WO2014095888A1

WO2014095888A1 - Electronic optical device

Info

Publication number: WO2014095888A1
Application number: PCT/EP2013/076935
Authority: WO
Inventors: Frédéric Andre
Original assignee: Thales
Priority date: 2012-12-19
Filing date: 2013-12-17
Publication date: 2014-06-26
Also published as: FR2999796A1; FR2999796B1

Abstract

The invention concerns an electronic optical device for generating an electron beam (6) comprising a cold cathode (9) extending in a main direction (Ox), an anode (10) arranged substantially facing the cold cathode (9), a substrate (17) extending the cold cathode (9) in the main direction (Ox) and a magnetic circuit (14), means (11) for generating a difference in electric potentials between the anode (10) and the cold cathode (9) are designed to generate a macroscopic electric field (Emacro) for extracting electrons from the cold cathode (9). The magnetic field generated by the magnetic circuit (14) is designed to redirect the electrons extracted from the cold cathode (9) towards the cold cathode (9), the electrons drifting in the main direction (Ox) until they exit a volume located between the cold cathode (9) and the anode (10), the magnetic field being perpendicular to the macroscopic electric field.

Description

ELECTRONIC OPTICAL DEVICE

The invention is aimed essentially at microwave amplifiers for telecommunication satellites and space probes, but can also be applied to other areas of microwave or vacuum electronics.

The paramount parameters for microwave amplifiers are electrical efficiency, reliability, bandwidth, size and mass. The first microwave link was made between the Moon and the Earth, using a Cross Field Amplifier, in English (CFA). The term "cross-field amplifier" means an apparatus in which the amplification is obtained using a constant magnetic field perpendicular to the electric field. The CFAs developed at the time include secondary emission cathodes for which the emission of electrons is induced by a flow of electrons. However, the coefficient of retransmission of this type of cathode drops very rapidly over time. A first alternative is the replacement of secondary emission cathodes such as a thermoelectronic emission cathode. The term "cathode with thermoelectronic emission" means a conductive surface from which electrons are extracted by application of an electric field. When brought to high temperature, electrons are released in the vacuum. However, when the emitting surface of this type of cathode is very large, the heating power required for such a cathode is very important which degrades the electrical yield unacceptably.

Subsequently, the CFA were therefore abandoned in favor of traveling wave tubes (TOP). For applications such as X-ray tubes, electrons are accelerated by a high voltage to a target. During their impact on the target, the electrons emit X radiation. The electron flow is focused by a succession of lenses so that the electron impact spot is of the smallest possible size so as to improve the resolution of the electrons. image X, as is the case for a mammogram.

For some applications, such as x-ray and CT, it may be helpful to have a short pulse x-ray tube. Currently, the proposed solutions are based on a switching of the high voltage or switching of the heating of the cathode. The switching of the high voltage is slow because of the very high electrical potential to be switched, typically of the order of several tens of kilovolt, and the switching of the heating is limited by the thermal inertia of the cathode.

It has been proposed to replace the thermoelectronic cathode of X-ray tubes with a field effect cathode. However, the X-ray tubes using field effect cathodes are limited in current and therefore also in X-ray intensities. Indeed, cathode-type field effect cathodes with carbon nanotubes or "Carbon Nano Tube" ( CNT), in English, have a maximum current density of 0.1 to 1 A.cm ^"2 for reliable operation.In addition, this type of cathode requires the use of a control gate whose integration In addition, the control gate and the CNTs are ionically bombarded by the ionization of the residual gas in the tube and the gate intercepts a large fraction of the electrons emitted by the CNTs.

An object of the invention is to provide an electronic optics for generating a very dense electron beam for pulsed emission from a cold cathode without resorting to the use of a control gate. According to one aspect of the invention, there is provided an electron optical device for generating an electron beam. The electronic optical device comprises a cold cathode extending in a main direction, an anode disposed substantially opposite the cold cathode, a substrate extending the cold cathode in the main direction and a permanent magnet, means for generating a difference of electrical potentials between the anode and the cold cathode are adapted to generate a macroscopic electric field for extracting electrons from the cold cathode, the magnetic field generated by the permanent magnet being adapted to redirect the electrons extracted from the cold cathode to the cold cathode, the electrons drifting in the direction of the main direction until leaving a volume located between the cold cathode and the anode, the magnetic field being perpendicular to the macroscopic electric field. Such a device makes it possible to obtain high current densities from a cathode with carbon nanotubes, typically of the order of 20 mm long and less than 1 mm wide.

