EP0473233A1 - High-flux neutron tube - Google Patents

Abstract

The invention relates to a neutron tube containing an ion source possessing at least one anode (6), at least one cathode (7), possessing at least one extraction orifice (12), and also containing an acceleration device (2) arranged such as to project at least one ion beam from the ion source onto a target in order to produce there a reaction bringing about an emission of neutrons. The ion source is arranged according to at least one portion of a first surface of revolution (8', 41, 51) and configured to produce a radial emission of ions directed outwards from the said first surface (8', 41, 51), in that the acceleration device (2) is arranged according to at least one portion of a second surface of revolution surrounding the said first surface (8', 41, 51), and in that the target (4) is arranged according to at least one portion of a third surface of revolution surrounding the said second surface. <IMAGE>

Description

The present invention relates to a neutron tube comprising an ion source having at least one anode and at least one cathode having at least one extraction orifice and also comprising an acceleration device arranged so as to project at least one beam ion source ion on a target to produce a reaction leading to emission of neutrons.

Neutron tubes are most often in the form of sealed tubes containing a gaseous mixture of deuterium and tritium under low pressure from which the ion source forms a confined ionized gas. The emission (or extraction) orifice is made in the cathode, the acceleration (and extraction) electrode making it possible to project the ion beam axially on a target electrode.

Plasma confinement can be obtained using magnetic and / or electric fields. Neutron tubes are used in techniques for examining matter using fast, thermal, epithermal or cold neutrons: neutrography, activation analysis, spectrometry analysis of inelastic scattering or radiative captures, neutron scattering, etc.

The type of ion source that is most used is the Penning type source which has the advantage of being robust, of being cold cathode (hence a long service life), of giving currents significant discharge for low pressures (of the order of 10 A / torr), to have a high extraction efficiency (from 20 to 40%) and to be of small dimensions. This type of source requires a magnetic field of the order of a thousand gauss, parallel to the axis of the ionization chamber, introducing a significant transverse inhomogeneity of current density of the ions inside the discharge and at the level of the extraction which takes place along the common axis of the field and the source.

The fusion reaction d (3 _H ), 4 _He ) n delivering 14 MeV neutrons is usually the most used because of its large cross section for relatively low ion energies. However, whatever the reaction used, the number of neutrons obtained per unit of charge passing through the beam is always increasing as the energy of the ions directed towards a thick target is itself increasing and this largely at the beyond the energies of the ions obtained in the sealed tubes currently available and supplied by a THT rarely exceeding 250 kV.

Among the main factors limiting the life of a neutron tube, the erosion of the target by ion bombardment is one of the most determining.

Erosion is a function of the chemical nature and structure of the target on the one hand, the energy of the incident ions and their density distribution profile on the impact surface on the other.

In most cases, the target is made of a hydrurable material (Titanium, Scandium, Zirconium, Erbium etc ...) capable of fixing and releasing large quantities of hydrogen without unacceptable disruption of its mechanical strength; the total quantity set is a function of the target temperature and the hydrogen pressure in the tube. The target materials used are deposited in the form of thin layers, the thickness of which is limited by problems of adhesion of the layer to its support. One way to delay erosion of the target is, for example, to form the absorbent active layer from a stack of identical layers isolated from each other by a diffusion barrier. The thickness of each of the active layers is of the order of the depth of penetration of the deuterium ions coming to strike the target.

Another way of protecting the target and therefore of increasing the lifetime of the tube consists in acting on the ion beam so as to improve its density distribution profile on the impact surface. At a constant total ion current on the target, which results in a constant neutron emission, this improvement results from a distribution as uniform as possible of the current density over the entire surface offered by the target to the ion bombardment.

A drawback results from the fact that the ions extracted and accelerated towards the target react with the molecules of the gas contained in the tube at a pressure, at first contrant order, to produce effects of ionization, dissociation and exchange of charges resulting on the one hand a decrease in the average energy on the target, that is to say a reduction in the production of neutrons and on the other hand the formation of ions and electrons which are then accelerated and bombard the ion source or the tube electrodes.

