WO1998036287A1

WO1998036287A1 - Device for detection of microwave pulses

Info

Publication number: WO1998036287A1
Application number: PCT/SE1998/000263
Authority: WO
Inventors: Kent Madsen
Original assignee: Försvarets Forskningsanstalt
Priority date: 1997-02-14
Filing date: 1998-02-13
Publication date: 1998-08-20
Also published as: SE9700526L; AU6235198A; SE511573C2

Abstract

A device for detection of high-power pulsed microwave radiation, which can be used to ascertain whether an object has been exposed to microwaves having high pulse power which can cause damage to the object. The device includes a vessel (1), for instance in the form of a glass tube, which encloses an ionizable gas. When the vessel is exposed to a microwave pulse, the gas is ionized by the electric field from the microwave pulse. If the field intensity is high enough to cause the gas to reach the level of ionization at which a discharge is generated, a pulse of light appears in the vessel (1). This pulse of light is recorded with a means (2) having 'memory function', for instance a film, which is arranged in connection with the vessel (1) and gives a remaining recording and thereby indicates that the vessel (1) has been exposed to a microwave pulse having a pulse power exceeding a certain level.

Description

Device for detection of microwave pulses

The present invention relates to a device for detection of pulsed microwave radiation, which can be used to ascertain whether an object has been exposed to micro- waves of high pulse power which can cause damage to the object.

The possibilities to generate short microwave pulses of extremely high pulse power by means of more and more compact equipment implies a new threat for both the national defence and the civil society. Equipment for generation of high-energy mi- crowave pulses can be mounted on different types of vehicles, and can thereby easily be moved for achievement of hidden sabotage against establishments of different kinds, for instance against stores containing defence electronics and systems forming part of the defence telecommunication network. An aggressor could use such mobile microwave weapons for carrying out sabotage activities, for in- stance in order to eliminate stored equipment or systems which are not activated at the moment. A forewarning that irradiation has occurred could deprive the aggressor of the time advantage and chaos he hopes to gain from this kind of hidden sabotage activities. Therefore, there is a military need of a sensor which can detect that an object has been exposed to harmful microwave pulses. Such a sensor could also have civil applications. It could, for instance, be included in the equipment of aircrafts, so that in the event of crashes or incidents it could be ascertained whether exposure to powerful microwave pulses might have caused the accident. Computer installations and other sensitive systems could also be provided with such sensors in order to render possible a check whether system errors could be due to micro- wave exposure.

Different types of sensors for detection of microwave pulses have been designed or are being developed. What these have in common is that they include relatively complicated electronics. Therefore, they are expensive to manufacture.

The object of the invention is to provide a sensor for detection of microwave pulses, which can stand high field intensities and which, at the same time, is uncomplicated and inexpensive to manufacture and has high reliability. Another object is to provide a sensor of the kind in question which can work without power supply.

The device according to the invention includes a vessel, for instance in the form of a glass tube, which encloses an ionizable gas. When the vessel is exposed to a microwave pulse, the gas is ionized by the electric field from the microwave pulse. If the field intensity is high enough to cause the gas to reach the level of ionization at which a discharge is generated, a pulse of light appears in the vessel. This pulse of light is recorded by a means having "memory function", for instance a film, which is arranged in connection with the vessel and gives a remaining recording and thereby indicates that the vessel has been exposed to a microwave pulse having a pulse power exceeding a certain level. As the means gives a remaining recording, it is afterwards possible to check whether the device and the objects near it have been exposed to a microwave pulse of a pulse power exceeding a certain level. The pulse power which is required to cause a discharge in the vessel is inter alia de- pendent on the gas mixture and the pressure in the vessel.

When a film is used as a recording means, a passive sensor is obtained which functions without power supply.

According to one embodiment of the invention, the device is provided with a metal envelope which screens off the microwave field and which can be pushed over the vessel to a desired position for regulation of the volume of gas that is exposed to the microwave pulses. In this way, the sensitivity of the device, i.e. the pulse power required to cause a discharge, can easily be regulated.

According to another embodiment, the vessel is provided with means for carrying an electric current of adjustable power through the gas enclosed in the vessel, whereby the sensitivity can be regulated in that the gas is pre-ionized by the current to a desired level of ionization below the level of discharge. A pre-ionization can also be achieved by means of a low radioactive isotope.

In the following, the invention will be further described by way of example with reference to the accompanying drawings, in which

figure 1 shows in section a first embodiment of the invention, figure 2 shows a second embodiment of the invention and figure 3 shows an alarm unit including several separate sensors.

Figure 1 shows in section a vessel in the form of a tube 1 of a material pervious to microwave effects, preferably glass, which encloses an ionizable gas. The gas can be argon, neon, xenon or the like. A mixture of gases can also be used. The gas is temporarily ionized when it is exposed to an electric field, and at a certain level of ionization a discharge is generated in the gas, whereby a pulse of light is emitted. The field intensity which is required to cause a discharge is inter alia dependent on the type of gas and the pressure in the vessel. The ratio between the electric field intensity which is required for a discharge and the gas pressure, is given by the so- called Paschen curves.

