Abstract:
A minefield is irradiated with coherent microwave energy at a first power level. Back radiation provides an indication of the position of the landmines. Coherent microwave radiation at a second power level high than the first is directed at the hidden mines to destroy the same. The microwave energy is produced by gyrotron oscillator.

Description:
This is a continuation of application Ser. No. 09/057,324, filed Apr. 9, 1998, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention concerns a method to clear minefields and an apparatus to use it. Together with a mine-detection method this leads to an automatic mechanism for land mine clearance. 
     The hidden land mines are an almost world-wide problem. Their number is estimated to be more than 110 million. The minefields sown during wars are often not cleared afterwards. This fact becomes understandable when you consider that one mine costs about 10 $, but its clearance costs about 200 $ up to 1,000 $ (cf. [7], p. 21). 
     To solve this problem, it is necessary to develop a method that clears minefields efficiently, quickly, cheaply and reliably. 
     A further main problem is the great risk to health of persons clearing the minefields. Statistically, 0.1% of the mines being cleared cause an accident. In Kuwait more than 80 people clearing minefields lost their lives as reported in the press. So the clearance of an evacuated minefield from a distance is advantageous, because there is no damage to men or material. Especially the anti-tank mines and the anti-helicopter mines often sown together with anti-personnel mines are a great risk for the vehicle respectively the airplane clearing the mines. 
     SUMMARY OF THE INVENTION 
     The purpose of the invention is a civil clearance of generally-hidden landbased explosive devices, for instance land mines, especially anti-personnel mines (so called APM&#39;s). This means a clearance rate of more than 99.6% as demanded by the UN (cf. [6], p. 25). 
     This method works, even if the mine is a plastic mine or contains little metal. Fully metal-enclosed mines cannot be destroyed, but can be found with this method. Even other munitions, such as ammunition can be found and cleared. 
     The method consists in emitting specific concentrated high-power microwave radiation onto the minefield or onto the mine&#39;s location. To locate the hidden mines the minefield is irradiated by low-power radiation and the back-radiation is received in the microwave and/or the infrared spectrum. Then specific concentrated high-power microwave radiation is used to burn down the mine. 
     In order to locate the mines you can also use other well-known detecting methods, for instance as described in U.S. Pat. No. 5,592,170 and other patents (see list) or Sieber et al. [4]. 
     This method intends to destroy mines automatically from a distance. 
     The wavelength of microwaves is within the cm up to the dm band, that is frequencies in the SHF and EHF- band according to DIN 40015. 
     A land mine consists of a cover that is often made of plastic and no longer of metal, which is why their detection is difficult. The mine consists essentially of a fuse with the detonator and the explosive charge with a lower sensitivity to impact, friction and temperature-change. 
     Physically, the explosive has a dielectric loss P d  when irradiated by an electromagnetic wave, depending on the frequency f, the power of the electromagnetic wave, the dielectric number ∈ r  of the material and the dissipation factor tan(δ) of the explosive. So when irradiated by electromagnetic energy, the explosive of a not completely metal-enclosed mine absorbs part of the wave&#39;s energy (cf. FIG.  4 ). The dielectric power loss P d  of the mine being directly exposed to the radiation is proportional to the product of the frequency f, the dielectric number ∈ r  and the dissipation factor tan(δ) of the explosive, whereas ∈ r  and tan(δ) also depend on the temperature and on the frequency f. (For highly polar materials such as water, the variation of the dielectric number ∈ r  with frequency is dramatic, going from 80 at 60 Hz to 20 at 10 kHz, because permanent dipols can no more follow the rapid field change.) 
     At various discrete values of temperature and frequency there are quasi-resonances where the loss is locally very high. A sharpe rise in dielectric loss, usually accompanied by a rapid temperature increase, is an indication of impending breakdown. If the absorbed power is high enough, the mine will burn down. The effect depends on the dissipated power inside the mine: 
     1) The explosive will gas out at lower temperatures and be defused, 
     2) the explosive will be heated above the deflagration point and burns down, or detonates when heated above the fusion point, 
     3) the explosive detonates immediately because the electromagnetic effect leads to electrical breakdown of the explosive depending on it&#39;s dielectric strength. 
     The deflagration points and the fusion points of most explosives (as pure substances) are between 300 F. and 720 F. (cf. [5]). 
     If the electric field irradiating the explosive is increased to some critical magnitude, depending on the dielectric strength of the medium, the temperature, the medium surrounding the dielectric and on other factors, the material abruptly becomes conducting, a large current flows and local destruction occurs. 
     This electrical breakdown leads to detonation of an explosive device which is not metal-enclosed. 
     A further point of the invention is the use of microwave frequencies between 3 GHz and 80 GHz, so that the dielectric loss inside the explosive is high enough and the radiation penetrates the soil deeply enough. 
     Especially Industrial-Scientific-Medical (ISM) frequency bands according to EN 55011 and VDE 0875 are of advantage because there is no interference with radio operators and because they can effectively be generated by gyrotron oscillators according to claim 11 (see also FIG.  1 ). Using the ISM-frequency band of 61 GHz up to 61.5 GHz another advantage is the relatively high absorption of the stray radiation by the air at this frequency to avoid scattered radiation (no window frequency). The frequency of radiation in the known ISM-frequency band is around 5.800 GHz; 24.125 GHz; and 61.250 GHz. 
     The advantages of a gyrotron device besides the coherent and bundled high-power radiation compared to a magnetron radiation at say 0.91 GHz or 2.45 GHz are: 
     1) Because of the shorter wavelength at the higher frequency, the microwave radiation generated by a gyrotron device can be deflected and focused by relatively small quasi-optical reflectors (see ( 5 ) and ( 5 ′) in FIGS. 1 to  4 ). 
     2) The absorbed power in the dielectric medium (here: the explosive in the mine) being directly exposed to the radiation increases with increasing frequency and dissipation factor which also grows with higher frequency. So, compared to 2.45 GHz the absorbed power at 61.25 GHz is about 400 times higher. 
     3) The higher frequency establishes a higher resolution as ground-penetrating radar. 
     A further possibility is the pulsed emission of microwave radiation even at a higher power level. This can be done by a gyrotron device. We suggest emitting more than one pulse on each mine. 
     According to claim 3 you get a frequency band which penetrates the soil deeply enough and is absorbed adequately by the explosive. 
     Even higher frequencies do not penetrate the ground deeply enough because of their short wavelength (i.e. the function P 1  decreases with decreasing λ). 
     Because microwaves in the frequency band of 3 GHz up to 80 GHz are well absorbed by dielectric substances and because gyrotrons are masers with high power output of more than 10 kW cw, and total efficiency η of more than 50%, whose coherent radiation can be deflected and focused by small quasi-optical reflectors, we suggest to clear landbased minefields by defusion or explosion of the hidden mines by high-power microwave radiation. 
     Especially the use of radiation at a ISM-frequency of 61.250 GHz has the advantage that the stray radiation will be well absorbed by the air (no window frequency), so the scattered radiation is low. 
     We suggest using a gyrotron oscillator with a permanent magnet and a water edge-cooled microwave transmission window. This equipment is easy to handle and of low volume. The invented apparatus will be provided with quasi-optical reflectors in order to reflect and focus the emitted microwave radiation according to claim 13. 
     Gyrotrons with kW power rating and frequencies around 30 GHz are lately used for technological applications to heat ceramics by dielectric loss in a closed cavity (cf. [1]), p. 32 or [2], p. 22). To use the maser for technical applications, you even can focus the radiation on an area of less than 0.2 square inch (cf. [8], p. 8). 
     Last but not least, the invention suggests to fit the microwave emitter in a vehicle or an airplane, for instance a helicopter. An electric power generator supplies the equipment with electric energy. 
     The electronics of the vehicle must be electromagnetically shielded against the stray radiation. In addition, the microwave equipment is shielded except for the radiation exit (see FIG.  3 ). 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The effect of the invention is shown by four drawings. They show in detail: 
     FIG. 1 the principle of a conventional gyrotron oscillator (current state of technology, cf. [1], p. 5), 
     FIG. 2 the use of quasi-optical reflectors to reflect and focus the microwave beam, 
     FIG. 3 the installation of the equipment in the helicopter and the method of irradiating the mine, 
     FIG. 4 the principal method to clear the minefields and 
     FIG. 5 the decrease of emitted power in the soil (showed by function P 1 ) and in the mine (explained by function P 2 ) and the resulting dielectric loss P d =P 1 −P 2  within the mine. 
    