This results in deposits of energy which increase the temperature of the materials of the electrodes such as molybdenum or stainless steel. The heating of these materials causes the desorption of impurities such as the carbon monoxide which they contain and thus disturbs the quality of the atmosphere of the tube. The impurity ions formed in the tube, CO` for example, bombard the target with a spray coefficient greater by a factor of 10 ² to 10 ³ than that of the deuterium-tritium ions, resulting in a significant accentuation of the erosion. These effects increase with the operating pressure in the neutron tube.

• - des cibles de grandes surfaces,
• - des densités de bombardement ionique des cibles compatible avec un refroidissement efficace et une faible pulvérisation,
• - des pressions de fonctionnement réduites nécessitant, par conséquent, des sources d'ions efficaces en production d'ions.

These general considerations valid whatever the nature of the ion source show that obtaining a high neutron flux with long periods of use (for example several thousand hours) requires using:

• - targets of large areas,
• - ion bombardment densities of the targets compatible with efficient cooling and low sputtering,
• - reduced operating pressures therefore necessitating efficient ion sources in ion production.

By way of illustration, an average bombardment density of 0.5 mA, with a maximum of the order of 1 mA should make it possible to exceed one thousand hours of operation: as for the neutron level, for an acceleration voltage 250 kV, it would be around 3.10 ¹⁰ n / cm. ² s of 14 MeV neutrons. Obtaining a level of 10 ¹³ n / s would require a target area of 300 cm ² and 3000 cm ² for 10 ¹⁴ n / s.

Other known types of ion sources, with electrostatic confinement of ions, such as that described in French patent application No. 88 13188 filed by the Applicant on October 7, 1988 and published under No. FR 26 37 727 present similar characteristics with regard to the wear of the target.

It has also been proposed by the Applicant in French patent application No. 88 13187 filed on October 7, 1988 and published under No. FR 26 37 726 an ion source of multicellular type having a Penning cell comprising a multi-hole anode arranged inside the cathode cavity to increase the ion current. It is thus possible to obtain a higher current homogeneity on a larger target, but emission levels as mentioned above would however require prohibitive dimensions.

The basic idea of the invention consists in carrying out an extraction of ions no longer axial, but radial on the one hand, starting from the recognition of the fact that it allows a reduction of the electric fields producing the cold emission of electrodes, and the resulting number of breakdowns, thanks to an asymmetry in the distribution of the electric field and other by the fact that it makes it possible to arrange the target cylindrically around the source of ions, from where an extremely gain important with regard to the size of a source with a high neutron flux.

A neutron tube according to the invention is thus characterized in that the ion source is arranged along at least a portion of a first surface of revolution and arranged to produce an emission of radial ions and directed towards the outside of said first surface, in that the acceleration device is arranged along at least a portion of a second surface of revolution surrounding said first surface, and in that the target is arranged along at least part of a third surface of revolution surrounding said second surface.

It will also be noted that with regard to the ion source, the radial extraction mode towards the outside partially eliminates the sheath effect due to the perimeter of the extraction electrode and results, all things being equal. moreover, an increase in the extraction efficiency of the source.

The tube according to the invention may comprise a device for suppressing secondary electrons known per se and arranged along at least a portion of a fourth surface of revolution comprised between the second and the third surface.

The acceleration device can advantageously be a cylindrical electrode.

According to a first embodiment, with magnetic confinement, the ion source consists of at least one elementary source with a Penning structure, which can in particular comprise a plurality of elementary sources arranged in at least portions of superimposed rings. According to an advantageous embodiment, the first surface of revolution is a first cylinder and it comprises a first cylindrical magnet disposed on the smallest radius of the first cylinder, and at least a second cylindrical magnet contained in said cathode along the largest radius of the first cylinder, so as to produce a radial magnetic field.

An anode can be cylindrical or frustoconical of revolution. It can preferably be made up of two parallel discs or with a frustoconical section, which makes it possible to produce a single anode per ring, hence simplification of the production. The extraction orifice can be an annular slot, which is favorable to the extraction efficiency.

According to a second embodiment, with magnetic confinement, the ion source consists of a structure of the inverted magnetron type. Such a structure is usually used only as a measuring instrument (ionization gauge). On this point one will refer to the work The Physical Basis of Ultrahigh Vacuum (Redhead et al National Research Council Ottawa, CDN edited by Chapman and Hall Ltd LONDON (GB), in its pages 333 and 334. Such a device is used here as an ion source by providing at least one extraction opening in the cathode. At least one anode can be annular. A third annular magnet can be arranged so as to produce a longitudinal magnetic field. The magnetic field can be obtained by a solenoid surrounding the third (or if necessary the fourth) cylindrical surface and arranged to produce a longitudinal magnetic field. In this case and according to a preferred variant of the invention, a cylindrical anode can be arranged along the smallest radius of the first cylinder and extend substantially over the height of the first cylinder. It is thus possible to obtain with a single anode and a single cathode an emission on a surface of revolution, in particular cylindrical, elongated.

