Patent Application
20200194213
The invention relates to a source for generating ionizing radiation and in particular x-rays, to an assembly comprising a plurality of sources and to a process for producing the source.
At the present time, x-rays have many uses in particular in imaging and in radiotherapy. X-ray imaging is widely employed in particular in the medical field, in industry to perform nondestructive tests and in the security field to detect dangerous materials or objects.
The production of images from x-rays has progressed a lot. Originally only photosensitive films were used. Since, digital detectors have appeared. These detectors, associated with software packages, allow two-dimensional or three-dimensional images to be rapidly reconstructed by means of scanners.
In contrast, since the discovery of x-rays by Rontgen in 1895, x-ray generators have changed very little. Synchrotrons, which appeared after the Second World War, allow an intense and well-focused emission to be generated. The emission is due to acceleration or deceleration of charged particles, which optionally move in a magnetic field.
Linear accelerators and x-ray tubes implement an accelerated electron beam that bombards a target. The deceleration of the beam due to the electric fields of the nuclei of the target allow bremsstrahlung x-rays to be generated.
An x-ray tube generally consists of an envelope in which a vacuum is produced. The envelope is formed from a metal structure and an electrical insulator normally made of alumina or glass. Two electrodes are placed in this envelope. A cathodic electrode, biased to a negative potential, is equipped with an electron emitter. An anodic second electrode, biased to a positive potential with respect to the first electrode, is associated with a target. Electrons accelerated by the potential difference between the two electrodes produce a continuous spectrum of ionizing radiation by deceleration (bremsstrahlung) when they strike the target. The metal electrodes are necessarily of large size and possess large radii of curvature in order to minimize the electric fields on the surface.
Depending on the power of the x-ray tube, the latter may be equipped either with a stationary anode or with a rotating anode that makes it possible to spread the thermal power. Stationary-anode tubes have a power of a few kilowatts and are in particular used in low-power medical, safety and industrial applications. Rotating-anode tubes may exceed 100 kilowatts and are mainly employed in the medical field for imaging requiring a high x-ray flux allowing contrast to be improved. By way of example, the diameter of an industrial tube is about 150 mm at 450 kV, about 100 mm at 220 kV and about 80 mm at 160 kV. The indicated voltage corresponds to the potential difference applied between the two electrodes. For medical rotating-anode tubes, the diameter varies from 150 to 300 mm depending on the power to be dissipated on the anode.
The dimensions of known x-ray tubes therefore remain large, of the order of several hundred mm. Imaging systems have seen the appearance of digital detectors with increasingly rapid and high-performance 3-D reconstruction software packages whereas, at the same time, x-ray tube technologies have remained practically unchanged for a century, and this is a major technological limitation on x-ray imaging systems.
Several factors are an obstacle to the miniaturization of current x-ray tubes.
The dimensions of the electrical insulators must be large enough to guarantee a good electrical insulation with respect to high voltages of 30 kV to 300 kV. Sintered alumina, which is often used to produce these insulators, typically has a dielectric strength of about 18 MV/m.
The radius of curvature of the metal electrodes must not be too small in order to keep the static electric field applied to the surface below an acceptable limit, typically 25 MV/m. Thereabove, the emission of parasitic electrons via a tunneling effect becomes difficult to control and leads to heating of walls, to the emission of undesirable x-rays and to micro-discharges. Thus, at high voltages, such as encountered in x-ray tubes, the dimensions of the cathodic electrodes are large in order to limit parasitic emission of electrons.
Thermionic cathodes are often used in conventional tubes. The dimensions of this type of cathode and their operating temperature, typically above 1000° C., lead to expansion problems and to the evaporation of electrically conductive elements such as barium. This makes miniaturization and integration of this type of cathode in contact with a dielectric insulator difficult.
Surface charge effects related to the coulomb interaction appear on the surface of the dielectrics (alumina or glass) used when this surface is in the vicinity of an electron beam. In order to prevent proximity between the electron beam and the dielectric surfaces, either an electrostatic shield is formed using a metal screen placed in front of the dielectric, or the distance between the electric beam and the dielectric is increased. The presence of screens or this increased distance also tends to increase the dimensions of x-ray tubes.