In one embodiment, the cold cathode is a cathode comprising carbon nanotubes. Advantageously, the device further comprises a laser control means adapted to modulate the operation of the cathode.

Advantageously, a ratio between the height in a direction perpendicular to the main direction of a carbon nanotube and the diameter of a nanotube is between 100 and 1000. The ratio between the height of the nanotube and the diameter determines the beta factor. The local electric field at the tip of the CNT, which determines the emission intensity of this CNT by field emission, is equal to the macroscopic field amplified by this beta factor. A high beta factor makes it possible to limit the standard of the electric field to be applied to reach the desired emission intensity, and thus to reduce the risk of electrical breakdown between the cathode and the anode. Advantageously, the distance between two carbon nanotubes is equal to the order of magnitude of the height of the nanotubes, which ensures a better compromise between the emitted current and the beta factor defining the standard of the electric field to be applied. Advantageously, a face of the anode is located substantially opposite the surface of the cathode which allows easier extraction of the electrons.

Advantageously, the face of the anode is concave. Preferably, the concave face of the anode has a radius of curvature about ten times greater than a distance between the anode and the cathode in the direction perpendicular to the main direction. The concave face of the anode makes it possible to exert a restoring force on the electrons, which makes it possible to focus the electron beam.

Advantageously, a surface of the substrate is concave. Preferably, the concave substrate has a radius of curvature approximately ten times greater than a distance between the anode and the cathode. This order of magnitude is a good compromise between the diameter of the electron beam and the transverse velocities of the electrons constituting the beam.

By transverse velocity is meant the acceleration produced by a transverse force modifying the trajectory of the electron. The transverse velocity here reflects the notion of dispersion of the electron beam, it is expressed in electronvolt (eV). The transverse velocity is decisive for the emittance then expressed in radian meter (m.rad) or brightness expressing in ampere per square meter per steradian (A. m ^"2, Srad ^{" 1} ). Advantageously, a microscopic electric field at the top of the carbon nanotube is between 1 and 5 V / nm, the microscopic electric field being the product of the standard of the macroscopic electric field and a beta factor depending on the height and the diameter of the a nanotube. The invention will be better understood by studying a few embodiments described by way of non-limiting examples, and illustrated by the appended drawings in which:

FIG. 1 represents a diagram of a microwave wave amplifier, according to the prior art,

FIG. 2 represents a schematic diagram of an electron gun comprising a field effect cathode, according to the prior art,

FIG. 3 represents a diagram of the electronic optical device according to one aspect of the invention, and FIG. 4 represents a permanent magnet making it possible to generate a magnetic field, according to one aspect of the invention.

FIG. 5 represents the trajectories of the electrons in an X-ray tube provided with an electronic optical device according to one aspect of the invention; FIG. 6a represents the trajectories followed by the electrons situated in a volume comprised between cathode and the anode and FIG. 6b shows the transverse velocities of the electrons according to one aspect of the invention.

Figure 1 shows a microwave wave amplifier. In this case, it is a traveling wave tube 1.

The traveling wave tube 1 comprises four main parts: an envelope 2 maintained under vacuum inside which is an electronic gun 3, a helix 4, collectors 5. The electronic gun 3 is the source of the electrons which constitute the electron beam 6 which propagates inside the helix 4. It can consist of a thermoelectronic cathode, a cold cathode or a cathode effect field and an anode, these different elements being not shown in FIG.

An electric field is applied between the cathode 9 and the anode 10 making it possible to tear electrons from the cathode 9 to form a beam 6. The beam 6 formed is subjected to a magnetic field B generated by a magnet 7 perpendicular to the electric field . The beam 6 forms a cylinder inside the helix 4.

The helix 4 is a spiral, typically tungsten, inserted inside the casing 2 kept under vacuum. Typically, the helix 4 is a few tens of centimeters long and the inside of the helix 4 forms a cylinder of diameter generally between 50 μιτι and 5 mm. The propeller 4 and the casing 2 are insulated from each other by insulating materials. Advantageously, the diameter of the cylinder formed by the electron beam 6 corresponds to one third of the diameter of the helix 4, it is therefore between 15 m and 1.5 mm.

