EP3238225B1 - High-energy electron source made from cnt with offset electromagnetic wave control element - Google Patents

Description

The invention relates to a high energy electron source, between 20 and 500 kV, for example, comprising at least one switchable or modulatable cathode or electron source and an electromagnetic wave control element external to the cathode structure. switchable.

It is used in the field of electron tubes incorporating an electron gun, and more particularly in the field of X-ray tubes. It relates to switchable or modulatable cathodes comprising one or more field effect emitters, or based on nanotubes. carbon nanofibers or CNT, associated with an electromagnetic wave (SCCO) controlled current source that can be physically deported out of the X-ray tube. It relates to an X-ray source, RX, delivering a wave-controlled RX flux. electromagnetic, for example an optical illumination source, and can be switched between an ON state and an OFF state or be regulated between these two states.

Switchable cathodes with an electromagnetic wave proposed today are optically controlled cathodes (photocathodes). One of the problems in CNT photocathodes is that photoelements physically associated with CNTs are subject to X-rays and ionizing bombardment within the tube enclosure. Their integration therefore requires a hardening technology. In addition, their integration in the form of a network homologous to the network of CNTs constrains the possible dimensions of these photoelements, which can limit their breakdown voltage, for example typically 40V, while the use of larger photoelements allows voltages of breakdown up to several hundred volts.

The document US 2006/0002514 discloses a device comprising an array of electronic transmitters associated with an extraction grid, a photosensitive component connected to a voltage source and to the extraction grid, and to a resistor connected to the mass. In this configuration, the gate is positively polarized with respect to the tip so as to allow the emission of electrons from this tip. The emission of electronic transmitters depends on the difference in voltage between the gate voltage and the tip voltage. This voltage difference depends on the on or off state of the photosensitive device. The emission current then follows the Fowler-Nordheim law known to those skilled in the art which is, as a first approximation, an exponential of the gate voltage. As a result, the emission current can not be finely controlled.

The patent US 5,804,833 discloses a structure comprising a photocathode and an anode. The photocathode comprises an emitter structure made on a detector structure. The bias voltage is typically 10 kV. Such a configuration does not make it possible to manufacture an RX source (operating voltage of 50 to 500 kV) having a low RX flux in the OFF state corresponding to the unlit detector structure. This patent discloses a second configuration that involves the use of a voltage source to bias the gate relative to the contact to activate the sensing structure. In the ideal configuration, anode to the ground to facilitate its cooling and high-voltage photocathode, the addition of a source of voltage at the photocathode complicates the high voltage supply of the photocathode, by adding a photocathode transformer. isolation, for example. The detector and emitter structures are made in a continuous piece of semiconductor with the photoconductive element located under the emitter. It is therefore exposed to X-rays generated in the tube. The photoelements always having a leakage current, there is an electron emission current that generates on the target X-rays. These X-rays in turn generate a current in the photoconductor. This loop induces the appearance of a residual flow of X-rays. It is therefore not possible in this configuration to obtain an extremely low residual X-ray flux in the OFF state, ie, without illumination of the photoelement.

In the following description, in the state "ON" the photoelement is illuminated, while in the state "OFF" the photo element is not illuminated. The same element will be referred to as "a current source controlled by an electromagnetic wave" or "a current control element".

The subject of the invention relates to a new high energy electron source structure controllable by an electromagnetic wave based on field effect transmitters, for example carbon nanotubes / nanofibers (CNT) where the configuration of the electrodes of the switchable cathode allows a dynamic reconfiguration of the potential in the vicinity of the CNTs. The nanotube or nanotubes are electrically connected to a base, all placed on a surface. The reconfiguration of the potential is in particular ensured by the coupling of the CNTs with a current source controlled by an electromagnetic wave (SCCO) which is externalised from the substrate and which, in fact, can be physically removed from a tube incorporating the switchable cathode (or modular) as an electronic source. This integration makes it possible in particular to avoid the direct exposure of the photoelement to the X-rays generated in the tube and the effect of the high-energy ion flux on the cathode which may lead to erosion or a modification of the electrical properties of the substrate, for example , the hydrogenation of silicon.