The anode forming the target must dissipate a high thermal power. This dissipation may be achieved with a flow of heat-transfer fluid or by producing a rotating anode of large size. The need for this dissipation also requires the dimensions of x-ray tubes to be increased.
Among emerging technological solutions, the literature describes the use of carbon-nanotube-based cold cathodes in x-ray tube structures, but currently proposed solutions remain based on conventional x-ray tube structures implementing a metal wehnelt encircling the cold cathode. This wehnelt is an electrode raised to a high voltage and is always subject to severe dimensional constraints, with regard to limiting the parasitic emission of electrons.
The invention aims to mitigate all or some of the aforementioned problems by providing a source of ionizing radiation, for example taking the form of a high-voltage triode or diode, the dimensions of which are much smaller than those of conventional x-ray tubes. The mechanism of generation of the ionizing radiation remains similar to that implemented in known tubes, namely an electron beam bombarding a target. The electron beam is accelerated between a cathode and an anode between which a potential difference, for example higher than 100 kV, is applied. For a given potential difference, the invention allows the dimensions of the source according to the invention to be substantially decreased with respect to known tubes.
To achieve this aim, the invention provides a source of ionizing radiation comprising a vacuum chamber in which the anode performs a number of functions.
More precisely, one subject of the invention is a source for generating ionizing radiation, comprising:
The getter is advantageously made from a material that is different from the material of the cavity.
The source advantageously comprises at least one magnet or electromagnet encircling the cavity. The walls of the cavity are then made of amagnetic material.
The source advantageously comprises a mechanical holder that holds the getter and that is made of magnetic material. The mechanical holder is placed in the cavity so as to guide the magnetic flux generated by the magnet or electromagnet.
The at least one magnet or electromagnet is advantageously arranged so as to deviate the parasitic ions towards the at least one getter.
At least one of the walls of the cavity advantageously forms a wall of the vacuum chamber.
The walls of the cavity are advantageously arranged coaxially to the axis.
The walls of the cavity advantageously comprise a cylindrical portion around the axis, extending between the target and a ring-shaped portion containing a hole and closing the cylindrical portion. The electron beam then penetrates into the cavity via the hole in the portion.
Advantageously, the source comprises a mechanical part that is made of dielectric and that forms a wall of the vacuum chamber. The anode is sealably fastened to the mechanical part.
The target may be inclined with respect to a plane perpendicular to the axis.
The source advantageously comprises an active magnetic system that generates a magnetic field transverse to the axis in the cavity and that is configured to modify the shape of an electron spot formed by the electron beam on the target.
The walls of the cavity advantageously form a shielding screen with respect to parasitic ionizing radiation generated in the interior of the vacuum chamber.
The invention will be better understood and other advantages will become apparent on reading the detailed description of one embodiment that is given by way of example, which description is illustrated by the appended drawing, in which:
For the sake of clarity, elements that are the same have been given the same references in the various figures.
X-ray generating tubes conventionally employ a thermionic cathode operating at a high temperature, typically about 1000° C. This type of cathode is commonly called a hot cathode. This type of cathode is composed of a metal or metal-oxide matrix that emits an electron flux that is caused by vibrations of atoms due to the high temperature. However, hot cathodes suffer from a plurality of drawbacks, such as a slow dynamic response of the current to control, related to the time constants of the thermal processes, and such as the need to use, to control the current, grids located between the cathode and the anode and biased to high voltages. These grids are thus located in a zone of very high electric field, and they are subjected to high operating temperatures of about 1000° C. All of these constraints greatly limit the options with regard to integration and lead to electron guns of large size.
