2916 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 39, NO. 11, NOVEMBER 2011 High-Speed Imaging in PAC: Multiple View and Tomographic Reconstruction of Pilot Arcing Transients Vittorio Colombo, Alessia Concetti, Emanuele Ghedini, Matteo Gherardi, Paolo Sanibondi, Marco Boselli, and Gianmatteo Cantoro Abstract—High-speed imaging of the pilot arc transient phase in plasma arc cutting has been carried out synchronizing the use of two high-speed cameras in conjunction with an optical system suitably designed for producing multiple synchronized views of the same phenomenon. Such imaging techniques have allowed a deeper understanding of the pilot arcing process, owing to the simultaneous visualization of anode and cathode attachments and through the development of tomographic reconstruction of the pilot arc. Index Terms—Plasma applications, plasma diagnostics, plasma sources. P LASMA ARC cutting (PAC) is a process where an electric arc is transferred from a cathode placed inside the torch to the anode workpiece to be cut. During the start-up phase, a pilot arc is ignited between the cathode and the nozzle by means of a short high-frequency pulse; the pilot arc is subsequently blown out of the nozzle by the gas flow. Deeper understanding of the nontransferred start-up transient can provide useful information on consumables wear. High-speed imaging has been widely used in PAC diagnos- tics for the investigation of cathodic and anodic attachments [1]–[3]. In this paper, we report some new results obtained with an imaging apparatus composed of two high-speed cameras, a NAC Memrecam K3R and GX3 (180 and 200 mm focal length lenses), by an optical system with four plane mirrors and by a digital oscilloscope (LeCroy LT374M) synchronizing the start acquisition time of both cameras. Experiments were carried out with the Cebora multigas plasma torch CP251G during a low-current (25 A) pilot arc. In a first setup [Fig. 1(a)], one camera is focusing on the cathode surface along the axis of the torch, while the other one can capture the arc loop attachment on the nozzle tip from two side-view points. In a second setup [Fig. 1(b)], both cameras are used to capture the pilot arc behav- ior outside of the nozzle from three points; a 3-D tomographic Manuscript received December 8, 2010; accepted April 13, 2011. Date of publication May 27, 2011; date of current version November 9, 2011. This work was supported by Cebora S.p.A., 40057 Bologna, Italy. The authors are with the Department of Mechanical Engineering (DIEM) and the Research Centre of Applied Mathematics, Alma Mater Studiorum Università di Bologna, 40123 Bologna, Italy (e-mail: colombo@ciram.unibo.it; alessia.concetti@mail.ing.unibo.it; emanuele.ghedini@ciram.unibo.it; matteo. gherardi2@virgilio.it; paolo.sanibondi@unibo.it; antagnod@libero.it; gianmatteo.c@hotmail.it). Digital Object Identifier 10.1109/TPS.2011.2146795 Fig. 1. Scheme of (a) the first experimental setup and (b) the second experi- mental setup. reconstruction [4] of the arc has then been accomplished. On this basis, the behavior in time of the anodic attachment during the pilot arcing phase could eventually be fully investigated, also highlighting the spot location and the rotation frequency. Some selected results will be here presented. Fig. 2 shows the axial view focused on the cathode surface, together with the synchronized side views showing the behavior of the arc loop attachment on the nozzle tip; with the high-frequency impulse being at t =0, the first frame in which the anodic attachment becomes visible in side views is at t =1 ms. The behavior of the cathodic attachment seen by the axial view has been previously described in [3]. For t =1.2 ms, Hf vapor emissions from inside the plasma chamber are visible. For t =5.3 ms, the left side view shows a double anodic attachment, which is hidden in the right-side visualization; it can also be put in correlation with the two bright spots evidenced by the axial view. For t =7.7 ms, massive Hf vapor emissions are shown on both views and can be related to the transition event described in [3]. In Fig. 3(a), three synchronized images of the pilot arc are shown. Fig. 3(b) shows the slices of the reconstructed total emissivity field on four planes located at different heights along the torch axis. Fig. 3(c) shows the 3-D total emissivity field reconstructed using the images in Fig. 3(a) and visualized by a single isocontour surface and volume rendering method, to qualitatively show the arc shape. 0093-3813/$26.00 © 2011 IEEE