• La SCCO est disposée en dehors de l'enceinte à vide,
• Une borne d'entrée de la SCCO est reliée à l'alimentation haute tension et à l'électrode d'écrantage de la cathode commutable,
• Une borne de sortie de la SCCO est reliée à l'électrode de base entre l'émetteur à effet de champ et le substrat,
• Le potentiel de la borne de sortie étant supérieur ou égal au potentiel de la borne d'entrée, l'électrode d'écrantage est adaptée à diminuer le champ électrique induit par l'anode sur l'émetteur,
• L'électrode d'écrantage étant localisée dans un plan P comprenant la surface conductrice située sous le pied de l'émetteur à effet de champ ou localisée sous ce même plan, une zone isolante électriquement existe entre l'électrode d'écrantage et cette surface conductrice.

The invention relates to a high-energy electron source according to claim 1, controlled by an electromagnetic wave comprising a vacuum chamber, a switchable or modulatable cathode based on field effect emitters comprising at least one screening electrode, at least one field effect transmitter connected to a base electrode disposed on a substrate, a grounded anode, a high voltage power supply, said high voltage power supply delivering a potential to create a field sufficient anode to induce emission from the field effect transmitter, at least one control circuit of a current source controlled by an electromagnetic wave or SCCO connected to said switchable cathode, characterized in that:

• The SCCO is arranged outside the vacuum chamber,
• An input terminal of the SCCO is connected to the high-voltage power supply and the switchable cathode shielding electrode,
• An output terminal of the SCCO is connected to the base electrode between the field effect transmitter and the substrate,
• Since the potential of the output terminal is greater than or equal to the potential of the input terminal, the screening electrode is adapted to reduce the electric field induced by the anode on the transmitter,
• Since the screening electrode is located in a plane P comprising the conductive surface located under the foot of the field effect emitter or located in the same plane, an electrically insulating zone exists between the screening electrode and this surface. conductive.

According to an alternative embodiment, the SCCO is disposed in a high voltage connector associated with the vacuum chamber, said connector comprising a window transparent to the electromagnetic wave, at least one source of electromagnetic waves controlled by the control circuit.

According to one embodiment, the source of electromagnetic waves is an optical source such as a laser source, a laser diode, a light emitting diode, and the window is transparent to the wavelength of the optical source.

According to another variant, the source of electromagnetic waves is a radiofrequency source comprising a transmission module and an RF radiofrequency transmission antenna, and the SCCO comprises an RF reception antenna connected to an RF reception module, and a source current controlled by this receiving module.

The SCCO includes, for example, an RF receiving antenna connected to an RF receiving module, two cathodes and a microprocessor adapted to drive the current generation.

According to one embodiment, a switchable or modulatable cathode based on field effect transmitters comprises at least two zones, each of these zones is connected to an output of a corresponding current source and one or more connected laser sources. to a control circuit.

The transport of the optical wave can be carried out using an insulating optical fiber inserted into a solid material.

According to one embodiment, the substrate comprises a screening electrode having on one part an opening Oi, on which an encapsulation insulator is deposited, the base electrode and the emitter being arranged opposite the the opening made in the screening electrode.

According to a variant, a base electrode having a radius R, the distance between the base electrode and the screening electrode is of the order of R.

The electron source may comprise a substrate covered with an insulating layer comprising a via allowing contact of the base electrode of the field effect transistor, a screening electrode positioned around a transmitter effect of field, a layer of encapsulation insulator deposited to cover the screening electrode and at least partially the base electrode of the nanotube.

The source may also include an array of field effect transmitters connected to the substrate through the presence of through contacts.

According to one embodiment, the substrate comprises a continuous screening electrode, an encapsulation insulator on which are positioned the base electrode and the associated field effect transmitter.

According to one embodiment, a field effect transmitter is a carbon nanotube or a carbon nanofiber.