More recently, cathodes employing a field-emission mechanism have been developed. These cathodes operate at room temperature and are commonly called cold cathodes. They for the most part consist of a conductive planar surface equipped with relief structures, on which an electric field is concentrated. These relief structures emit electrons when the field at the tip is sufficiently high. The relief emitters may be formed from carbon nanotubes. Such emitters are for example described in the patent application published under the number WO 2006/063982 A1 and filed in the name of the applicant. Cold cathodes do not have the drawbacks of hot cathodes and are above all much more compact. In the example shown, the cathode 14 is a cold cathode and therefore emits the electron beam 18 via a field effect. The means for controlling the cathode 14 are not shown in
Under the effect of a potential difference between the cathode 14 and the anode 16, the electron beam 18 is accelerated and strikes the target 20, which for example comprises a membrane 20a, which is for example made of diamond or beryllium coated with a thin layer 20b made from an alloy based on a material of high atomic number such as, in particular, tungsten or molybdenum. The layer 20b may have a variable thickness that is for example, depending on the energy of the electrons of the beam 18, comprised between 1 and 12 μm. The interaction between the electrons of the electron beam 18, which electrons are accelerated to high speed, and the material of the thin layer 20b allows x-rays 22 to be produced. In the example shown, the target 20 advantageously forms a window of the vacuum chamber 12. In other words, the target 20 forms a portion of the wall of the vacuum chamber 12. This arrangement is in particular implemented for a target operating in transmission. For this arrangement, the membrane 20a is formed from a material of low atomic number, such as diamond or beryllium, for its transparency to x-rays 22. The membrane 20a is configured to ensure, with the anode 16, the vacuum tightness of the chamber 12.
Alternatively, the target 20, or at the very least the layer made from a high-atomic-number alloy, may be entirely placed in the interior of the vacuum chamber 12, the x-rays then exiting from the chamber 12 by passing through a window forming a portion of the wall of the vacuum chamber 12. This arrangement is in particular implemented for a target operating in reflection. The target is then separate from the window. The layer in which the x-rays are produced may be thick. The target may be stationary or rotate so as to allow the thermal power generated during the interaction with the electrons of the beam 18 to be spread.
Advantageously, it is possible to relax a severe constraint on the electric-field level at the surface of the cathodic electrode or wehnelt. This constraint is related to the metal nature of the interface between the electrode and the vacuum present in the chamber through which the electron beam propagates. Specifically, on the electrode, the metal/vacuum interface is replaced with a dielectric/vacuum interface that does not allow parasitic emission of electrons via a tunneling effect. It is then possible to accept much higher electric fields than those acceptable with a metal/vacuum interface. Initial internal trials have shown that it is possible to achieve static fields much higher than 30 MV/m without parasitic emission of electrons. This dielectric/vacuum interface may, for example, be obtained by replacing the metal electrode, the external surface of which is subjected to the electric field, with an electrode consisting of a dielectric the external surface of which is subjected to the electric field and the internal surface of which is coated with a perfectly adherent conductive deposit performing the electrostatic wehnelt function. It is also possible to cover the external surface of a metal electrode subjected to the electric field with a dielectric in order to replace the metal/vacuum interface of known electrodes with a dielectric/vacuum interface, there where the electric field is high. This arrangement in particular allows the maximum electric field below which parasitic emission of electrons does not occur to be increased.
The increase in the permissible electric fields allows x-ray sources, and more generally sources of ionizing radiation, to be miniaturized.
To this end, the source 10 comprises an electrode 24 that is placed in the vicinity of the cathode 14 and that allows the electron beam 18 to be focused. The electrode 24 forms a wehnelt. In the case of what is called a cold cathode, the electrode 24 is placed in contact with the cathode. A cold cathode emits an electron beam via a field effect. This type of cathode is for example described in document WO 2006/063982 A1 filed in the name of the applicant. In the case of a cold cathode, the electrode 24 is placed in contact with the cathode 14. The mechanical part 28 advantageously forms a holder of the cathode 14. The electrode 24 is formed from a continuous conductive area placed on a concave face 26 of a dielectric. The concave face 26 of the dielectric forms a convex face of the electrode 24 facing the anode 16. To perform the wehnelt function, the electrode 24 has an essentially convex shape. The exterior of the concavity of the face 26 is oriented toward the anode 16. Locally, where the cathode 14 and the electrode make contact, the convexity of the electrode 24 may be zero or slightly inverted.
