Experiments with Microwave Heat Sources for Thermal Stimulation of Anti-Personnel Landmines

Transcription

1 Special Publication SPI.0251 Experiments with Microwave Heat Sources for Thermal Stimulation of Anti-Personnel Landmines B. Hosgood, G. Andreoli (HS Unit / IPSC / JRC Ispra) November 2001 Abstract This report describes the development of a microwave heat source at the JRC, Ispra during 2000 and 2001 in support to a series of infrared measurements made in the context of the JRC demining research activity and program. The safety aspects of the system are first considered, followed by a description of the development of two prototype heat sources. Some results are shown and the some of the advantages and disadvantages of such systems are discussed. In another series of tests, some of the effects of microwave heating on antipersonnel mines and surrogates are investigated and the results are described in the report. The information contained in this document may not be disseminated, copied or utilized without the written authorization of the European Commission. The European Commission reserves specifically its rights to apply for patents or to obtain other protection for the matter open to intellectual or industrial protection.

2 Contents 1. Introduction 2. Safety considerations 3. Development of the first prototype source and results of measurements 4. Development of a second prototype source and results of measurements 5. Results of laboratory tests 6. Conclusions 7. Acknowledgements 8. References List of Figures Fig.1 The interior of the original MW oven Fig.2 The 2450 MHz magnetron Fig.3 The first prototype source under test Fig.4 Water beaker in hole Fig.5 The waveguide covered by a damp towel Fig.6 The heat pattern obtained after 3 minutes Fig.7 The wave guide swinging over target Fig.8 Soil target on polythene sheet Fig.9 The stirrer or rotating vane system Fig.10 The original position of mode stirrer in oven Fig.11 The second prototype source (V2) Fig.12 Suspended mode stirrer inside box Fig.13 The damp towel before heating Fig.14 The heat pattern obtained with V2 Fig.15 Damp towel before heating Fig.16 Heat pattern on towel after heating Fig.17 A surrogate mine damaged by heating Fig.18 A wire ring overheats at one end only Fig.19 Thermal image of overheated wire Fig.20 Fused wire ring after MW heating 2

3 1. Introduction One of the methods investigated at the JRC, Ispra in the context of its demining program has been to stimulate the thermal contrast between a buried object and its surroundings using microwave (MW) heating [1]. The contrast thus created can be exploited by an infrared imaging system to provide indications on the presence of buried objects in certain conditions. Materials respond to microwave processing through several mechanisms. Among them are dipole rotation, resistive heating, electromagnetic heating and dielectric heating. Depending on the substance, the response may be one mechanism exclusively, or to a combination of the processing mechanisms. Metals tend to reflect microwaves, a quality that enables them to be used as waveguides and as containers to hold, direct and apply microwave fields. On the other hand, many plastics are transparent to microwaves. Although anti-personnel landmines (APLs) are mainly composed of plastic, their presence in the ground can interfere, in particular, with the upwelling heat flow in the ground and in principle cause a cool spot on the ground surface corresponding with their shape, size, material composition and depth. The thermal contrast eventually created however, also depends on several other factors such as soil moisture, composition, compactness, surface roughness and cover. Microwave heating theoretically offers several advantages over conventional heating methods such as hot air, steam, tungsten filament lamps and solar energy - all of which have been tested in one way or another in various experiments at the JRC, Ispra. Some of these advantages are described below. Some disadvantages are described in section 6 of this report. Energy Penetration: Microwave energy penetrates to generate heat internally as well as at the surface of the material in question. Other methods apply heat only to the surface. Consequently, the maximum applied temperature must be limited in order to avoid burning the surface and heating time is mainly controlled by thermal conductivity. Selective Energy Absorption: Some materials absorbs microwaves readily, others do not. This phenomenon, and its implications in the case of anti-personnel landmines, is discussed in more detail in section 5 (Results of laboratory tests) of this report. Efficiency & Speed: Microwave systems convert AC power to heat the target with efficiencies greater than conventional systems. Instantaneous Control: Most conventional heating systems require considerable amounts of time to effect temperature changes whereas microwave power levels can be adjusted in a fraction of a second. Initial tests made in the laboratory at JRC using a microwave oven showed that microwave heating could indeed rapidly heat a buried object depending on the size and material of the object. Consequently, it was decided to investigate the possibility of detecting buried objects in the field by means of a similar system. Since this is not a standard application of microwave heating, the cheapest and quickest way to achieve and test such a system was to modify a standard microwave oven. Researchers investigating the same problem at the University of Auckland, New Zealand had come to the same conclusion and had already developed a microwave system mounted on wheels in a laboratory set-up. Thus, in 3