According to a third embodiment, with electrostatic confinement, the ion source is of the orbitron type comprising a second cylindrical anode disposed along the smallest radius of the first cylinder and extending substantially over the height of said first cylinder. The ion source may also include a hot cathode.

According to a fourth embodiment, with electrostatic confinement, the ion source is of the electrostatic reflex type (SIRE) and has at least one annular anode, or advantageously a multiannular electrode.

• - la figure 1, un tube neutronique à extraction axiale du type Penning, selon l'art antérieur (demande FR 2637725).
• - les fig. 2a et 2b, dans une même structure cylindrique, deux variantes d'un tube à source d'ions de type Penning, à extraction radiale selon l'invention, les fig. 2c et 2d étant des détails des fig. 2a et 2b.
• - les fig. 3a et 3b, dans une même structure cylindrique, deux variantes préférées d'un tube à source Penning, à extraction radiale selon l'invention, les fig. 3c et 3d étant des détails des fig 3a et 3b, et la fig. 3e une variante de la fig.3a correspondant à une émission tronconique.
• - la fig. 4 une première variante d'un tube à source d'ions de type magnétron inversé, à extraction radiale selon l'invention, la fig. 4b représentant le cheminement des électrons ionisants dans une telle source d'ions.
• - les fig. 5 et 6 deux variantes d'un tube à source magnétron inversé, à extraction radiale selon l'invention, avec aimant ou solénoïde.
• - les fig. 7 et 8 deux variantes d'un tube à source d'ions du type orbitron, à extraction radiale selon l'invention.
• - et les fig. 9 et 10 deux variantes d'un tube à source d'ions de type Reflex électrostatique, à extraction radiale selon l'invention.

The invention will be better understood on reading the description which follows, given by way of nonlimiting example, in conjunction with the drawings which represent:

• - Figure 1, a neutron tube with axial extraction of the Penning type, according to the prior art (application FR 2637725).
• - figs. 2a and 2b, in the same cylindrical structure, two variants of an ion source tube of the Penning type, with radial extraction according to the invention, FIGS. 2c and 2d being details of FIGS. 2a and 2b.
• - figs. 3a and 3b, in the same cylindrical structure, two preferred variants of a Penning source tube, with radial extraction according to the invention, FIGS. 3c and 3d being details of figs 3a and 3b, and fig. 3rd a variant of fig.3a corresponding to a frustoconical emission.
• - fig. 4 a first variant of an inverted magnetron type ion source tube, with radial extraction according to the invention, FIG. 4b representing the path of ionizing electrons in such an ion source.
• - figs. 5 and 6 two variants of an inverted magnetron source tube, with radial extraction according to the invention, with magnet or solenoid.
• - figs. 7 and 8 two variants of an ion source tube of the orbitron type, with radial extraction according to the invention.
• - and fig. 9 and 10 two variants of an ion source tube of the electrostatic Reflex type, with radial extraction according to the invention.

The drawing of Figure 1 shows the main basic elements of a sealed neutron tube 11 containing a gaseous mixture under low pressure to be ionized such as deuterium-tritium and which comprises an ion source 1 and an acceleration electrode 2 between which exists a very high potential difference allowing the extraction and the focusing of the ion beam 3 and its projection on the target 4 where the fusion reaction takes place resulting in an emission of neutrons at 14 MeV for example.

The ion source 1 secured to an insulator 5 which can allow the THT supply connector, for example 250 kV (not shown) to pass, is a Penning type source for example, consisting of a cylindrical anode 6, of a cathode structure 7 in which is incorporated a magnet 8 with an axial magnetic field which confines the ionized gas 9 around the axis of the anode cylinder and whose lines of force 10 show a certain divergence. An ion emission channel 12 is formed in said cathode structure opposite the anode.