It is on this convex face of the electrode 24 that high electric fields develop. In the prior art, a metal-vacuum interface existed on this convex face of the electrode. Therefore, it was possible for this interface to be the seat of emission of electrons under the effect of the electric field in the interior of the vacuum chamber. This interface of the electrode with the vacuum of the chamber is removed and replaced with a dielectric/vacuum interface. A dielectric, since it contains no free charge, cannot therefore be the seat of a sustained emission of electrons.
It is important to prevent an air-filled or vacuum cavity from forming between the electrode 24 and the concave face 26 of the dielectric. Specifically, in case of an uncertain contact between the electrode 24 and the dielectric, the electric field could be very highly amplified at the interface and electron emission could occur or a plasma could be generated there. For this reason, the source 10 comprises a mechanical part 28 made from the dielectric. One of the faces of the mechanical part 28 is the concave face 26. In this case, the electrode 24 consists of a deposit of a conductor that adheres perfectly to the concave face 26. Various techniques may be employed to produce this deposit, such as in particular physical vapor deposition (PVD) or chemical vapor deposition (CVD) that is optionally plasma-enhanced (PECVD).
Alternatively, it is possible to produce a deposit of dielectric on the surface of a bulk metal electrode. The dielectric deposit, which adheres to the bulk metal electrode, again allows an air-filled or vacuum cavity to be avoided at the electrode/dielectric interface. This dielectric deposit is chosen to withstand high electric fields, typically higher than 30 MV/m, and to possess a sufficient suppleness compatible with potential thermal expansion of the bulk metal electrode. However, the inverse arrangement, implementing the deposition of a conductor on the internal face of a bulk part made of dielectric has other advantages, in particular that of allowing the mechanical part 28 to be used to perform other functions.
More precisely, the mechanical part 28 may form a portion of the vacuum chamber 12. This portion of the vacuum chamber may even be a preponderant portion of the vacuum chamber 12. In the shown example, the mechanical part 28 forms, on the one hand, a holder of the cathode 14, and, on the other hand, a holder of the anode 16. The part 28 ensures the electrical insulation between the anode 16 and the cathodic electrode 24.
With regard to the production of the mechanical part 28, just using a conventional dielectric, such as for example sintered alumina, allows any metal/vacuum interface to be avoided. However, the dielectric strength of this type of material, about 18 MV/m, still limits miniaturization of the source 10. To further miniaturize the source 10, a dielectric possessing a dielectric strength higher than 20 MV/m and advantageously higher than 30 MV/m is chosen. The value of the dielectric strength for example remains above 30 MV/m in a temperature range comprised between 20 and 200° C. Composite nitride ceramics allow this criterion to be met. Internal trials have shown that one ceramic of this nature even allows 60 MV/m to be exceeded.
On miniaturization of the source 10, surface charges may accumulate on an internal face 30 of the vacuum chamber 12, and in particular on the internal face of the mechanical part 28, when the electron beam 18 is established. It is useful to be able to drain these charges, and for this reason the internal face 30 has a surface resistivity measured at room temperature comprised between 1×109 Ω·square and 1×1013 Ω·square and typically in the vicinity of 1×1011 Ω·square. Such a resistivity may be obtained by adding to the surface of the dielectric a conductor or semiconductor that is compatible with the dielectric. By way of semiconductor, it is for example possible to deposit silicon on the internal face 30. In order to obtain the right resistivity range, for example for a nitride-based ceramic, it is possible to modify its intrinsic properties by adding thereto a few percent (typically less than 10%) of a powder of titanium nitride, which is known for its low resistivity, of about 4×10−3 Ω·m, or of semiconductors such as silicon carbide SiC.
It is possible to disperse the titanium nitride in the volume of the dielectric in order to obtain a uniform resistivity throughout the material of the mechanical part 28. Alternatively, it is possible to obtain a resistivity gradient by diffusing the titanium nitride from the internal face 30 via a high-temperature heat treatment at a temperature above 1500° C.