The microwave signal comprising electromagnetic waves to be amplified is applied to the helix 4 at a first end 8a on the side of the electron gun 3. The electromagnetic waves move along the helix 4. When the interaction between the electromagnetic waves and the electron beam 6 is optimal, or, in other words, when the speed of displacement of the electromagnetic waves along the helix 4 and the speed of displacement of the electron beam 6 inside the helix 4 are substantially equal, the electrons communicate their kinetic energy to electromagnetic waves. The amplitude of the electromagnetic waves at the output end 8b is then greater than the amplitude of the wave at the input end 8a. The collectors 5 then collect the electrons after their passage through the helix 4. They thus receive all the residual energy that is not transmitted to the electromagnetic waves.

FIG. 2 represents an electron gun 3 of cathode type with carbon nanotubes, according to the known art.

The electronic gun 3 comprises a cathode 9 comprising carbon nanotubes 9a, an anode 10 facing the cathode 9 and a means 11 for generating an Emacro potential difference between the cathode 9 and the anode 10. Electrons are tunneled out of carbon nanotubes. They are then accelerated towards the anode 10 by the positive polarization thereof. Advantageously, an extraction grid 12 polarized with respect to the cathode 9 can make it possible to control the current.

This type of electrode gun 3 has a certain number of drawbacks: the cathode 10 / gate 12 capacity limits the maximum switching frequency,

the current emitted by the cathode 9 varies exponentially with the voltage applied to the gate 12 degrading the quality of the servocontrol of the current emitted by the cathode 9, and the gate 12 not being entirely transparent to the electronic flux, it intercepts 30 to 40% of the current emitted by the cathode 1.

the fraction of current intercepted generates an increase in temperature which limits the use of this type of electron gun 3 for high currents; the beam generated has a high transverse velocity of the order of 30 eV much greater than the transverse velocity of 0.5 eV obtained with a thermoelectronic cathode. Such a transverse speed makes the focusing of the electron beam 6 inside a standard helix 4 impossible from linear optics for example. Indeed, the linear optics allow a beam surface convergence of up to 100 in conventional optics ("Pierce" gun). However, the convergence factor is also applied to the transverse speed which can reach 3000 eV which is much higher than the capabilities of the focusing means currently existing with permanent magnets.

FIG. 3 represents a first embodiment of the electronic optical device 13 making it possible in particular to generate an electron beam 6 inside a helix 4 from a cathode 9 comprising carbon nanotubes 9a.

The electronic optical device 13 comprises a cathode 9 comprising carbon nanotubes 9a. An anode 10 is disposed substantially vis-à-vis the cathode 9. A means for generating a potential difference 1 1 generates an electric field E between the anode 10 and the cathode 9. Electrons are torn off by effect tunnel and are then accelerated to the anode 10. A magnetic field B perpendicular to the macroscopic electric field Emacro generated by a magnetic circuit 14 flap the electrons from the cathode 9 to cathode 9. Under the influence of the magnetic field B, the electrons derive in the direction of the main direction Ox until the volume defined by the cathode 9 and the anode 10 is exceeded. The electrons thus form an electron beam 6.

The diameter of the electron beam 6 generated by the electronic optical device 13 is substantially equal to the width of the cathode 9 comprising carbon nanotubes. Advantageously, the diameter of the electron beam 6 corresponds to one-third of the diameter of the helix 4 used in the TOP or in the X-ray tube in which the electron gun 3 is integrated. Typically, the diameter of the helix 4 is between 50 μιτι and 5 mm, the width of the cathode 9 is therefore substantially equal to one third of the diameter of the helix 4. The width of the cathode 9 is advantageously between 0 , 15 m and 1, 8 mm. The current density obtained depends on the length of the cathode 9. The longer the cathode 9 is and the higher the current density obtained, typically the cathode length is a few tens of millimeters. It will be readily understood that the length of the cathode is optimized according to the applications for which the electronic optical device 13 is intended. However, it should be noted that an excessively high current density generates an electric charge field which limits the macroscopic Emacro electric field applied between the cathode 9 and the anode 10.

When a cathode 9 comprising carbon nanotubes is subjected to an intense electric field E, the electrons are torn off the surface of the material by tunnel effect. A macroscopic Emacro electric field between 10 and 20 kV per millimeter between the cathode 9 and the anode 10 is required to tear the electrons in a cross-field device. Advantageously, the carbon nanotubes 9a form a tip at their end so as to reinforce the electric field E at the tip. The microscopic electric field Emicro observed at the tip of the carbon nanotube 9a is much higher than the macroscopic electric field Emacro applied. The Emicro microscopic field is a function of a beta factor and the macroscopic Emacro electric field, the beta factor corresponding to the ratio between the height of a carbon nanotube 9a and the diameter of the carbon nanotube 9a. Advantageously, the factor is between 100 and 1000. Typically, the microscopic electric field Emicro is less than ten volts per nanometer. An Emicro microscopic electric field greater than 10 volts per nanometer could damage the nanotubes 9a. Typically, the carbon nanotubes 9a have a height of between 1 and 10 micrometers and a diameter of between 1 and 50 nanometers.