The anode is brought to a higher potential of the order of a few (1 to 6 for example) kV than that of the cathode, itself brought to the very high voltage THT. The acceleration electrode 2 and the target 4 are generally at ground potential.

According to Figures 2a and 2c, a neutron tube is, according to the invention with radial emission and extraction. The ion source consists of a plurality of sources of the Penning type, arranged according to a cylindrical symmetry (as shown) or else conical. To do this, it has an annular structure, or else a plurality of superimposed annular structures 20 (and of the same section in the case of a cylindrical symmetry). Each annular structure 20 mechanically fixed on a central axis 18 brought to a high potential (200 to 250 kV) comprises a cylindrical magnet 8 on the smallest radius of the annular structure 20, and a flat ring 14, as well as a cylindrical part 8 'disposed on the largest radius of the annular structure 20. The flat ring 14 forms a part of metallic structure holding the cylindrical magnet 8 and the cylindrical part 8' together, which can itself be constituted by a cylindrical magnet contained in the cathode structure 7. The cathode 7 is then formed by the internal cylindrical surfaces corresponding on the one hand to the inner radius of lower value and on the other hand to the outer radius of higher value. The cylindrical magnet 8 has a height at least equal to that of the cathode 7. The flat ring, due to the fact that it serves as a magnetic circuit, itself consists of magnetic material (soft iron or magnetic alloy for example).

A plurality of cylindrical anodes 6 are distributed radially around the periphery of the annular structure 20, and have substantially the same axis as the extraction openings 12 formed in the cylindrical part 8 'of the cathode structure 7. An acceleration electrode 2 is in the form of a cylinder (or a cone) having acceleration openings 21 located opposite the openings 12. The target has a cylindrical (or conical) support 4 on which the acceleration electrode 2 can be connected mechanically and electrically. A tapered high-voltage insulator 5 mechanically holds the assembly. The ion source can be arranged in such a way that the emission takes place over the entire periphery or only over a part or sector thereof. To do this, the ring can extend over 360 or only over a more limited angle, and has openings 12 only at the useful places. The openings 12 of two superimposed rings can be angularly offset for example for better homogeneity of the beam on the target. A deuterium-tritium reservoir is shown at 23 as well as a pressure measurement gauge 22. Electrodes 24 suppressing secondary electrons are arranged in intermediate planes between the rings, apart from the ion beams 3. Insulating bushings 25 distributed around the perimeter allow their mechanical fixing and / or their electrical supply. The electrodes 24 are brought to a negative potential (-5 kV for example) relative to those of the acceleration electrode 2 and the target 4 grounded, and is advantageously made of a refractory material. For more information, reference is made to French patent application No. 88 13186 filed on October 7, 1988 by the Applicant and published under No. FR 2 637 725. The electrodes 24 are preferably toric in V section to match the better the profile of the ion beams 3.

In Figures 2b and 2d, the anodes 6 'are conical and not cylindrical. These two variants have been shown for convenience on the same cylindrical structure. More information on this form of anode can be found in French patent application No. 88 13185 filed on October 7, 1988 by the Applicant and published under No. FR 2 637 724.

A second model of ion source structure, still of the Penning type, consists in integrating the n cylindrical (or conical) ion source modules in an annular structure having close electrical mapping, the distribution of the magnetic field being similar to the former. To do this, the anode of the structure consists of two parallel discs 16 or inclined 16 'relative to each other to better match the lines of force of the magnetic field. These structures are shown in Figures 3a to 3d. The cathode 7 of the structure is formed by the internal cylindrical surfaces corresponding on the one hand to the inner radius of lower value and on the other hand to the outer radius of higher value, this latter surface is pierced over its entire length with a extraction slot 32 of height and depth coupled so as to avoid excessive penetration of the electric field applied by the acceleration electrode. As in a classic Penning structure, the magnetic field inside the structure must be greater than the cut-off field (value linked on the one hand to the geometric structure: distance between the two anode rings and to a lesser degree at the intercathode distance and on the other hand to the voltage applied between anode and cathode) that is to say to the magnetic field preventing electrons from reaching the anode from oscillations without ionizing shock.

The magnets used to produce this magnetic field consist as above of rings distributed in two assemblies held mechanically by metal carcasses 14 serving as a magnetic circuit (magnetic material). The first set consists of two rings 8 'arranged on either side of the extraction slot. The second magnet consists of a cylinder 8 whose thickness depends on the magnetic field necessary for the proper functioning of the source and the nature of the material used. Its height is at least equal to the height of the cathode 7.