4 collaboration with L.J. Carter from the University of Auckland, at that time a visiting scientist at the JRC Ispra, a prototype microwave heater was designed and constructed at the JRC. 2. Safety considerations Microwaves occupy the spectral region between 300 GHz and 300 MHz, while RF or radio frequency waves include 300 MHz to 3 khz. RF/MW radiation is non-ionizing in that there is insufficient energy (less than 10 ev) to ionize biologically important atoms. The primary health effects of RF/MW energy are considered to be thermal and the absorption of RF/MW energy in a material varies with frequency. One of the first considerations in modifying a microwave source must be the increased possibility of radiation leakage since the radiation is no longer confined to a closed box. Although microwave frequencies produce a skin effect, radiation can also penetrate the body and be absorbed in deep body organs without the skin effect which can warn an individual of danger. Microwave energy can produce heat in body tissue faster than it can be safely dissipated. Skin burns, internal burns, and organ damage, especially to the eye, testes and brain, are all potential hazards. Internal effects from microwave exposure, especially low-level exposure, may not be immediately noticeable. Other effects, labeled as "nonthermal" interactions, have been associated with the interaction of the microwave fields with the central nervous system of the body. Thus, two microwave radiation leak detectors were acquired and used in all testing of the modified system. Furthermore, the system was only tested outdoors or in an area of restricted access in order to reduce accidental exposure to the operators and other persons to a minimum. 3. Development of the first prototype microwave source. The first prototype used practically all the parts of a commercially available oven (2450 MHz, 1250 W, =12.2 cm in air) with the exception of the oven walls and door. The main difference between the modified source and the original oven was that the magnetron would have to project into an open horn antenna or waveguide rather than into a closed cavity. Figure 1 shows the main components of the oven before disassembling. Figure 2 shows the heart of the system: the magnetron. The high voltage (typically 3kV to 4kV) which powers the magnetron is produced by a step-up transformer rectifier and filter. Fig. 1 The interior of the original MW oven Fig.2 The 2450 MHz magnetron 4

5 Based on the recent experience of the New Zealand group, it was decided to construct a copper waveguide whose dimensions would be suitable for transport in the field and which would ideally allow heating of an area of about 50*50 cm on the ground. The ideal height of the antenna was estimated to be about 80 cm in this case. The magnetron and associated electrical components were dismounted from a conventional microwave oven and remounted in a metallic box in a configuration which allowed the source to project into an open waveguide. Figure 3 shows the first prototype source being tested for leakage in the field at JRC. To decrease the possibility of accidental exposure to radiation, an acoustic alarm was fitted to the system. Whenever power was applied to the system, the alarm would sound and the operator maintained a certain safety distance from the system. However, consequent measurements with the radiation leak detectors showed that leakage was surprisingly low and well within recommended safety limits (<5 mw/cm²) when used with precaution. (~ 0.2 mw/cm² at 100 cm distance as described in [2]). Typical levels of radiation leakage from microwave ovens is about 0.2 mw/cm² which cannot be sensed by the body. Fig. 3 The first prototype source under test Before mounting the magnetron on the copper waveguide, tests were made using a flat perforated metallic plate as a base and mounting the system over a hole in the ground. A beaker of water was placed in the hole (Fig.4) and its initial temperature noted. The system was allowed to irradiate the hole for some minutes and the water temperature was noted again. This first test showed that the system was capable of heating the water in the beaker but that the temperature rise depended on the position of the beaker in the hole, apart from the duration of heating. Thermal images of the system itself were also made in order to 5 Fig. 4 Water beaker in hole visualize the heat losses from the source and waveguide. The source was then mounted on the copper waveguide and further tests were made. The next test of the system was to verify the distribution of radiated energy from the waveguide. Since energy at this frequency interacts with water in particular, a method using a damp cloth was devised. A cotton towel was immersed in water. Excess water was removed by wringing manually. The towel was then stretched over the waveguide aperture (Fig. 5) and the system was allowed to heat the towel for several minutes. In this type of test, it