The density of carbon nanotubes 9a on the surface of the cathode 9 is as high as possible so as to maximize the current emitted by the cathode 9. However, a density of carbon nanotubes 9a too large decreases the beta factor β. However, the higher the β-beta factor, the weaker the macroscopic Emacro electric field to be applied, and consequently the weaker the magnetic field to be applied. facilitates the implementation of the electronic optical device 13. In fact, the applied magnetic field B is used to fold the electrons removed from the cathode 9 and accelerated to the anode 10 to the cathode 9. It will be easily understood that the more the macroscopic electric field Emacro is high and the magnetic field B must be high to fold the electrons before they intercept the anode 10. A distance between two carbon nanotubes 9a in the two directions of the space Ox and Oy , between one and two times the height of carbon nanotubes 9a, provides the best compromise between the maximum current emitted and the maximum beta factor β.

The distance between the cathode 9 and the anode 10 must be as small as possible to reduce the risk of electrical breakdown. However, the smaller the distance between the cathode 10 and the anode 9, the greater the standard of the magnetic field B to be applied, so as to fold the electrons coming from the cathode 9 before they intercept the anode 10 We must therefore find a compromise between a short distance and a limited magnetic field. A distance of 0.3 mm seems to be an acceptable distance with a magnetic field of the order of 0.5 to 1 tesla.

The anode 9 typically comprises molybdenum or copper. These are materials that have good resistance to strong electric fields. They allow a surface without protrusion that could locally strengthen the electric field and generate electrical breakdowns.

Advantageously, the face of the anode 9 facing the cathode 10 is of concave shape. It has a radius of curvature for creating an electric return field on the electrons for focusing the electron beam 6 generated. Thus, the smaller the radius of curvature and the greater the standard of the electric return field, the smaller the diameter of the electron beam 6. However, the smaller the radius of curvature, the greater the transverse velocities of the electrons. It is necessary to find a compromise between focusing the electron beam 6 and increasing the transverse speeds. Advantageously, the radius of curvature is approximately equal to ten times the distance between the cathode 9 and the anode 10 which limits the magnification of the beam to a factor of 1.5 and limits the increase in transverse speeds by a factor of 2. The applied magnetic field is perpendicular to the electric field applied between the cathode 9 and the anode 10. This magnetic field is used to fold the electrons from the cathode 9 and accelerated to the anode 10. The standard of the magnetic field depends on the beta factor and macroscopic electric field. Advantageously, the electronic optics device comprises a control means for modulating the operation of the device 13. In this case, the control means is a photoconductive element 15 placed in series with the cathode 9. The photogenerated current by the Photoconductive element 15 following illumination 16 of the laser type, for example, is equal to the current of the cathode 9. It is then possible to generate an electron beam 6 in a pulsating manner.

Advantageously, the extension in the main direction Ox of a substrate 17 which extends the cathode 10 has a concave surface. Advantageously, the radius of curvature of the substrate 17 is approximately ten times greater than the distance between the cathode 9 and the anode 10.

Figure 4 shows a magnetic circuit as used in the invention.

The magnetic field is generated by a magnetic circuit 14 comprising a permanent magnet comprising for example Samarium Cobalt 18. The cathode 9 and the anode 10 are disposed in the gap 19. The gap 19 of the magnetic circuit is 3 mm. The magnetic field created is almost constant in the gap 19 and decreases very rapidly when one moves away from the gap 19 by a few millimeters.

FIG. 5 represents the trajectory of an electron beam 6 generated by the electronic optical device 13 in an X-ray tube. The trajectories of the electrons of the beam 6 can be divided into three zones. A first zone defining a volume between the cathode 9 and the anode 10 within which the electrons are extracted, a second zone with decreasing magnetic field and a third zone defining a volume between the cathode and a target anode 20 in which electrons are accelerated by an intense electric field.

In the first zone, the electrons are extracted from the cathode 9 in the vacuum tunnel and accelerated to the anode 10 by the macroscopic electric field Emacro. The magnetic field B perpendicular to the electric field folds the electrons towards the cathode 9 thus avoiding a current intercepting the anode 10. The path followed by the electrons is a cycloid.

