high-speed camera (IDT MotionPro Y6) is focused on a small region close to the bottom plane of one of the polymer walls with an exposure time of 1 µs. A second monochromatic (Photron FASTCAM) camera is used to record a dual image of the arc for a line of sight opposite to that of the spectrograph. For better characterization of the arc, two different band-pass filters are used for each image. One image is filtered with a central wavelength (CWL) of 510 nm (20 nm width) bandpass filter and a neutral density filter ND03. The second image is taken through a bandpass filter with CWL of 656 nm (20 nm width). The Photron camera is recording with an exposure time of 2 µs. Figure 3 Typical current and voltage measured in the experiments. 3. RESULTS Fig 3 shows the typical current and arc voltage measured during a complete cycle of a 50 Hz AC current with a peak of 1.9 ka. Fig 4 shows the images from the Photron camera taken at 4 ms when POM walls (Fig 4(a)) and PA6 walls (Fig 4(b)) are used. The image corresponds to the 656 nm CWL bandpass filter with 40% enhanced contrast and brightness. Interestingly a layer of diffuse brightness is observed right in front of the surface of the POM walls (as indicated by the arrow in Fig 4(a). Between this diffuse layer and the arc jet there is a darker region with low brightness. The image does not show any distinct visible bright layer close to the PA6 surface (Fig 4(b)). Fig 5 shows the colour images of the bottom section of the POM and PA6 polymers at 4 ms. The brightness saturated area at the bottom left of both images is the arc jet blowing downwards. Solid ablation fragments forming a dusty region can be seen in front of the POM bottom surface in Fig 5(a), as indicated by the arrow. This observation confirms the release of large solid fragments when POM is ablated by an arc plasma, as recently shown in [6]. Further-more, this indicates that the solid ablated material could be responsible for the detected layer of diffuse brightness seen close to the surface in Fig 4(a). The corresponding area in Fig 5(b) does not show any visible material being ablated from PA6. Fig 6(a) shows the visible spectrum of the arc jet over the wavelength interval between 495 nm and 669 nm Figure 2 Schematic picture of the experimental setup, including all the measuring equipment. Figure 4 Photographs of the arc jet at 4 ms for (a) POM walls and (b) PA6 walls. The images have been filtered by a bandpass filter at 656±10 nm and 40% contrast and brightness intensified.

probably caused by the ablation of the quartz windows. Unsurprisingly, the intensity of the spectrum decays rapidly in space towards the polymer wall. The spectrum detected in front of the polymer boundary is close to the noise level of the spectrometer for the given exposure time. Figure 5 High-speed colour images (taken at 4 ms) from the bottom section of (a) POM walls and (b) PA6 walls. obtained with POM walls. Non-saturated spectra are taken along the unblocked observation line with exposure time of 3 µs at 4 ms. The vertical axis correspond to the spatial resolution over the dashed red line in Fig 1(a). The spectrum at the arc centre is also shown in Fig 6(b). As it can be seen, the characteristic 505 nm Cu II peak is clearly visible together with other Cu I peaks (515, 521.8 nm) in the arc core. The presence of the 505 nm Cu II emission suggests that the arc temperature is larger than 14 000 K [2]. In addition, an intense hydrogen Balmer-alpha line H α at 656.3 nm is observed, which is produced by the ablation of the polymer [7]. Two Si II peaks at 634 nm and 636.5 nm are also measured in the arc, In order to study the diffuse layer in front of POM walls, the spectrum close to the polymer surface is obtained with an improved signal-to-noise ratio by blocking the arc emission with the aluminium tape. The spectrum shown in Fig 6(c) is detected with a larger exposure time of 10 µs. The vertical axis corresponds to the spatial resolution over the dashed red line in Fig 1(b). The boundaries of the polymer walls are indicated by the white dotted lines and the aluminium tape by the white dashed line. Unexpectedly, the spectrum detected at 0.1 mm from the polymer surface (Fig 6(b)) presents similar peaks as the spectrum at the arc core. Since the temperature near the polymer surface is expected to be significantly lower than in the arc core [5], emission of ionic copper is unlikely. Careful evaluation of the experiments showed that the detected intensity close to the wall is not caused by reflection or Figure 6 Typical emission spectra using polymer walls of POM. Fig 6(a) shows the space resolved spectra of the uncovered arc with 3 µs exposure time. Fig 6(b) shows the spectra at the centre of the uncovered arc and the spectra 0.1 mm from the polymer surface with the covered arc. Fig 6(c) shows the space resolved spectra of the covered arc with 10 µs exposure time. Figure 7 Typical emission spectra using polymer walls of PA6. Fig 7(a) shows the space resolved spectra of the uncovered arc. Fig 7(b) shows the spectra at the centre of the uncovered arc and the spectra 0.1 mm from the polymer surface with the covered arc. Fig 7(c) shows the space resolved spectra of the covered arc.