The source 10 comprises a stopper 32 that ensures the seal tightness of the vacuum chamber 12. The mechanical part 28 comprises a cavity 34 in which the cathode 14 is placed. The cavity 34 is bounded by the concave face 26. The stopper 32 closes the cavity 34. The electrode 24 comprises two ends 36 and 38 that are distant along the axis 19. The first end 36 makes contact with the cathode 14 and is in electrical continuity therewith. The second end 38 is opposite the first. The mechanical part 28 comprises an interior conic frustum 40 of circular cross-section placed about the axis 19 of the beam 18. The conic frustum 40 is located at the second end 38 of the electrode 24. The conic frustum widens with distance from the cathode 14. The stopper 32 has a shape that is complementary to the conic frustum 40 in order to be placed therein. The conic frustum 40 ensures the positioning of the stopper 32 in the mechanical part 28. The stopper 32 may be implemented independently of whether, as in this embodiment, the electrode 24 takes the form of a conductive area placed on the concave face 26 of the dielectric.
Advantageously, the stopper 32 is made from the same dielectric as the mechanical part 28. This allows potential effects of differential thermal expansion between the mechanical part 28 and the stopper 32 during use of the source to be limited.
The stopper 32 is for example fastened to the mechanical part 28 by means of a brazing film 42 produced in the conic frustum 40 and more generally in an interface zone between the stopper 32 and the mechanical part 28. It is possible to metallize the surfaces intended to be brazed of the stopper 32 and of the mechanical part 28, then to carry out the brazing by means of a metal alloy the melting point of which is higher than the maximum temperature of use of the source 10. The metallization and the brazing film 42 are placed in electrical continuity with the end 38 of the electrode 24. The frustoconical shape of the metallized interface between the stopper 32 and the mechanical part 28 allows shapes that are too pronouncedly angular for the electrode 24 and for the conductive zones extending the electrode 24 to be avoided in order to limit potential edge effects on the electric field.
Alternatively, it is possible to avoid the need to metallize the surfaces by incorporating into the brazing alloy an active element that reacts with the material of the stopper 32 and with the material of the mechanical part 28. For nitride-based ceramics, titanium is integrated into the brazing alloy. Titanium is a material that reacts with nitrogen and allows a strong chemical bond to be created with the ceramic. Other reactive metals may be used such as vanadium, niobium or zirconium.
Advantageously, the brazing film 42 is conductive and is used to electrically connect the electrode 24 to a power supply of the source 10. The electrical connection of the electrode 24 by means of the brazing film 42 may be implemented with other types of electrode, in particular metal electrodes covered with a dielectric deposit. To reinforce the connection with the electrode 24, it is possible to embed a metal contact in the brazing film 42. This contact is advantageous for connecting a bulk metal electrode covered with a dielectric deposit. The electrical connection of the electrode 24 is ensured by this electrical contact. Alternatively, it is possible to partially metallize a surface 43 of the stopper 32. The surface 43 is located at the end of the vacuum chamber 12. The metallization of the surface 43 makes electrical contact with the brazing film 42. It is possible to braze on the metallization of the surface 43 a contact that may be electrically connected to a power supply of the source 10.
The brazing film 42 extends the axisymmetric shape of the electrode 24 and thus contributes to the main function of the electrode 24. This is particularly advantageous when the electrode 24 is formed from a conductive area placed on the concave face 26. The brazing film 42 extends the conductive area forming the electrode 24 directly and without discontinuity or angular edges extending away from the axis 19. The electrode 24, which is associated with the brazing film 42 when the latter is conductive, forms an equipotential area that is used to help focus the electron beam 18 and to bias the cathode 14. This allows local electric fields to be minimized with a view to increasing the compactness of the source 10.
The face 26 may contain locally convex zones, such as for example at its junction with the conic frustum 40. In practice, the face 26 is at least partially concave. The face 26 is concave on the whole.
In
With a source 10 such as shown in
A bipolar operating mode is achieved with a source such as illustrated in
That surface 43 of the stopper 32 which is located on the exterior of the vacuum chamber 12 may be metallized in two separate zones: a zone 43a centered on the axis 19 and a peripheral annular zone 43b around the axis 19. The metallized zone 43a is in electrical continuity with the metallized via 68. The metallized zone 43b is in electrical continuity with the brazing film 42. A central contact 70 bears against the zone 43a and a peripheral contact 71 bears against the zone 43b. The two contacts 70 and 71 form a coaxial connector that electrically connects the cathode 14 and the electrode 24 by way of the metallized zones 43a and 43b and by way of the metallized via 68 and of the brazing film 42.