According to Figure 3e, annular structures corresponding to fig.3a, but of different radii, are stacked to form a frustoconical structure. The acceleration electrode 2 and the target 4 can also be frustoconical.

For Figure 3a, we can have the following values: ri = 4 cm, r ₂ = 7 cm, r ₃ = 10.5 cm, r ₄ = 15 cm; thickness of magnet 8: 1 cm; thickness of the magnet 8 ': 1.5 cm; height of a ring h = 6 cm.

According to FIGS. 4a and 4b, the ion source is produced from a so-called "inverted magnetron" structure, known for producing an ionization gauge (book by Redhead et al supra). The dimensions are practically identical to those of the Penning structure as well as the operating pressure and tensions.

In this structure (Figure 4a), the anode is constituted by a ring 40 (for example 3 cm high, on a radius of 5 cm) located inside the cathode cavity 42, the main element of which is constituted by the cylindrical cathode wall 41 separated into two parts by the extraction slot 32. The height of an elementary cell can for example be 6 to 8 cm. The electric field is, in this zone, radial and the confining magnetic field is generally perpendicular and therefore parallel to the axis of symmetry of the structure. The electrons accelerated towards the anode are deflected towards the cathode by the magnetic field and describe cycloOlaes (figure 4b) on the basis of the cylindrical surface (or the equipotential surface) on which they were created.

The confining magnetic field can be created by magnets 48 in the form of discs arranged symmetrically with respect to the plane of symmetry of the structure; these magnets 48 can be held mechanically on a metal support 43 acting as a magnetic circuit and whose diameter is less than the anode diameter. It can also be created by a coil 50 arranged outside the tube structure (Figures 5 and 6) and leading to the production of a magnetic field greater than the cut-off field. The coil 50 has a height which can advantageously be 1.5 to 2 times the total height of the cathode structures. This configuration can be advantageous in certain uses requiring braking of the neutrons, use of a heavy coiling material, cooled by circulation of water which can also be used for cooling the target. In this configuration, an important advantage is that the secondary electrons of the target are trapped (returned to the target 4) by the magnetic field and the suppressor electrode 24 is no longer strictly necessary in operation at low pressure (some 10- ⁴ to 10- ² Torr). In the case of FIGS. 5 and 6, the anode can be constituted (FIG. 5) by a ring 40 disposed in each cathode cavity 42 delimited by flat rings 52 of conductive material, the cathode being constituted by conductive rings 51 ( for example 3 to 4 cm in height) integral with the flat rings 52 (for example 2 mm in height) between which there are extraction slots 32. The anode is preferably made up (FIG. 6) by a single cylinder ( or truncated cone) 55 fixed by spacers 56, the flat rings 52 being removed.

The structures presented now include an ion source, radially extracted according to the invention, with an electric confining field.

Figures 7 and 8 show an orbitron structure having an anode 70 of small dimension (diameter for example between 0.05 and 0.1 cm), located on the axis of the cathode 51 (diameter for example between 10 and 15 cm). This structure may be with a cold cathode (FIG. 7) and therefore requiring a high anode voltage and an operating pressure as well as possible within the range of 10- ^4- 10- ³ torr or also having a hot cathode 71 (FIG. 8) , resulting in a larger extension of the operating range to low pressures. The operating principle is as follows: the electrons emitted by the filaments or the cathodes are attracted to the anode; according to their angle of emission and their initial energy, they can "miss" the anode and thus oscillate for a long time inside the structure, the probability of ionization is thus strongly increased and a discharge, with formation of a plasma is created. The ions are attracted to the cathode and their extraction is done through one where several cylindrical slots 32. The extraction and the position of the slots 32 can be carried out in a similar manner to the inverted magnetron structure with solenoid. The acceleration 2 and suppression 24 structure of the target's secondary electrons are similar to that of ion source systems with a confining magnetic field. The shape and position of the suppressor electrode 24 must take account of the higher operating pressures, in accordance with the provisions taken in the aforementioned French patent n ° 88 13186.