The invention also relates to a source of electrons where the electrons strike an anode for the production of X-rays.

• La figure 1, un exemple de structure selon l'invention,
• La figure 2, un exemple de structure selon l'invention avec le CNT,
• La figure 3, un exemple de réalisation avec un réseau de CNTs,
• La figure 4, une illustration du fonctionnement de la cathode commutable,
• La figure 5, la différence de tension entre le nanotube et l'électrode d'écrantage qui permet une annulation du champ au sommet d'un nanotube,
• La figure 6, une première variante de réalisation du système avec commande à fibre optique,
• Les figures 7 et 8, deux exemples de réalisation avec une commande radiofréquence,
• La figure 9, une deuxième variante comprenant plusieurs sources,
• La figure 10, une variante permettant de supprimer les fuites de courant à la surface de l'isolant,
• La figure 11, une variante de réseaux de CNTs connectés au substrat et différentes électrodes surfaciques de contrôle,
• La figure 12, une variante où les CNTs sont polarisés individuellement et ont une électrode surfacique de contrôle commune,
• La figure 13, un exemple d'intégration à un niveau surfacique,
• La figure 14, un exemple de réalisation de différentes zones d'émission avec contrôle individuel du courant émis,
• La figure 15A et la figure 15B représentent deux exemples de structure à électrode d'écrantage enterrée,
• La figure 16, schématise une autre variante de structure à électrode d'écrantage enterrée,
• La figure 17, représente un circuit électronique de contrôle, et
• La figure 18, un exemple de réseau à trois connexions.

Other features and advantages of the present invention will emerge more clearly on reading exemplary embodiments given by way of illustration and in no way limitative, appended to the figures which represent:

• The figure 1 an example of structure according to the invention,
• The figure 2 , an example of structure according to the invention with the CNT,
• The figure 3 , an exemplary embodiment with a network of CNTs,
• The figure 4 , an illustration of how the switchable cathode works,
• The figure 5 , the difference in voltage between the nanotube and the screening electrode which allows a cancellation of the field at the top of a nanotube,
• The figure 6 , a first variant embodiment of the system with optical fiber control,
• The Figures 7 and 8 two embodiments with radio frequency control,
• The figure 9 a second variant comprising several sources,
• The figure 10 , a variant for eliminating current leakage on the surface of the insulation,
• The figure 11 , a variant of networks of CNTs connected to the substrate and different surface control electrodes,
• The figure 12 a variant where the CNTs are individually polarized and have a common control surface electrode,
• The figure 13 , an example of integration at a surface level,
• The figure 14 an embodiment of different emission zones with individual control of the emitted current,
• The figure 15A and the figure 15B represent two examples of buried shield electrode structure,
• The figure 16 , schematizes another variant of buried buried electrode structure,
• The figure 17 , represents an electronic control circuit, and
• The figure 18 , an example of a network with three connections.

The following examples are given for the use of CNTs, but could be implemented for any type of micrometric field effect transmitter, for example, silicon or metal micropoints, diamond, zinc oxide ZnO, etc.

The figure 1 discloses a first exemplary embodiment of a switchable or modulatable electromagnetic wave high energy electron source 100 which comprises a grounded vacuum enclosure 101 including an X-ray transparent window 102, a high voltage power supply 103, ( -30 to -500 kV), a switchable cathode 104 based on field effect emitters, for example carbon nanotubes / nanofibers CNT 105, integrating one or more screening electrodes 111, the conductive layers on the other hand, else or around the nanotube are connected. The electromagnetic wave current control element (SCCO) is arranged outside the vacuum chamber, the switchable cathode and the SCCO being polarized at the high negative voltage, an anode 106 at ground, a wave source electromagnetic 107, for example an optical source such as a laser, a laser diode or a light emitting diode, a window transparent to the electromagnetic wave 108 and a control circuit 109 of this source of electromagnetic waves, for example an optical source . The supply of the source is galvanically decoupled from the high voltage supply 103. The high voltage supply 103 delivers a potential having a value chosen to create an anode field sufficient to induce transmission from the transmitter 105.