The cathode 14 may comprise a plurality of separate emitting zones that are separately addressable. The back face 66 then has a plurality of separate electrical contact zones. The holder 60 and the spring 64 are modified accordingly. A plurality of contacts similar to the contact 69 and a plurality of metallized vias similar to the via 68 allow the various zones of the back face 66 to be connected. The surface 43 of the stopper 32, the contact 69 and the spring 64 are partitioned accordingly in order to provide therein a plurality of zones similar to the zone 43a and in electrical continuity with each of the metallized vias.
At least one getter 35 may be placed in the cavity 34, between the cathode 14 and the stopper 32, in order to trap any particles liable to degrade the quality of the vacuum in the chamber 12. The getter 35 generally acts by chemisorption. Alloys based on zirconium or titanium may be employed to trap any particles emitted by the various components of the source 10 encircling the cavity 34. The getter 35 is, in the example shown, fastened to the stopper 32. The getter 35 is made up of ring-shaped discs that are stacked and that encircle the contact 69.
During the bombardment of the target 20 by the electron beam 18, the increase in the temperature of the target 20 may lead to molecules degassing from the target 2, which, under the effect of the x-rays 22, are ionized. Ions 91 that appear at the interior face 84 of the target 20 may damage the cathode if they migrate in the accelerating electric field located between the anode and the cathode. Advantageously, the walls of the cavity 80 may be used to trap the ions 91. To this end, the walls 88 and 90 of the cavity 80 are electrical conductors and form a faraday cage with respect to the parasitic ions that may be emitted by the target 20 into the interior of the vacuum chamber 12. The ions 91 possibly emitted by the target 20 into the interior of the vacuum chamber 12 are to a large extent trapped in the cavity 80. Only the hole 89 of the portion 90 allows these ions to exit from the cavity 80 and to then possibly be accelerated toward the cathode 14. To better trap the ions in the cavity 80, at least one getter 92 is placed in the cavity 80. The getter 92 is separate from the walls 88 and 90 of the cavity 80. The getter 92 is a specific component placed in the cavity 80. Just like the getter 35, the getter 92 generally acts by chemisorption. Alloys based on zirconium or titanium may be used to trap the emitted ions 91.
In addition to trapping ions, the walls of the cavity 80 may form a shielding screen with respect to parasitic ionizing radiation 82 generated in the interior of the vacuum chamber 12 and optionally an electrostatic shield with respect to the electric field generated between the cathode 14 and the anode 76. The x-rays 22 form the useful emission emitted by the source 75. However, parasitic x-rays may exit from the target 20 via the internal face 84. This parasitic emission is neither useful nor desirable. Conventionally, shielding screens that block this type of parasitic radiation are placed around x-ray generators. This type of embodiment however has a drawback. Specifically, the further the shielding screens are placed from the x-ray source, i.e. the further they are from the target, the larger the area of the screens must be because of their distance. This aspect of the invention proposes to place such screens as close as possible to the parasitic source, thereby allowing them to be miniaturized.
The anode 76 and in particular the walls of the cavity 80 are advantageously made from a material of high atomic number such as, for example, from an alloy based on tungsten or molybdenum, in order to stop the parasitic emission 82. Tungsten or molybdenum have almost no effect with respect to the trapping of parasitic ions. Producing the getter 92 separately from the walls of the cavity 80 allows the materials thereof to be freely chosen with a view to ensuring that the function of trapping parasitic ions performed by the getter 92 and the function of screening the parasitic emission 92 performed by the walls of the cavity 80 are both performed as well as possible without compromise therebetween. For this reason, the getter 92 and the walls of the cavity 80 are made from different materials each of which is suitable for the function that is assigned thereto. The same goes for the getter 35 with respect to the walls of the cavity 34.
The walls of the cavity 80 encircle the electron beam 18 in the vicinity of the target 20.
Advantageously, the walls of the cavity 80 form a portion of the vacuum chamber 12.