FIGS. 9 and 10 show electrostatic reflex structures (SIRE) with cold cathode. The anode 90 is close to the cylindrical cathode 51 (diameter of the cathode for example between 2 and 3 cm) and the electrons oscillate between the two plane sections of the cathode; the ion current density is much higher on the two flat sections of the cathode, in particular at low pressure (p Ê 10- ³ torr). The radial extraction takes place via cylindrical slots 32 formed in the cylindrical wall of the cathode 51, under conditions similar to those of the inverted magnetron structure. Their relative surface (compared to the total surface of the cylindrical part of the cathode) can be important because most of the discharge is due to the plane sections. The number of slots depends on the height of the structure of the ion source and its dimensions. The number of annular anodes (circular or cylindrical section) cooled or not and arranged in the middle part between the extraction surfaces is a function of the height of the structure. Figure 9 shows a structure with four extraction "rings", while Figure 10 shows a much higher neutron tube with N extraction structures (N> 4). In this case, an anode having several rings 91 is used. The parts "acceleration" 2 and "suppression of secondary electrons" 24 are similar to those of structures with magnetic fields. The diameter of the SIRE structure can be of the order of 10 to 15 cm. Their operating pressures are generally between 10 ^-3 torr and some 10- ² torr, and tensions between some kV and 12 kV.

• - structure de source d'ions de type Penning : les circuits magnétiques en forme d'anneaux sont communs à deux structures consécutives et chaque structure a ses aimants propres (figures 2a à 2d, 3a à 3d.
• - structure de source d'ions de type magnétron inversé avec aimants : deux structures consécutives ont les mêmes aimants 48 et les circuits magnétiques 41 sont empilés les uns et les autres et par conséquent propres à chaque structure (figure 4a),
• - structure de sources d'ions de type magnétron inversé avec bobine extérieure ; la bobine extérieure 50 est plus longue que les structures de source d'ions empilées les unes sur les autres. La densité d'enroulement par unité de longueur est approximativement constante (figure 5).

A significant increase in the neutron emission, with a much smaller increase - in relative value - in the volume can be obtained by placing several similar structures along the same axis, as shown in FIGS. 2a to 2d, 3a to 3d, 4a , 5, 9 and 10. Indeed, the isolation parts electric and possibly the magnetic supports, remain the same, only the active parts composed of the electrodes and (possibly) of the magnets are multiplied. The stacking can be carried out so as to form cylinders or truncated cones. The following solutions can be given by way of example.

• - Penning-type ion source structure: the magnetic circuits in the form of rings are common to two consecutive structures and each structure has its own magnets (Figures 2a to 2d, 3a to 3d.
• - inverted magnetron type ion source structure with magnets: two consecutive structures have the same magnets 48 and the magnetic circuits 41 are stacked one and the other and consequently specific to each structure (FIG. 4a),
• - structure of ion sources of inverted magnetron type with external coil; the outer coil 50 is longer than the ion source structures stacked on top of each other. The winding density per unit length is approximately constant (Figure 5).

As for electrostatic structures, their larger volume and their own configuration only allows a reduced number of complementary cells, knowing that the dimensions of the tube are close to those of magnetic field structures and that electrostatic structures are equipped with several extraction slots. It is also advantageous to modify the structures themselves (position and number of anodes in the SIRE structure, height of the cylindrical cathodes in the SIRE and orbitron structures).

All of the structures described and shown above have the advantages of radial extraction. As the extraction takes place along a cylindrical (or tapered) surface, the structures benefit, independently of the divergence effect of the ion beam, from an increase in the bombarded surface (target 4) corresponding to the ratio of the rays of the target 4 and the extraction electrode (8 ', 41). As far as the ion source itself is concerned, the radial extraction, in particular through a cylindrical slot 32, partially eliminates the sheath effect due to the perimeter of the extraction electrode (i.e. say the part of the cathode where the extraction takes place) and leads to an increase in the extraction efficiency of the source, all other things being equal.

E_ex = champ d'extraction ; E_acc = champ d'accélération r_ex = rayon d'extraction, d = distance d'accélération, k et k' sont des constantes.A second advantage of structures with radial extraction is to lead to a reduction in the electric fields producing the cold emission of the electrodes and in the number of breakdowns resulting therefrom thanks to an asymmetry in the distribution of the electric field: for a distance between two electrodes , the average applied electric field varies in 1 / r:

E _ex = extraction field; E _acc = acceleration field r _ex = extraction radius, d = acceleration distance, k and k 'are constants.

