494 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 33, NO. 2, APRIL 2005 Imaging of Negative Polarity dc Breakdown Streamer Expansion in Transformer Oil Due to Variations in Background Pressure Michael D. Cevallos, Member, IEEE, Michael Butcher, Member, IEEE, James Dickens, Member, IEEE, Andreas Neuber, Member, IEEE, and Hermann Krompholz, Senior Member, IEEE Abstract—The breakdown physics of transformer oil is investi- gated using high speed electrical and optical diagnostics. Exper- iments are done in self-breakdown mode utilizing a needle/plane geometry. Shadowgraphy combined with high-speed electrical di- agnostics are aimed at measuring streamer expansion as a function of external pressure. Assuming a breakdown mechanism for nega- tive needle based on bubble formation with subsequent carrier am- plification in the gas phase implies a pressure dependence, which is observed in the experiments, i.e. the expansion velocity decreases with increasing pressure. Index Terms—Breakdown, liquid dielectrics, plasma, streamers. E LECTRICAL breakdown in liquid dielectrics is of high technical interest since it is widely used for switching in the development of compact pulsed power technologies. The phenomenology of liquid breakdown has been an area of in- terest for many years but is still not fully understood. The main line of observations discussed in the literature focuses on the breakdown development consisting of electron injection, bubble creation/expansion, and electron amplification in the gas phase leading to breakdown [1]. The following experiments concen- trated on the influence of low-hydrostatic pressure on negative streamers and whether the low density region obeys the ideal gas law which states that volume should be inversely proportional to pressure. The setup employs a cable (RG 220) discharge into a coaxial system with point/plane axial discharge. For the gap region, the charging and load lines outer conductor is removed and the di- electrics of both cables are tapered under a 15 angle exposing the center conductor. The outer conductor is then replaced with a rigid aluminum cylinder. This outer conductor is machined to maintain the 50 impedance across the 15 taper where the rigid coax system makes its transition from a polypropy- lene dielectric to transformer oil [2]. The rigid charging and load lines are then fed into the discharge chamber from opposite sides. The center conductor of the charging line is fitted with a needle holder and the center conductor of the load line is fitted with a 10.2-mm-diameter circular plane. The gap distance be- Manuscript received July 30, 2004; revised January 17, 2005. This work was supported by the Compact-Pulsed Power MURI Program funded by the Director of Defense Research and Engineering (DDR&E) and managed by the Air Force Office of Scientific Research (AFOSR). The authors are with the Department of Electrical and Computer En- gineering, Texas Tech University, Lubbock, TX 79409 USA (e-mail: mic.cevellos@p3e.ttu.edu). Digital Object Identifier 10.1109/TPS.2005.845901 tween the needle and the plane is 2.5 mm. The two aluminum outer conductors of the charging and load lines are connected with a removable aluminum outer conductor with two small windows placed horizontally to allow for shadowgraphy and imaging. The load line is terminated with a 50- load to sup- press reflections. The discharge chamber was constructed with two 15 cm quartz viewports that allow for ultraviolet emissions to be recorded. The windows are inset and 108 mm away from the gap for good optical resolution [3]. The load line is charged through a 900- charging resis- tance until self-breakdown occurs. A voltage amplifier is used to trigger a pulser/delay generator for initiating the camera gate. All pulser/delay generators are set to the minimum internal delay of 80 ns. The delay generator triggers a fast 650-nm solid state diode laser producing a 20-ns pulse and a second pulser/delay generator. The second pulser/delay generator triggers the shutter of the intensified charge coupled device (ICCD) camera. The laser beam is expanded and fed into the camera through a laser line filter. The shadowgraphs in Fig. 1 show the temporal development of the negative streamers as a function of pressure. The needle cathode (shaft diameter 0.51 mm, radius of curvature 5.0 m) and the copper plane anode at a gap distance of 2.5 mm are shown in black. The negative streamer extending from the needle tip is also shown in black. Once full breakdown occurs, the luminosity from the discharge is much more intense than the laser and thus is detected in the shadowgraphs and appears white. The individual images at 500 and 800 ns are of predis- charge events, and our results show that the general expansion characteristics are reproducible. The initial direction of the streamers, however, is not reproducible. The images at 2.4 s are of full breakdown. Zero time corresponds to the beginning of streamer formation, and breakdown typically occurs at 2.4 s. It is seen that the average velocity in field direction, propagation velocity, of about 1 km/s is virtually independent of hydrostatic pressure. The lateral expansion velocity increases from close to 0 at 300 torr to about 0.5 km/s at 0.01 torr. The structures of the streamer formations shown for 300 torr are identical to streamer formations observed at atmosphere. At 300 torr and above, a uniform, luminous channel is observed after breakdown, displaying no expanded regions. The images at 30 torr and below begin to show some expansion and once breakdown occurs luminosity is seen throughout the expanded region. Ongoing research focuses on identifying the critical 0093-3813/$20.00 © 2005 IEEE