Advantageously, the walls of the cavity 80 are arranged coaxially to the axis 19 so as to be located about the axis 19 radially at a constant distance and therefore as close as possible to the parasitic radiation. At the end 88a, the cylindrical portion 88 may partially or completely encircle the target 20, thus preventing any parasitic x-rays from escaping from the target 20 radially with respect to the axis 19.
Thus, the anode 76 performs several functions: its electrical function, a faraday-cage function blocking parasitic ions that may be emitted by the target 20 into the interior of the vacuum chamber 12, a function of shielding against parasitic x-rays and, also, the function of a wall of the vacuum chamber 12. By performing several functions by means of a single mechanical part, in this case the anode 76, the compactness of the source 75 is increased and its weight decreased.
Moreover, it is possible to place, around the cavity 80, at least one magnet or electromagnet 94 allowing the electron beam 18 to be focused on the target 20. Advantageously, the magnet or electromagnet 94 may also be arranged so as to deviate parasitic ions 91 toward the one or more getters 92 in order to prevent these parasitic ions from exiting from the cavity via the hole 89 in the portion 90 or, at the very least, to deviate them with respect to the axis 19 passing through the cathode 14. To this end, the magnet or electromagnet 94 generates a magnetic field B that is oriented along the axis 19. In
The means for trapping the parasitic ions 91 that may be emitted by the target 20 are multiple: faraday cage formed by the walls of the cavity 80, presence of getters 92 in the cavity 80 and presence of a magnet or electromagnet 94 for deviating the parasitic ions. These means may be implemented independently or in addition to the function of shielding against parasitic x-rays and the function of a wall of the vacuum chamber 12.
The anode 76 advantageously takes the form of a one-piece mechanical part that is axisymmetric about the axis 19. The cavity 80 forms a central tubular portion of the anode 76. The magnet or electromagnet 94 is placed around the cavity 80 in an annular space 95 that is advantageously located outside of the vacuum chamber 12. In order to ensure the magnetic flux of the magnet or electromagnet 94 affects the electron beam 18 and the ions degassed by the target 20 into the interior of the chamber 12, the walls of the cavity 80 are made of an amagnetic material. More generally, the entire anode 76 is made, and for example machined, from the same material.
The getter 92 is located in the cavity 80 and the magnet or electromagnet 94 is located on the exterior of the cavity. Advantageously a mechanical holder 97 of the getter 92 holds the getter 92 and is made from a magnetic material. The holder 97 is placed in the cavity so as to guide the magnetic flux generated by the magnet or electromagnet 94. In the case of an electromagnet 94, it may be formed about a magnetic circuit 99. The holder 97 is advantageously placed in the extension of the magnetic circuit 99. The fact of using the mechanical holder 97 to perform two functions: holding the getter 92 and guiding a magnetic flux, allows the dimensions of the anode 76 and therefore of the source 75 to be further decreased.
On the periphery of the annular space 95, the anode comprises a zone 96 that bears against the mechanical part 28. This bearing zone 96 for example takes the form of a flat ring that extends perpendicularly to the axis 19.
In
It is possible to implement each and every variant of the anode 16 and 76 irrespectively of whether the electrode 24 takes the form of a conductive area placed on the concave face 26 of the dielectric and irrespectively of whether the stopper 32 is employed.
In the variants illustrated in
More precisely, the mechanical part 28 made of dielectric and on which various metallizations have been produced, in particular the metallization forming the electrode 24, forms a monolithic holder. It is possible to assemble the cathode 14 and the stopper 32 on one side of this holder. On the other side of this holder, it is possible to assemble the anode 16 or 76. The anode 16 or 17 and the stopper 32 may be fastened to the mechanical part by ultra-high vacuum brazing. The target 20 or 21 may also be assembled with the anode 76 by translation along the axis 19.