The current control element (SCCO) remote from the enclosure, in this example, is a phototransistor or photodiode illuminated by an optical source through an optically transparent window and a gas optically transparent dielectric. The SCCO 120 is located in a high voltage connector 121, comprising a ground-tight envelope 122 and composed of electrical insulators 123 and pressurized gas with high dielectric strength and optically transparent 124.

The switchable cathode 104 ( figure 2 , figure 3 ) comprises at least one multi-wall carbon nanotube / nanofiber 105 (CNT), comprising a conductive surface 105s located under the foot of the emitter, the CNT is oriented vertically relative to the plane of the cathode, a screening electrode 111 of the field induced by the anode 106 located on either side or around the nanotube, these elements being arranged on a substrate 112. The electrical insulator 115 has openings at the base electrodes 110 so as to connect electrically the CNTs 105 to the substrate 112.

A CNT has an important aspect ratio, for example, in the range [100-200], between its length one hundred nanometers to several microns, and its diameter taken at the apex or equivalent for apex surfaces of Non spherical CNTs, one nanometer to several tens of nanometers. The distance between the screening electrode 111 and the nanotube 105 is close to the height h _CNT of the nanotube. The screening electrode 111 is preferably disposed in a plane P comprising the foot conductive surface 105p of the transmitter or located below this plane.

The insulating zone 115 supports the potential difference between the disk at the base of the nanotube and the screening electrode. The reduction of this voltage makes it possible to limit the induced electrical stress.

The conductive electrode between the nanotube 105 and the substrate 112 is connected to the output terminal 131 of the SCCO. The screening electrode 111 on the surface of the substrate is connected to the input terminal 132 of the SCCO. The input terminal 132 of the SCCO is connected to the high voltage HT. The optical source 107 illuminates the current source SCCO with a power controlled by the electronic control circuit 109. The potential of the output terminal 131 is in this example greater than or equal to the potential of the terminal The screening electrode 111 only serves to reduce or eliminate the electric field induced by the anode 106 on the transmitter 105, in normal operation.

• le CNT 105 présente un rapport d'aspect de 100 à 200 entre sa longueur I et son diamètre au sommet,
• V représente la tension entre le nanotube 105 et son électrode de base 110 par rapport à l'électrode d'écrantage 111,
• R est le rayon de l'ouverture dans l'électrode d'écrantage.

The model in this example is defined for the following assumptions:

• the CNT 105 has an aspect ratio of 100 to 200 between its length I and its diameter at the top,
• V represents the voltage between the nanotube 105 and its base electrode 110 with respect to the screening electrode 111,
• R is the radius of the opening in the screening electrode.

When the tube RX is energized, a negative voltage is applied to the switchable cathode 104 and the SCCO 120 relative to the anode 106. This potential difference induces an electric field at the cathode 104. An electric field is then applied to CNT 105 which can induce the emission of electrons. The current I _CNT delivered by the carbon nanotube is equal to the current I _SCCO delivered by the current source SCCO, it adjusts to the current. This operating point results in a self- _polarization phenomenon of the CNT 105: when the current I _SCCO delivered by the current source SCCO decreases, this increases the positive voltage on the nanotube 105 relative to the screening electrode 111. Screening electrode then screens the anode field applied locally to the nanotube, which automatically reduces the _CNT emission current I CNT, until the _CNT current I delivered by the CNT is equal to the current I _SCCO delivered by the current source SCCO. The _CNT current I delivered by the nanotube or nanotubes automatically adjusts to the current I _SCCO delivered by the SCCO. This operating mode makes it possible to control the emission current of the nanotube according to a quasi-linear law of the optical power, in this exemplary embodiment (the SCCO being a photodiode or a phototransistor).