    
     DESCRIPTION OF THE INVENTION 
     In FIG. 1 we see the principle of a conventional gyrotron oscillator: The electrons emitted and accelerated by the electron gun ( 1 ) are forced by a magnetic field to oscillate transverse to the direction of beam motion within the resonator ( 2 ). So the braked electrons partially deliver their energy to the electromagnetic field that is built by stimulated emission of radiation. The magnetic field is generated by (often superconducting) magnets ( 3 ). The generated coherent electromagnetic radiation ( 6 ) leaves the gyrotron through a microwave transmission window ( 4 ) and will be reflected and focused afterwards by quasi-optical reflectors ( 5 ) and ( 5 ′). 
     FIG. 2 shows schematically the use of quasi-optical reflectors ( 5 ) and ( 5 ′) to reflect and to focus the microwave beam ( 6 ). Here the radiation successively meets the flat reflector ( 5 ) and then the focusing parabolic reflector ( 5 ′). Let the x-axis of a co-ordination system be towards the spectator, then the microwave beam ( 6 ) will be deflected by turning the second reflector ( 5 ′) around its x-axis or its z-axis. So the microwave beam ( 6 ) reaches a restricted area. The quasi-optical reflector  5  may be formed of suitable materials such as copper, aluminum and graphite. 
     Simultaneous moving of the parabolic reflector ( 5 ′) along its y-axis and a suitable rotation of the reflector ( 5 ) around its x-axis leads to a change in focusing and therefor in the focal length of the microwave beam. 
     We see in FIG. 3 the installation of the equipment in a helicopter, especially a quasi-optical reflector ( 5 ), the microwave radiation beam ( 6 ), the microwave emitter ( 7 ), the electromagnetic shielding ( 8 ) around the equipment inside the helicopter, and a hidden mine ( 9 ) actually burned down by the microwave radiation. 
     In FIG. 4 we see the principal method to clear the minefields: The microwave radiation ( 6 ) emitted by the microwave generator ( 7 ) and reflected with the help of a mirror ( 5 ) burns down the hidden mine ( 9 ) in the soil. 
     Now FIG. 5 demonstrates by function the dielectric loss within the mine. Here P(x) means the actual power effective in the depth x of the soil. Let Po be the power emitted by the microwave source. Now we assume the mine is lying in the depth d 1  under the soil. Then a part of the power is absorbed by the soil, and so the power decreases to P 1 =P 1 (d 1 ) being          P1   =     Po                   e1       -       2                 π     λ              ɛ   r          tan                   (     δ      1     )        d           ,                          
     with dielectric number ∈ r  and dissipation factor tan(δ 1  ) of the soil and λ=wavelength of the microwave in vacuum, for instance 1 cm for a frequency f=30 GHz. On condition that the radiation permeates the mine&#39;s cover without significant loss, especially when the mine is not metal-enclosed, the power within the mine of width d 2  decreases to a level P 2 =P(d 2 ) with power factor P 1 :          P2   =     P1                   e       -       2                 π     λ              ɛ   r   ′          tan                   (     δ      2     )        d2           ,                          
     with dielectric number ∈ r ′ and dissipation factor tan(δ 2  ) of the explosive. As a result, the dielectric loss within the mine is P d =P 1 −P 2 . 
     REFERENCES CITED 
     