scattering of light by the bulk polymer. Instead, it is found that the detected spectrum originates from the arc core, scattered by the vapour layer created by ablation of the polymer. The intensity of scattered light is highest at the polymer boundary and rapidly decreases with distance, reaching a minimum at about 1 mm from the surface. The detected intensity of the spectra then increases again towards the arc core due to the emission of the arc at outer radial positions. A similar set of spectra obtained in the presence of PA6 walls at 4 ms is shown in Fig 7. The exposure time for the unblocked and blocked spectra is 1 µs and 10 µs respectively. Comparing the spectra in Fig 6(c) and 7(c), taken with the same exposure time, shows that the intensity of scattered light in front of the PA6 walls is significantly lower than in the case of the POM walls. Furthermore, there is no stronger intensity of the scattered light close to the PA6 surface as with POM. Instead, the detected intensity in the area in front of PA6 boundary is rather uniform, even 1 mm from the surface. This could indicate a difference in size and spatial distribution of ablated particles generated by ablation of these two polymers. Evaluation of the scattering signature in front of the polymers can be used to make a rough assessment about the size of the particles released. Since the type of scattering depends on the size of the scattering particles, let us define the non-dimensional parameter xx = 2ππππ/λλ, where r is the radius of the scattering particles and λ is the wavelength of the scattered light. For values xx 1, the scattering is categorized as Rayleigh scattering [4]. If the ablated particles are assumed to be spherical and the detected light is Rayleigh scattering, the scattered light would be given by [3] 1 + cos 2 Θ II ssss = II 0 2RR 2 2ππ 4 λλ nn2 2 1 nn 2 + 2 rr 6 (1) where II ssss is the intensity of the scattered light, II 0 is the intensity of the incoming light, Θ is the scattering angle, RR is the distance to the observer, nn is the relative refractive index. Considering two different lines emitted from the arc core, the ratio of their scattered intensities from Eq. (1) gives: II aa aa 4 ssss / II 0 II bb bb ssss / II = λλbb 0 λλ aa (2) Using the lines 505 nm (Cu II line) and 635 nm (Si II line) for POM, which have roughly the same intensity (approximately 1400 WW/mm 2 nnnn ssss ), the ratio (2) is approximately 2.5. Since the ratio between the measured scattered light of said lines is only 1.28, there are strong arguments against the observed scattering being in the Rayleigh regime. This means that the scattering particles in the experiment have a size close to or larger than 100 nm (i.e. x >~ 1). A similar observation can be made with the same wavelengths for the PA6 walls, giving the ratio 1.2. Further evaluation of other regimes of scattering and particle size is unfortunately not possible based on the measurements reported in this experiment. Even though evaluation of the size of large particles based on other scattering regimes is a standard method [8], it requires measurements at different scattering angles Θ. For this reason, the measurement of scattering intensity at the single viewing angle of the spectrograph does not allow the estimation of the actual size of the scattering particles in the experiment. CONCLUSION The presented results show that scattering of light adjacent to POM or PA6 polymer surface is not caused by Rayleigh scattering, hence the size of the ablated particles is close to or larger than about 100 nm. The measured intensity and decrease of the scattered light in front of the polymer surfaces with the spectrograph are significantly different between PA6 walls and POM walls. The difference is further highlighted by the high-speed images, showing visible ablated particles using POM walls but not using PA6 walls, suggesting a significant difference in the size of the ablated particles and the corresponding type of scattering. ACKNOWLEDGMENT The authors would like to acknowledge the financial support of ABB AB and SweGRIDS. M.B. would like to acknowledge the financial support of the Swedish strategic research program StandUp for Energy. REFERENCES [1] C. B. Ruchti and L. Niemeyer, "Ablation Controlled Arcs", IEEE Trans. On Plasma Science, 14, 4, 423-434 (1986) [2] St. Franke et al, "Temperature determination in copper-dominated free-burning arcs", Journal of Physics D: Applied Physics, 47, 1-12 (2014) [3] M. Kerker, The Scattering of Light, Academic Press (1969) [4] J. H. Seinfeld and S. N. Pandis, Atmospheric Chemistry and Physics, John Wiley & Sons, 696 (2006) [5] E. Z. Ibrahim et al, "The ablation dominated polymethylmethacrylate arc", Journal of Physics D: Applied Physics, 13, 2045-65 (1980) [6] N. Aminlashgari, M. Becerra and M. Hakkarainen, Characterization of degradation fragments released by arc-induced ablation of polymers in air, Journal of Physics D: Applied Physics, 49, 9pp (2016) [7] M. Becerra, D. Piva, R. Gati and G. Dominguez 2011, On the optical radiation of ablation dominated arcs in air, Proc. 19th Symposium the Physics of Switching Arc (Brno, Czech Republic) pp 113 116 [8] David. Sinclair, Victor K. La Mer, Light Scattering as a Measure of Particle Size in Aerosols. The Production of Monodisperse Aerosols, Chem. Rev., 1949, 44 (2), pp 245 267