The position of the SCCO 120 outside the vacuum chamber prevents its exposure to generated X-rays. The residual flow of X-rays emitted when the power source is not illuminated, OFF state, is then very weak. This configuration does not require an active voltage source to handle the voltage of the shielding electrode or to activate the SCCO. As a result, the high voltage power supply generates only one signal to bias the switchable cathode and the SCCO relative to the anode. It is thus possible to design a very compact high voltage power supply that does not require an isolation transformer in normal operation.

Because of the provision of the SCCO outside the enclosure, it is possible to obtain a dark current I _obsc equal to the intrinsic dark current (<1nA) of the SCCO and therefore a current of emission of extremely low nanotubes (<1nA), which is essential for medical applications, for example.

The anode 106 is grounded which facilitates its cooling. According to one embodiment, the anode 106 may include an opening for the passage of electrons, the anode 106 being connected to a vacuum chamber according to a scheme known to those skilled in the art. The source according to the invention is a source of high energy electrons, for example from 20 to 500 kV.

The figure 3 illustrates an exemplary embodiment of the invention with a CNT network, 105i. The elements referenced in this figure have been described previously.

The figure 4 schematically illustrates the operation of the cathode controlled by the SCCO. It includes the emission current I _CNT of a CNT as a function of the potential difference between the nanotube 105 and the screening electrode 111, and this for a constant anode field. It also includes the current I _SCCO delivered by the SCCO according to the bias voltage of this SCCO and according to the optical power Popt received by the SCCO.

In the configuration shown, the difference in voltage between the nanotube and the screening electrode, which is equal to the voltage difference between the output terminal and the input terminal of the SCCO source.

Since the _CNT delivered current by the nanotubes is equal to the delivered current I _SCCO by the SCCO, the current value is the intersection, I _s , between the curve 200 of the current delivered by the SCCO and the current curve 201. emission of the nanotube. For a optical power P _opt1 of illumination of the SCCO corresponding to a current delivered by this SCCO source of 10 μA, the emission current of the CNT is equal to this current of 10 μA.

When the transmission power of the source SCCO decreases, curve P _opt2 , the current I _SCCO delivered by the SCCO decreases, in the example, 5 μA. The electrons initially accumulated at the top of the CNT will partly be emitted by field effect, thereby reducing the extraction field at its peak. The emission current I _CNT will be reduced. This process stops when the emission current becomes equal to 5 μA. It is possible to temporally modulate the illumination power of the source SCCO and thus the emission current I _CNT of the nanotubes and thus the RX flux emitted on the object to be examined.

When the SCCO is no longer illuminated, state OFF, the delivered current I _SCCO by the source is equal to the current value corresponding to the intersection of the curve describing the dark current I _obsc and the emission curve. nanotube. In order to obtain an extremely low OFF state current, the dark current of the SCCO source must be extremely low and a voltage across the SCCO current source must be lower than the avalanche voltage of the SCCO source. SCCO.

The figure 5 represents the voltage difference between a CNT and a shielding electrode allowing a cancellation of the field at the top of the CNT. For a CNT with a height of 5 μm and an opening radius in the 1 μm screening electrode, this voltage is 110 V. In this configuration, an SCCO that has an avalanche voltage greater than 110V and which has an extremely low dark current. There are photodiodes with avalanche voltages of 200 V with a dark current of less than 1 nA. It is therefore possible with this configuration to make an RX tube with an electron current in the OFF state less than 1 nA. The current source SCCO can also feed a network of nanotubes, as will be schematized later. The current in the ON state, illuminated current source, can reach for example 1 mA. An ON / OFF ratio of 10 ^{6 is} then obtained. It is particularly advantageous to use short and fine CNTs, for example of 2 μm in height and 20 nm in diameter at the top. For the same electrode radius of 1 μm, the voltage that makes it possible to cancel the field at the top of the nanotube is of the order of 50 V. It is then possible to use SCCOs having a lower avalanche voltage.

For a buried screening electrode, the insulation thickness will be adjusted according to the voltages to be held and the insulating material. For example, 1 .mu.m of thermal silica can hold a voltage of 200V and theoretically 1000V. The operating principle of the switchable cathode described above remains the same for this variant embodiment.