In the embodiment according to figure 1, the gas vessel 1 is surrounded by a film 2 which reacts to the pulse of light emitted at a discharge. It could be suitable to use a gas that emits UV-light at a discharge. For instance, mercury vapour under low pressure can be used, which is often used in UV-radiating fluorescent lamps. The film could then for instance be a thin film of chromium, which exhibits a readable mark in the form of cracks when it is exposed to ultraviolet light. The film 2 is fastened to the gas vessel 1 for instance by means of an adhesive tape. When a discharge has been recorded, the film can easily be removed and replaced by a new one, whereby the sensor can be used over again. To prevent the film from being affected by UV-radiation for instance from sunlight, the sensor can be placed inside a box of dark plastic or any other material that stops the unwanted UV-radiation but is pervious to microwave pulses. Furthermore, such a box gives protection against contamination and against the effects of weather and wind. The box is suitably provided with a hatch which can be opened when the sensor is to be checked. If the gas used is such that it emits visible light at a discharge, a photographic film, preferably of Polaroid type, can be used as recording means.

Figure 2 shows an embodiment where the means for recording the pulse of light is constituted by a circuit including a photocell, a so-called SR flip-flop ("set-reset flip- flop") and a light emitting diode 5, which is mounted in a unit 4 that is arranged in connection with the gas vessel 1. When the photocell is exposed to the pulse of light from a discharge, it gives off a voltage pulse to the flip-flop which thereby changes-over and switches on the light emitting diode 5. The unit 4 is also provided with a resetting button 6, for resetting the flip-flop to inactivated mode, and a battery or some other power supplying means. The gas vessel 1 can be covered by a material which is impervious to light but lets through microwaves.

The gas vessel 1 can, as shown in figure 2, co-operate with a metal envelope 3 which surrounds part of the vessel and is displaceable along said vessel. By varying how far the metal envelope 3 is pushed over the vessel 1 , it is possible to regulate the volume of the gas that is exposed to the microwave pulses that may occur. Hereby, the sensitivity of the sensor can be adjusted to a desired level. In order to control the sensitivity of the sensor, the gas vessel 1 can also be provided with means, not shown, for carrying an electric current of adjustable power through the gas enclosed in the vessel, whereby the gas can be pre-ionized by the current to a desired level of ionization below the level of discharge. The higher the level of ionization of the gas, the lower microwave power is required to cause a discharge. A pre-ionization can also be produced by arrangement of a low radioactive isotope in the vessel 1 , which emits free electrons to the gas enclosed in the vessel.

In certain applications it can be desirable to have a central alarm unit with input terminals from several sensors. Figure 3 shows an embodiment with several sensors, where the gas vessel 1 of each sensor is connected to an optical cable 7 comprising one or several optical fibres. The optical cables are connected in their other end to a common alarm unit 8. The alarm unit 8 is provided with a photocell for each op- tical input terminal. Each photocell is, via a flip-flop, connected to an alarm lamp 5 in the form of a light emitting diode. Since each sensor is associated with a separate alarm lamp 5, it is possible to record in which of the sensors a discharge has been generated. The alarm unit 8 also includes a resetting button 6 for resetting the flip- flops to inactivated mode.

Claims

Claims:

1. A device for detection of pulsed microwave radiation, characterized i n that it includes a vessel (1) made of a material pervious to microwave effects, which encloses an ionizable gas or gas mixture; and a means (2) arranged in connection with the vessel, which gives a remaining recording of the pulse of light that ensues from a discharge in the gas when the device is exposed to a microwave pulse having an electric field intensity which is high enough to cause the gas to reach the level of ionization at which a discharge is generated.

2. A device according to claim 1, characterized in that it includes an adjusting means in the form of a metal envelope (3), which can be displaced over the vessel (1) for regulation of the volume of the gas that is exposed to microwave pulses.

3. A device according to claim 1or2, characterized in that the vessel (1) is provided with means for carrying an electric current through the gas, whereby the sensitivity of the device can be regulated in that the gas is pre-ionized by the electric current to a desired level of ionization below the level of discharge.

4. A device according to claim 1 or2, characterized in that a radioactive isotope is arranged in the vessel (1), which isotope emits free electrons which affect the gas in the vessel so that it is pre-ionized to a level of ionization below the level of discharge.

5. A device according to anyone of claims 1-4, ch a racte rize d i n that the gas or the gas mixture is such that it emits ultraviolet light at a discharge, and that the means for recording of the discharge is a film (2) sensitive to ultraviolet light.

6. A device according to anyone of claims 1-4, characterized in that the gas or the gas mixture is such that it emits visible light at a discharge, and that the means for recording of the discharge is a photographic film (2).

7. A device according to anyone of claims 1-4, ch a ra cte rize d i n that the means for recording of the discharge is a photocell, which is connected to a light emitting diode (5) via a flip-flop, whereby the photocell when exposed to the pulse of light from a discharge, gives off a voltage pulse to the flip-flop which thereby changes-over and switches on the light emitting diode (5).

8. A device according to claim 7, characterized in that it comprises several vessels (1) which are connected to a common alarm unit (8) via an optical cable (7), and that a photocell is arranged in the alarm unit (8) in connection with each cable (7).

9. A device according to anyone of claims 1-8, characterized in that the vessel (1) is constituted by a glass tube.