Thus for acceleration distances d of the order of 20 mm and extraction electrodes of radius r ex 150 mm, the variation of the global electric field compared to a conventional structure (flat and parallel electrodes) would be around 5 to 10%. This small difference corresponds to a decrease in the cold emission current of the order of 5 to 10 compared to an axial emission.

The invention is not limited to the embodiments described and shown. It also applies for example to neutron tubes in a Deuterium atmosphere only (production of 2.6 MeV neutrons). In addition, pulsed operation is possible after placement in the ion source, in a manner known per se for axial emission sources, of an electron source or an emitter a and / or and / or producing the first electric particles therein, causing the ignition and the discharge in the ion source.

Claims (20)

1. Neutron tube comprising an ion source having at least one anode, at least one cathode, having at least one extraction orifice, and also comprising an acceleration device arranged so as to project at least one ion beam from the ion source onto a target to produce there a reaction leading to an emission of neutrons characterized in that the ion source is arranged along at least a portion of a first surface of revolution (8 ', 41, 51) and arranged to produce an emission of radial ions and directed towards the outside of said first surface ( 8 ', 41, 51), in that the acceleration device (2) is arranged in at least a portion of a second resolution surface surrounding said first surface (8', 41, 51), and in that the target (4) is arranged along at least a portion of a third surface of revolution surrounding said second surface. 2. Neutron tube according to claim 1 characterized in that it comprises a device for suppressing secondary electrons (24) arranged along at least a portion of a fourth surface of revolution comprised between the second and the third surface. 3. Neutron tube according to one of claims 1 or 2, characterized in that at least one said surface of revolution is a cylinder. 4. Electronic tube according to claim 3 characterized in that the acceleration device is a cylindrical electrode. 5. Neutron tube according to one of claims 1 to 4 characterized in that the ion source consists of at least one elementary source with Penning structure (6, 8, 8 ', 14). 6. neutron tube according to claim 5 characterized in that it comprises a plurality of elementary sources arranged in at least portions of superimposed rings (20). 7. Neutron tube according to one of claims 5 or 6 characterized in that it comprises a first cylindrical magnet (8) disposed along the smallest radius of the first surface of revolution and at least a second cylindrical magnet (8 ') contained in said cathode along the largest radius of the first surface of revolution, so as to produce a radial magnetic field. 8. Neutron tube according to one of claims 5 to 7 characterized in that at least one anode (6, 6 ') is cylindrical or frustoconical of revolution. 9. Neutron tube according to one of claims 5 to 7 characterized in that at least one anode consists of two parallel discs (16). 10. Neutron tube according to one of claims 6 or 7 characterized in that at least one anode consists of two discs with frustoconical section (16 '). 11. Neutron tube according to one of claims 9 or 10 characterized in that at least one extraction orifice is an annular slot (32). 12. Neutron tube according to one of claims 1 to 4 characterized in that the ion source consists at least of a structure of the inverted magnetron type (fig.4a, 5, 6). 13. Neutron tube according to claim 12 characterized in that it comprises at least a third annular magnet (48) arranged so as to produce a longitudinal magnetic field. 14. Neutron tube according to one of claims 12 or 13 characterized in that at least one anode (40) is annular. 15. Neutron tube according to claim 12 characterized in that it comprises a solenoid (50) of diameter greater than that of the third surface of revolution and arranged so as to produce a longitudinal magnetic field. 16. Neutron tube according to claim 15 characterized in that it comprises a first cylindrical anode (55) arranged along the smallest radius of the first surface of revolution and extending substantially on the height thereof. 17. Neutron tube according to one of claims 1 to 4 characterized in that the ion source is of the orbitron type (fig. 7, 8), comprising a second cylindrical anode (70) arranged along the smallest radius of the first surface of revolution and extending substantially over the height thereof. 18. Neutron tube according to claim 17 characterized in that it also comprises a hot cathode (71). 19. Neutron tube according to one of claims 1 to 4 characterized in that the ion source is of the electrostatic reflex type (SIRE) (Fif. 9 and 10) and has at least one annular anode (90), at least one orifice extraction being a slot (32). 20. Neutron tube according to claim 19, characterized in that it comprises a multiannular anode (91).

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