The figure 6 represents an alternative embodiment using an electrical insulating optical fiber for the propagation of the control signal. The elements of this variant identical to those described in figure 1 bear the same references. An insulating optical fiber 140 allows the propagation of the signal from the source 107. This fiber passes through a dielectric solid 141, such as a polymer, a ceramic, an epoxide, in order to excite the SCCO 120. The assembly is disposed in an electrical insulator 142. As in the example of the figure 1 , there is a direct optical link between the optical source and the remote SCCO source.

The figure 7 illustrates a radiofrequency electromagnetic wave source 180 for controlling the SCCO. The radiofrequency source comprises a transmission module 181 and an RF emission antenna 182. The SCCO comprises an RF reception antenna 183 connected to a module RF receiver 184, and a current source controlled by this receiving module. The SCCO is therefore a source of current controlled by the source of electromagnetic waves 180. Such a device does not require any direct link between the RF source and the SCCO. This device is particularly well suited for controlling many switchable cathodes carried at high voltage by an electromagnetic wave having different modulations and thus allowing transmission multiplexing and demultiplexing at the reception of each channel, CNT cathode. The control can be all or nothing (On / Off) or allow precise control of the current intensity of CNT pulse width modulation or Pulse Width Modulation PWM in Anglo-Saxon.

The figure 8 schematically a variant for controlling two cathodes C ₁ , C ₂ by multiplexing Mix. This device allows, for example, the RF control and the generation of PWM signals to control the current from a second microprocessor 185. The communication between the two RF microprocessors can be done using the SPI protocol for example.

The figure 9 represents a variant for which the switchable cathode comprises at least two zones 81, 82, or even more than two zones. Each zone comprises one or more CNTs 105 and each zone is connected to an output 83s, 84s, of a current source 83, 84 corresponding thereto. Each CNT 105 is associated with a screening electrode positioned on either side or around the nanotube as described above. One or more laser sources 85, 86 are connected to a control circuit. The operation of this variant is similar to that described for the preceding figures with a greater possibility in the modulation.

The figure 10 is a sectional view of an example of a solution for eliminating current leaks that may exist on the surface of the insulator 1001. In this example, the substrate 1000 is covered with an insulating layer 1001 comprising a via 1002 allowing the contact of the base electrode of the nanotube, a screening electrode 111 positioned around the nanotube 105 ( figure 2 ). An encapsulation insulator layer 1004 is deposited to cover the shielding electrode and at least partially the base electrode of the nanotube. This arrangement advantageously makes it possible to reduce or even cancel the leakage currents.

The figure 11 represents a network of nanotubes 105 connected to the substrate through the presence of through contacts 1100, known by the abbreviation TSV (through silicon vias). The presence of these TSV makes it possible to transfer contacts from the rear face 1101 to the front face 1102. In addition, being themselves isolated from the substrate, they make it possible to electrically control different areas on the chip surface. Thus, all the CNTs 105 are connected to the substrate 91. Electrically isolated shielding electrodes can be attached to different CNTs areas, thus independently controlling their emission currents.

The figure 12 schematically an example of individually polarized nanotubes 105 through the presence of through contacts TSV 1100 and the presence of a surface control electrode 1200 common to different nanotubes CNTs.

The figure 13 , describes an example of integration at a surface level. On the substrate 1300 an insulating layer 1301 is deposited. Then a conductive layer is cut into two disjoint conductive areas, 1303, 1304, but interlaced so as to obtain an interdigitated structure. One of the electrodes serves as a base electrode 110 at CNT 105 ( figure 2 ), the other electrode plays the role of screening electrode 111. Here the substrate no longer has an electrical role, only a role of mechanical support.

The figure 14 , gives an exemplary embodiment of different transmission zones 1401, 1402, 1403, 1404, 1405 with individual control of the emitted current. Each of the zones has a structure such as that described in figure 12 . The different areas are positioned next to each other according to the specifications of the intended application. It is possible carry out a transfer of contacts on the rear face without changing the operating principle.