6 can be assumed that the initial temperature distribution of the damp towel is homogeneous. This was verified by making a thermal image of the towel before heating (Fig. 13). During heating, eventual radiation leakage was monitored but in general it was found that the damp cloth tended to absorb most of the radiation at the aperture of the waveguide. Fig.5 The wave guide covered by a damp towel Fig. 6 The heat pattern obtained after 3 minutes Fig. 6 shows that the energy distribution was far from satisfactory and much less homogeneous than that observed with, for example, a 2kW tungsten halogen lamp used for the same purpose. However, since it was important to obtain information on the heating effect of the system on buried objects and since no apparent alternatives were readily available, several tests were then made using the microwave system over soil targets and buried mine surrogates in the test field. These tests are described in more detail in Chapter II, Part II of [2]. To reduce the unwanted heterogeneity of the heated area, the entire system was suspended at a height of about 20cm above the ground and caused to swing slowly over the target area of interest. (Fig. 7). This method does improve the homogeneity of the irradiated spot but also decreases the total energy absorbed by the ground and increases the stray radiation. In another test, a soil target with a known moisture content was isolated using a polythene sheet and irradiated with the MW source. During the tests however it was found that the copper waveguide was difficult to manoeuvre because of its size and weight and that it represented a danger in itself when suspended due to the combination of its weight and sharp edges. A second waveguide was thus designed and constructed while some modifications were made to the MW source itself. 6

7 Fig.7 The wave guide swinging over target Fig. 8 Soil target on polythene sheet Fig. 9 The stirrer or rotating vane system Fig. 10 The original position of mode stirrer in oven 4. Development of a second prototype microwave source. Based on the above considerations, a second prototype source was devised. One potentially important item which had not been included in the first prototype source was the mode stirrer, or rotating vanes system, (Fig. 9) which was mounted immediately in front of the magnetron cavity in the original oven. as shown in Figure 10 in the inverted position. The modified apparatus (V2) is shown in Fig. 11. The mode stirrer was suspended just below the magnetron in the second prototype (Fig. 12) and allowed to rotate freely due to convection after heating the target. The material and dimensions of the second system were also changed to a simpler and more convenient configuration. Aluminium was used in the place of copper, being lighter, and a rectangular box design with an aperture proportional to that of the FLIR-Agema 570 IR camera was chosen. The overall system was easier to construct and displace and also produced a more symmetrical if not homogeneous distribution of energy than the first prototype (Fig. 14) The system was 7

8 tested again for leakage and found to be similar in behaviour to the copper waveguide. Fig. 14 shows the heat pattern obtained on a damp towel after 3 minutes of heating. Clearly, most of the heating occurs in the four corners of the system with some occurring directly under the magnetron cavity. The pattern, although still not satisfactory, is more predictable than that shown in Fig. 6. Fig. 11 The second prototype source (V2) Fig. 12 Suspended mode stirrer inside box Fig. 13 The damp towel before heating Fig. 14 The heat pattern obtained with V2 8

