stainless steel vacuum components with a coaxial center conductor made out of brass. A capacitor followed by a triggered spark gap was used to energize the ...
HIGH SPEED OPTICAL DIAGNOSTICS OF AN EXPLODING WIRE FUSE FOR POWER CONDITIONING OF EXPLOSIVE FLUX COMPRESSION GENERATORS M. Giesselmann, T. Heeren, A. Neuber, J. Walter, M. Kristiansen The Center for Pulsed Power and Power Electronics Department of Electrical & Computer Engineering Texas Tech University Lubbock, TX 79409-3102
Abstract This paper presents high-speed optical diagnostics of an exploding wire fuse, which is used in the power conditioning system for an explosive flux compression generator. The images were taken using an IMACON® 790 high-speed framing camera utilizing a gated image intensifier tube. For our measurements, the camera was operated in the high-speed multiple frame mode, yielding 8-10 sequential, 2 dimensional pictures with 100 ns between exposures.
I. INTRODUCTION Typical Magnetic Flux Compression Generators (MFCGs) produce currents in the order of 100’s of kA at voltage levels of a few kV. MFCGs are a very potent and compact source for pulsed power applications. However a power conditioning stage is required, if voltages of 100’s of kV are needed, which is often the case. The power conditioning system described here consists of an energy storage inductor, which is connected in series with an exploding wire fuse. After current has build up to a
predetermined level, the fuse explodes and the inductor current will commutate into the load. For optimum voltage gain, the fuse should open as rapidly as possible and consume as little energy as possible in the process.
II. EXPERIMENTAL SETUP Figure 1 shows a detailed drawing of the complete system, which was used for the investigations reported in this paper. The system was build using standard 6-inch stainless steel vacuum components with a coaxial center conductor made out of brass. A capacitor followed by a triggered spark gap was used to energize the inductor, since the use of an explosive flux compression generator is prohibitive in a standard laboratory environment. The energy storage inductor is a solenoid, mounted in the center of a stainless steel tube. The value of the inductor is 3.5 µH. To achieve optimum isolation between the individual turns of the solenoid and between the solenoid winding and the coaxial return, the energy storage inductor was filled with transformer oil. The fuse was mounted vertically at an angle of 90 degrees to
Figure 1. Drawing depicting the setup used for this paper
facilitate access for changing the wires after each shot. The fuse could be mounted coaxially in the center of the energy storage inductor to achieve a more compact system. However, since the inductor is filled with oil, changing the fuse wires would be very cumbersome for a coaxial fuse. A water load, which contained an CuSo 4 solution, was used to represent the load. The value of the load resistor is 11.5 Ω, which was verified pulsed conditions. To analyze the performance of the system, we measured the voltage on the output of the triggered closing switch located directly behind the primary storage capacitor. The currents in the fuse and in the load were measured with Pearson® current monitors models 4418 and 4997, respectively. The fuse was constructed of a filament of thin wires. For the optical investigation presented here, the fuse wires and the support structure are contained in a transparent enclosure. To augment the suppression of the arc after the explosion, SF6 was used. The fuse section, consisting of the lower fuse-T and upper fuse cylinder, as well as the enclosure of the peaking gap were filled with a mixture of SF6 and Air at an approximate 1:1 ratio. For all of the results reported here, the peaking gap (item 14 in Figure 1) was closed. This was done to obtain a continuous load current signal in order to observe the heating phase of the fuse, and to calculate the resistance of the fuse at all times. The peaking gap was included in the system to be able to isolate the load from the initial phase of the discharge, when low voltages are present. To observe the temporal development of the explosion optically, a setup employing an IMACON® High-speed camera utilizing a micro channel plate image intensifier and a CCD-camera was used. The camera is capable of taking 8-10 pictures per frame with 100 ns between pictures. A delay timer is used to trigger the High-speed camera relative to the trigger signal for the primary spark gap. The CCD-camera integrates the phosphor luminescence of the IMACON® camera over approximately 20 seconds and stores it internally to be read by a computer. The rising voltage pulse from the inductive energy storage system measured between trigger gap and storage inductor triggers the high-speed camera and the micro channel plate image intensifier. The CCD-camera is entirely controlled by the computer. In addition a photo diode has been installed at the camera to record the spatially integrated intensity of the light emission due to the explosion of the wires. In addition to observing the entire fuse further investigations have concentrated on the top 25 mm of the fuse.
signal of the camera were recorded for every shot. The signal from the photo diode is available for most shots. Figure 2 shows a typical oscilloscope readout for copper wires. Load and fuse currents are shown in bold and boldbroken, respectively. The thin-broken curve indicates the photo diode signal and the monitor pulses (thin-sold) indicate pictures 1,3,5,7 with pictures 2,4,6,8 in between pulses.