The dispersion of the electrons along the axis Oz is limited by an electric return field generated by the rounded shape of the anode 10. The radius of curvature of the anode 10 is optimized to focus the electron beam 6 while limiting the increase of the transverse velocity.

Due to the strong magnetic field B prevailing in the second zone, the electrons remain confined near the substrate 17 of the cathode 9. They drift slowly in the main direction Ox. They are therefore less and less subject to the magnetic field B. The electrons are thus found in the third zone in which the magnetic field is weak. The electrons are then subjected to a strong electric field. They are then accelerated towards the target anode 20. The impact of the electrons on the target anode 20 forms a task of millimeter size.

FIG. 6a represents the trajectories of the electrons in the first zone, between the cathode 9 and the anode 10. The distance between the cathode 9 and the anode 10 is 25 mm and the length of the cathode 9 is 20 mm.

The trajectory of the electrons defines a cycloid, the cycloid pitch being 0.72 mm, the cycloid pitch being defined by the macroscopic Emacro electric field and the magnetic field B. The impact spot obtained on the target anode 20 is approximately 3.2 mm × 1.6 mm. It is possible to reduce it by using a substrate of concave shape as described above.

Another advantage of the electronic device 13 proposed according to the invention is that the ions present in the X-ray tube or in the traveling wave tubes can not go up the intense magnetic field and thus damage the cathode 9. These ions come from the ionization of the residual gases present in the tubes.

Figure 6b is a graphical representation of the speed of an electron versus time.

The transverse velocity of the electrons coming from a cathode 9 with carbon nanotubes is conventionally 30 eV. The transverse velocity of the electron in the proposed electronic optics device 13 increases by a factor of 2, it is about 70 eV. The electronic optical device 13 thus makes it possible to generate a pulse-dense electron beam from a carbon nanotube cathode and without using an extraction grid. He finds applications in the fields of cross-field traveling-wave tubes (CFA), X-ray tubes and neutralisers for ionic engines of artificial satellites.

RECTIFIED SHEET (RULE 91) ISA / EP

Claims

1. An electron beam generating optical device (6) comprising a cold cathode (9) extending in a main direction (Ox), an anode (10) disposed substantially opposite the cold cathode (9), a substrate (17) extending the cold cathode (9) in the main direction (Ox) and a magnetic circuit (14), means (1 1) for generating a difference of electric potentials between anode (10) and the cold cathode (9) are adapted to generate a macroscopic electric field (Emacro) for extracting electrons from the cold cathode (9), the magnetic field generated by the magnetic circuit (14) being adapted to redirect the electrons extracted from the cold cathode (9) towards the cold cathode (9), the electrons drifting in the direction of the main direction (Ox) to leave a volume located between the cold cathode (9) and the anode (10), the magnetic field being perpendicular to the electric field macroscopic stick.

2. Device according to claim 1 wherein the cold cathode is a cathode (9) comprising carbon nanotubes (9a).

3. Device according to claim 2 wherein a laser control means (16) is adapted to modulate the operation of the cathode so as to generate an electron beam (6) impulse.

4. Device according to one of claims 2 or 3 wherein a ratio between the height in a direction perpendicular to the main direction (Ox) of a carbon nanotube (9a) and the diameter of a nanotube (9a) is between 100 and 1000.

5. Device according to one of claims 2 to 4 wherein the distance between two carbon nanotubes (9a) is approximately equal to the order of magnitude of the height of the nanotubes.

6. Device according to one of the preceding claims wherein a face of the anode (10a) is located substantially opposite the surface of the cathode (9).

7. Device according to claim 5 wherein the face of the anode (10) is concave.

8. Device according to one of claims 6 or 7 wherein the concave face of the anode (10a) has a radius of curvature about ten times greater than a distance between the anode (10) and the cathode (9) according to a direction perpendicular (Oz) to the main direction (Ox).

9. Device according to claim 8 wherein the distance between the cathode (9) and the anode (10) is one third of the diameter of the electron beam (6) to be generated.

10. Device according to one of claims 1 to 9 wherein a surface of the substrate (17) is concave.

1 1. Device according to claim 10 wherein the concave substrate (17) has a radius of curvature about ten times greater than a distance between the anode (10) and the cathode (9).

12. Device according to one of the preceding claims wherein a microscopic electric field at the top of the carbon nanotube is between 1 and 5 V / nm, the microscopic electric field being the product of the standard of the macroscopic electric field and the ratio between the height and diameter of a nanotube.