       
         
               
               
             
               
               
               
               
             
               
               
             
               
               
               
               
               
             
               
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
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                 US-PS 5,592,170 
                 7/1997 
                 Price et al. 
               
             
          
           
               
                   
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     OTHER PUBLICATIONS 
     [1] Thumm, Manfred (1994): State-of-the-Art of High Power Gyro-Devices and Free Electron Masers 1994, Forschungszentrum Karlsruhe (FZKA), Germany, nr. 5564. 
     [2] Like [1], Update 1996, FZKA, Germany, nr. 5877. 
     [3] Thumm, Manfred et al. (1996): ITER ECH Window Development, Final Report, FZKA, Germany, nr. 5808. 
     [4] Sieber, A. et al. (1995): Anti-personnel mine detection by using polarimetric microwave imaging, JRC, Ispra, Italy/Daimler Benz, Germany. In: Detection Technology for Mines and minelike Targets, Orlando, USA, Apr. 17-21, 1995, Bd. 2496, pp. 14-19. 
     [5] Meyer, Rudolf (1973): Explosivstoffe, Verlag Chenie, Weinheim, Germany, third edition. 
     [6] International Expert Conference on Mechanical Mine Clearance, Petersberg/Bonn, Germany, Dec. 11-12, 1996. 
     [7] United Nations Institute for Disarmament Research (UNIDIR) NewsLetter, Number SI 3/97. 
     [8] N.N. (1994): Uniform materials quality by thorough heating, KfK, Karlsruhe, Germany. In: Blick durch die Wirtschaft, Oct. 12, 1994, Bd. 37, issue 197. 
     SIGN DECLARATION 
     P d  dielectric loss in watts 
     ∈ r  dielectric number of the material 
     tan tangent 
     tan(δ) dissipation factor of the material 
     cw continuous wave 
     η total efficiency