Figure 2. Typical oscilloscope readout (shot 134) Initially, the investigation focused on the entire fuse length of 50 cm. Figures 3 and 4 show the waveforms for the currents and the photographs, respectively. In almost all the observed cases the explosion starts at the top of the fuse, right below the fixture that hold the wires. Early emissions appear 700-800 ns before the actual explosion of the entire fuse initiates. The following appearance of hotspots (dark spots in the pictures) are m=0 type MHD (or sausage type) instabilities in the plasma as described in [2]. They also show no axial movement and stay fixed in space. The hotspots develop for all materials, whereas the observed spatial frequency is higher for aluminum. In addition it has been found that a fuse with aluminum
III. RESULTS For all reported results, the capacitor voltage was 30 kV, and the fuse consisted of 13 wires with 0.125 mm diameter of silver, copper, or aluminum. The delay times were adjusted in such a way that the framing sequence was initiated with the onset of the explosion of the fuse wires. The fuse and load current, as well as the monitor
Figure 3. Current waveforms (Fuse: broken, Load: bold), and exposure times (vertical lines) for Shot 127
Figure 4. Set of pictures showing the full fuse with 100 ns plug-in for shot 127, 48 cm object height
Figure 6. Set of pictures taken with zoom lens and 100 ns plug-in for shot 139, 25 mm object height
wires hardly ever recovers if only SF6 at atmospheric pressure and no quenching medium such as sand is used. This is in contrast to copper or silver wire fuses, which recover much more readily without sand. It has been learned from the observation of the entire exploding fuse that in most of the cases the explosion starts at the top of the fuse, which has also been found to be independent of the fuse material. Edelson and Korneff, investigating the explosion of wires in vacuum, describe a similar effect in [5]. In the paper they state that the arc was initiated by the voltage spike that appears across the wires at rupture and named this phenomenon the “end effect”. The fact that in our case the end-effect only occurs at the top of the fuse could be due to a lower local concentration of SF6.
To observe this behavior in greater detail a zoom lens has been installed to show only the upper-most 25 mm of the fuse. Additionally, two different camera plug-ins with 100 and 500 ns between pictures were used. Figure 6 shows the set of pictures taken with the zoom lens and the 100 ns plug-in. Figure 5 shows the corresponding current waveforms. The vertical lines indicate when the pictures were taken. The pictures indicate that the wires do not explode simultaneously, which is probably due to a current imbalance in the fuse. Simulations with Maxwell 3D® reveal that due to the deviation from the coaxial setup to the T-shaped system, a current imbalance in the fuse wires is caused, such that more current flows in the return-rods which are closer to the source than in the ones that are farther away. Simulations suggest that the imbalance is on of the order of 35%. This imbalance in the return currents would also create an imbalance in the magnetic field and the fuse currents, which leads to faster heating of wires that are facing the capacitor. The intensity traces show clearly that the peak intensity of light emission occurs at about the same time as the peak load current. They also show that in case of Shot 141 (Figure 7) the fuse does not recover completely and current continues to flow through the plasma (also indicated by the non-zero fuse current) heating it. In fact, the fuse current increases after about 9 µs and it appears that a new ringing of the circuit with higher damping resistance than initially starts. As mentioned above, it has been found that recovery of aluminum wire fuses is harder to achieve than for fuses with copper or silver. This is due to a much lower ionization potential of
Figure 5. Current waveforms (Fuse: broken, Load: bold), Intensity (thin), and exposure times (vertical lines)
aluminum (Al: 5.99 eV, Cu: 7.73 eV, Ag: 7.58 eV [3]). However, non-recovery occurred for all wire materials even though the transparent enclosure was flooded with SF6 and held slightly above atmospheric pressure. Higher pressures caused cracking of the enclosure leading to open air arcing at the fuse bottom.
study included an up-to-date gated image intensifier as well as a high resolution CCD-camera. Both, the entire fuse as well as only a small section of it were studied. High-resolution images show single wires exploding. Those studies were done for fuses using aluminum, copper, or silver wires. Pictures showed that even neighboring wires do not explode simultaneously. Current imbalances as well as inhomogeneities in the wires lead to this behavior. The performance of the fuse (rise time and peak load current) can appreciably improve if a fuse design is chosen that improves the simultaneous explosion of the wires. Hot spots due to m=0 type MHD instabilities form at distinct point along the axis of the fuse and show no axial movement. The hot spots form before the actual explosion, remain throughout the explosion, and last until the fuse completely recovers. The recorded light emission intensity peaks at the same time as the load current peak. The intensity trace also indicates quite clear whether the fuse has recovered or not.
Figure 7. Current waveforms (bold), Intensity, and exposure times (vertical lines) for shot 141
Figure 8. Set of pictures taken with zoom lens and 500 ns plug-in for shot 141, 25 mm object height
IV. SUMMARY We build and tested a pulse conditioning system for use with a high power microwave source containing an exploding wire fuse based pulse-shaping device. We performed a comprehensive optical study of the exploding wires. The study included high performance photography with down to 100 ns between pictures. The
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M. Giesselmann, T. Heeren, E. Kristiansen, J. Kim, J. Dickens, M. Kristiansen, “Experimental and Analytical Investigation of a Pulsed Power Conditioning System for Magnetic Flux Compression Generators”, IEEE Transactions on Plasma Science, IEEE Transactions on Plasma Science, Special Edition on Pulsed Power Science & Technology, October 2000, p. 1368…1376.
2.
Samuel Glasstone, Ralph H. Loveberg, “Controlled Thermonuclear Reactions”, pages 234-ff, D. Van Nostrand Company, Princeton, New Jersey, 1960
3.
CRC Handbook of Physics and Chemistry, E-69-ff.
4.
M. Giesselmann, T. Heeren, M. Kristiansen, “The Effect of Materials on the Performance of Exploding Wire Fuses”, submitted to IEEE Transactions on Plasma Science.
5.
H.D. Edelson, T. Korneff, “Conducting Mechanisms for Exploding Wires in Vacuum”, Exploding Wires, Volume 3, Proceedings of the Third Conference on the Exploding Wire Phenomenon, Pages 267-ff, Plenum Press, New York, 1964
ACKNOWLEDGEMENT This work is solely funded by the Explosive-Driven Power Generation MURI program funded by the Director of Defense Research & Engineering (DDR&E) and managed by the Air Force Office of Scientific Research (AFOSR)