The figure 15A and the figure 15B , two examples of insulating multilayer structure. The figure 15A represents a first embodiment which allows in particular to avoid the risk of current leakage on the surface of the insulation. Is deposited on an insulating substrate a screening electrode 151 having on one part an opening O, on which is deposited an encapsulation insulator 152. The base electrode and the nanotube are arranged vis-à-vis the opening made in the screening electrode. The figure 15B schematically a second variant in which is disposed on the insulating substrate, a continuous screening electrode, an encapsulation insulator 154 on which we will position the base electrode 110 and the associated nanotube 105.

In these two configurations, the conductor network at the potential of the nanotubes is separated from the control screening electrode by an insulating dielectric layer. The galvanic isolation between the two conductive elements is no longer surface but intrinsic. This device is interesting with regard to arcing phenomena, partially conductive deposits may appear in the vacuum electronic tubes and more particularly the RX tubes. The control screening electrode preferably operates in self-polarization thus ensuring electrostatic shielding of the main field created by the anode which is carried at high voltage.

The figure 16 schematizes an example of buried buried electrode structure optimized to minimize the coupling capacitors between the base electrode 110 connecting the CNTs 105 and the buried screening electrode 111 shown in dotted lines, it can take the form of a flat ring and has a certain surface extending outside the surface of the base electrode. This structure makes it possible to envisage operating frequencies greater than frequencies used at a continuous buried screening electrode which exhibits a stronger capacitive coupling with the base electrodes.

The figure 17 represents an example of an electronic circuit for controlling current of the nanotubes by an optically controlled current source. The screening electrode 111 is voltage controlled using a phototransistor, illuminated it is passing. The screening electrode 111, in dashed lines, is polarized at the high voltage HT (potential reference of the system). If the phototransistor 171 is unlit, it becomes blocking: the screening electrode 111 is found to be negatively polarized with respect to the high voltage HT thanks to a battery 172 (typical polarization 40V). This makes it possible to control the voltage level of the screening electrode with respect to the potential reference. The base electrode 110 is connected to the high voltage HT through a phototransistor 175 which acts as an optically controlled switch. Illuminated under strong flux, the phototransistor 175 is fully conducting, thereby providing a direct connection of the base electrode 110 to the high voltage HT. In the absence of light flux, the phototransistor 175 is blocking and the current emitted by the nanotubes 105 equals the dark current of the phototransistor (typically <1nA). With intermediate illumination, the current level of the phototransistor can be regulated precisely: the current emitted by the CNTs 105 then equals this current per operating point (cf. figure 2 ). The level of illumination of the phototransistor makes it possible to control the electronic emission level of the CNTs. A Zener diode 176 placed in parallel with the phototransistor 175 makes it possible to avoid overvoltages on the phototransistor 175 and avoids its destruction during uncontrolled events such as breakdowns in the X-ray tube.

The figure 18 schematizes an example of a network with three connections allowing the use of individual emitters and requiring electrostatic symmetry around the emission axis of a nanotube to minimize electron optical aberrations. Indeed the generated electric field has the symmetry of the electrodes which forms it (near the CNT). Thus a high symmetry is obtained by making a shielding electrode connection and base electrode connected by three channels 191, 192, 193 distributed at 120 °.

The offset of the SCCO out of the tube offers a greater margin of maneuver on the choice of the SCCO (photo element for example), dimensions, electrical characteristics, resistance in tension, etc. The SCCO is no longer subject to the direct environment of the tube, X-rays, bombardment and ion implantation, etc. The configuration of the electrodes notably allows a dynamic reconfiguration of the potential in the vicinity of the nanotubes.