9 Fig. 15 Damp towel before heating Fig. 16 Heat pattern on towel after heating 5. Results of laboratory tests Tests made with buried mine surrogates with both systems however had also demonstrated the fact that the buried objects could be easily damaged by the microwave radiation (Fig. 17) despite the fact that the irradiation time was never prolonged and that the ground surface temperature did not rise excessively during a few minutes of irradiation. Therefore, two further types of test were designed for the laboratory using a commercial microwave oven without modification to the system. The first test was to measure the energy pattern on a flat surface such as a damp cloth placed in the oven. To do this the rotating base plate of the oven was removed. The results are shown in Figs. 15 and 16. The test showed that even in the oven configuration, the energy distribution is far from uniform and that the problem is avoided in practice by continuously rotating the target, which is obviously not a viable option in the field. The second series of tests was designed to study the effect of microwave heating on selected pieces of metal, similar in size and shape to those found in the surrogates and in a real mine. A piece of steel wire (Fig. 18) identical to that used in the surrogate mine shown in Fig. 17 was placed on a bed of sand and heated in the oven for only 20 seconds. Arcing was produced and the heat produced at one end of the wire was sufficient to fuse the sand, its container and even to damage the Pyrex support plate underneath. Fig. 19 shows a thermal image of the same piece of steel wire where one end only has reached a temperature exceeding 250ºC in just a few seconds. The end result is shown in Fig. 20. It can be noted that the length of the wire was close to the wavelength of the MW radiation (i.e., ~ 12 cm) and probably causes a standing wave pattern to be formed, concentrating the MW energy at one extremity only. Similar tests were then performed using metallic targets of various composition, shape and size. In general, the tests showed that open ended light wire similar to that found in many antipersonnel landmines produced most arcing and often unpredictably. 9

10 Fig. 17 A surrogate mine damaged by heating Fig. 18 A wire ring overheats at one end only Fig. 19 Thermal image of overheated wire Fig. 20 Fused wire ring after MW heating 6. Conclusions The tests described above lead the authors to the following conclusions. - The construction of a suitable source for this particular application is not easy. The main problem has been the non-uniform distribution of the heat flux on the ground surface. The attempts made so far to overcome this problem have not been successful. The problem of nonuniform distribution of the heat flux should not be under estimated since it also gives rise to hot spots on the ground surface which are a major source of false alarms in the detection chain. On the other hand, the radiation leakage from the system has not represented a major obstacle. The tests show that the heat produced by the magnetron source can rapidly penetrate several centimetres into the ground but can also heat the target in an unpredictable manner. 10

11 - Further tests made in the laboratory have shown that the presence of metallic pieces in the target object can create serious arcing problems very rapidly. Due to the unstable condition of a mine and detonator, it is considered that this method of heating could be unpredictable and extremely dangerous if used in proximity to live mines or explosives. Therefore, despite certain advantages over conventional heating, it is not to be recommended for practical use especially where explosives are present. Further research in this area would be required for a more complete evaluation of the problem. - Although MW radiation can rapidly penetrate the ground, the time required for the thermal contrast between the buried object and its surroundings to return to the surface still depends on the thermal conductivity and diffusivity of the soil (~ 0.1 mm²/sec for dry sand) and thus the overall process can be considered in terms of several minutes rather than seconds. 8. References [1] L.J. CARTER*, A. KOKONOZI, B. HOSGOOD, C. COUTSOMITROS and A. SIEBER, Landmine detection using stimulated infrared imaging. IGARSS 2001 proceedings. Sydney, Australia [2] A. Kokonozi. A Study of Thermal Imaging Techniques Applied to Humanitarian Demining. Technical Note No. I JRC Ispra (May 2001) 7. Acknowledgements The authors acknowledge the guidance provided by Dr. Lawrence Carter of the University of Auckland, New Zealand in the design and construction of the microwave heat sources used in these tests and in the participation and performance of several of the tests. They also thank Athina Kokonozi for her help in the preparation of the tests and in the analyses of the results. 11