In order to prevent any air-filled cavity forming at the high-voltage interface between the holder 100 and the mechanical part 28, a supple seal 114 that is for example based on silicone is placed between the holder 100 and the mechanical part 28, and more precisely between the complementary conic frustums and crowns. Advantageously, the conic frustum 108 of the holder 100 has an angle at the apex that is more open than that of the conic frustum 102 of the mechanical part 28. Likewise, the conic frustum 110 of the holder 100 has an angle at the apex that is more open than that of the conic frustum 104 of the mechanical part 28. The difference in angular value at the apex between the conic frustums may be smaller than 1 degree and for example about 0.5 degrees. Thus, when the source 75 is mounted in its holder 100, and more precisely when the seal 114 is crushed between the holder 100 and the mechanical part 28, air may escape from the interface between the crowns 106 and 112 on the one hand toward the more flared portion of the two conic frustums 102 and 108 in the direction of the anode 16 and on the other hand toward the narrower portion of the two conic frustums 104 and 110 in the direction of the cathode 14 and more precisely in the direction of the stopper 32. The air located between the two conic frustums 102 and 108 escapes to the ambient environment and the air located between the two conic frustums 104 and 110 escapes to the stopper 32. In order to prevent the trapped air from being subjected to a high electric field, the source 75 and its holder 100 are configured so that the air located between the two conic frustums 104 and 110 escapes into the interior of the coaxial link formed by the two contacts 70 and 71 and supplying the cathode 14. To achieve this, the exterior contact 71 ensuring the supply of the electrode 24 makes contact with the metallized zone 43b by means of a spring 116 allowing a functional play between the contact 71 and the stopper 32. In addition, the stopper 32 may comprise an annular groove 118 separating the two metallized zones 43a and 43b. Thus, air escaping from between the conic frustums 104 and 110 passes through the functional play between the contact 71 and the stopper 32 to reach a cavity 120 located between the contacts 70 and 71. This cavity 120 is protected from the high electric field because it is located in the interior of the coaxial contact 71. In other words, the cavity 120 is shielded from the main electric field of the source 10, i.e. the electric field due to the potential difference between the anode 16 and the cathodic electrode 24.
After the mechanical part 28 equipped with its cathode 14 and its anode 76 has been mounted, a closing plate 130 may hold the mechanical part 28, equipped with its cathode 14 and its anode 76, in the holder 100. The plate 130 may be made of a conductive material or comprise a metallized face in order to ensure the electrical connection of the anode 76. The plate 130 may allow the anode 76 to be cooled. This cooling may be achieved by conduction by means of a contact between the anode 76 and for example the cylindrical portion 88 of the cavity 80 of the anode 76. To reinforce this cooling, it is possible to make provision for a channel 132 in the plate 130 and encircling the cylindrical portion 88. A heat-transfer fluid flows through the channel 132 in order to cool the anode 76.
In
In the variant of
In the example shown, the mechanical part 162 is made of dielectric and comprises a concave face 168 that is placed in the vicinity of the various cathodes 14. The electrode 166 is formed from a conductive area placed on the concave face 168. The electrode 166 performs all the functions of the electrode 24 described above.
Alternatively, it is possible for the electrode that is common to a plurality of sources to take the form of a metal electrode that is not associated with a dielectric, i.e. that possesses a metal/vacuum interface. Likewise, the cathodes may be thermionic. In this embodiment, the common metal electrode forms the holder of the various cathodes of the various sources. Since this electrode is large in size, it is advantageous to connect it to the ground of the generator of the multi-source assembly. The one or more anodes are then connected to one or more positive potentials of the generator.
The multi-source assembly 160 may comprise a stopper 170 that is common to all the sources. The stopper 170 may perform all the functions of the stopper 32 described above. The stopper 170 may in particular be fastened to the mechanical part 162 by means of a conductive brazing film 172 used to electrically connect the electrode 166.
As in the variant of
In
The arrangement of the cathodes 14 on an axis allows sources distributed in one direction to be obtained. It is also possible to produce a multi-source assembly in which the cathodes are distributed along a plurality of concurrent axes. It is for example possible to place the sources along a plurality of curved axes, each located in one plane, the planes being secant. By way of example, as shown in
In these two embodiments, the anodes 114 are common to all the sources 75 of the assembly 150 and their potential is the same, for example that of the ground 52. In both embodiments, each of the sources 10 may be driven separately. In
In the embodiment of
In
| Number | Date | Country | Kind |
|---|---|---|---|
| 1700743 | Jul 2017 | FR | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2018/068815 | 7/11/2018 | WO | 00 |