Claims (14)

1. A high-energy electron source adapted to be controlled by an electromagnetic wave, comprising a vacuum chamber (101), a switchable or modulatable cathode (104) based on field effect emitters (105) comprising at least one screening electrode (111), at least one field effect emitter (105) connected to a base electrode (110) disposed on a substrate (112), a grounded anode (106), a high-voltage power supply (103), said high-voltage power supply (103) being adapted to deliver a potential in order to create a sufficient anode field for inducing the emission from the field effect emitter (105), at least one circuit (109) for controlling a current source adapted to be controlled by an electromagnetic wave, called SCCO (120), connected to said switchable or modulatable cathode (104), characterized in that:

   • the SCCO (120) is disposed outside the vacuum chamber;

   • an input terminal (132) of the SCCO is connected to the high-voltage power supply and to the screening electrode (111) of the switchable or modulatable cathode (104) ;

   • an output terminal (131) of the SCCO is connected to the base electrode (110) between the field effect emitter and the substrate (112);

   • the potential of the output terminal (131) is adapted to be greater than or equal to the potential of the input terminal (132), the screening electrode (111) is adapted to reduce the electric field induced by the anode (106) on the emitter (105);

   • the screening electrode (111) is located in a plane P comprising the conductive surface (105p) located under the foot of the field effect emitter (105) or located under said plane, an electrically insulating zone (115) exists between the screening electrode and this conductive surface.
2. The electron source as claimed in claim 1, characterized in that the SCCO (120) is disposed in a high-voltage connector (121) associated with the vacuum chamber, said connector (121) comprising a window transparent to the electromagnetic wave, at least one electromagnetic wave source (107) adapted to be controlled by the control circuit (109).
3. The electron source as claimed in claim 2, characterized in that the electromagnetic wave source (107) is an optical source such as a laser source, a laser diode, a light emitting diode and the window (108) is transparent to the wavelength of the optical source.
4. The electron source as claimed in claim 2, characterized in that the electromagnetic wave source is a radiofrequency source comprising an emission module (181) and an RF emission antenna (182), and in that the SCCO (120) comprises an RF reception antenna (183) connected to an RF reception module (184), and a current source controlled by this reception module.
5. The electron source as claimed in claim 4, characterized in that the SCCO (120) comprises an RF reception antenna (183) connected to an RF reception module (184), two cathodes C₁, C₂, a microprocessor (185) adapted to control the generation of current.
6. The electron source as claimed in claim 1, characterized in that the switchable or modulatable cathode (104) based on field effect emitters (105) comprises at least two zones (81, 82), each of said zones is connected to an output (83s, 84s) of a current source (83, 84) corresponding thereto, and in that the one or more laser sources (85, 86) are connected to a control circuit (109).
7. The electron source as claimed in claim 3, characterized in that the optical wave is carried using an insulating optical fibre (140) inserted in a solid material (141).
8. The electron source as claimed in any one of the preceding claims, characterized in that the substrate comprises the screening electrode (151) having an opening Oi on one part, on which opening an encapsulation insulator (152) is deposited, the base electrode and the emitter being disposed opposite the opening made in the screening electrode.
9. The electron source as claimed in any one of the preceding claims, characterized in that, the base electrode (110) having a radius R, the distance between the base electrode (110) and the screening electrode (111) is of the order of the radius R.
10. The electron source as claimed in any one of the preceding claims, characterized in that the substrate (1000) is covered with an insulating layer (1001) comprising a via (1002) allowing contact with the base electrode of the field effect transistor, the screening electrode (111) is positioned around the field effect emitter (105), an encapsulation insulating layer (1004) is deposited so as to cover the screening electrode (111) and at least partially cover the base electrode (110) of the nanotube.
11. The electron source as claimed in any one of the preceding claims, characterized in that it comprises an array of field effect emitters connected to the substrate by virtue of the presence of through-contacts (1100).
12. The electron source as claimed in any one of the preceding claims, characterized in that the substrate comprises a continuous screening electrode, an encapsulation insulator (154), on which the base electrode (110) and the associated field effect emitter (105) are positioned.
13. The electron source as claimed in any one of the preceding claims, characterized in that the field effect emitter is a nanotube or a carbon nanofibre (105).
14. The electron source as claimed in any one of the preceding claims, characterized in that the electrons strike an anode in order to produce x-rays.

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