EP0362947B1 - Sealed neutron tube equipped with a multicellular ion source with magnetic confinement - Google Patents

Description

The invention relates to a sealed neutron tube containing a low-pressure deuterium-tritium gas mixture from which an ion source with two electrodes, an anode and a cathode, forms an ionized gas channeled by a confining magnetic field created by magnets or by any other means of creating said field, said source emitting from emission channels made in said cathode ion beams which pass through an extraction-acceleration electrode and are projected at high energy onto a target electrode for producing therein a fusion reaction resulting in the emission of neutrons, in which said ion source is of the multicellular type consisting of a structure of elementary cells of the Penning type comprising for all of said cells a cathode cavity inside which is disposed a multi-hole anode, the axes of said holes being aligned respectively on the corresponding axes desd its broadcast channels.

Such a tube is known from NL-A- 7707357.

Neutron tubes of the same kind are used in techniques for examining matter by fast, thermal, epithermal or cold neutrons: neutronography, analysis by activation, analysis by spectrometry of inelastic diffusions or radiative captures, neutron scattering, etc. .

Obtaining the full effectiveness of these nuclear techniques requires having, for the corresponding emission levels, longer tube lifetimes.

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 not 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 consists of a hydrurable material (Titanium, Scandium, Zirconium, Erbium etc ...) capable of fixing and releasing large quantities of hydrogen without significant disturbance 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 of 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 to protect the target and therefore increase the life of the tube is to act 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 will result from a distribution as uniform as possible of the current density over the entire surface offered by the target to the bombardment of the ions.

In a sealed neutron tube, the ions are generally supplied by a Penning-type ion source which has the advantage of being robust, of being cold cathode (hence a long service life), of give large discharge currents for low pressures (of the order of 10 A / torr; 1 torr = 133.3 Pa), to have a high extraction yield (from 20 to 40%) and to be of small dimensions.

However, this type of source has the disadvantage of requiring a magnetic field of the order of a thousand gauss (1Gs = 10⁻⁴T), parallel to the axis of the ionization chamber, introducing significant transverse inhomogeneity in density. current of the ions inside the discharge and at the level of the extraction which is carried out along the common axis of the field and the source.

Another disadvantage results from the fact that the ions extracted and accelerated towards the target will react with the molecules of the gas contained in the tube at a constant first order pressure to produce effects of ionization, dissociation and charge exchange resulting on the one hand a reduction in the 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 will bombard the ion source or the tube electrodes.

This will result in energy deposits which will increase the temperature of the electrode materials such as molybdenum or stainless steel. The heating of these materials will cause the desorption of impurities such as the carbon monoxide which they contain and thus disturb the quality of the atmosphere of the tube. The impurity ions formed in the tube, Co⁺ for example, will bombard the target with a spray coefficient higher by a factor of 10² to 10³ than that of the deuterium-tritium ions, resulting in a significant increase in erosion . These effects increase with the operating pressure in the neutron tube.

The object of the invention is to provide a source device enabling these disadvantages to be overcome.

To this end, the invention is remarkable in that, the number of said holes is optimized so as to increase the extracted ion beam for an equivalent bulk of said ion source, and the shape and / or dimensions and / or positioning of said holes are adapted to the topology of said magnetic field , so that the radius of said anode holes of said ion source is gradually increased from the center to the periphery of the structure.

Note that it is known, from patent application NL-A 77 07357, a multicellular type neutron tube comprising, for all of the cells a cathode cavity inside which is arranged a multi-hole anode, the axes of the holes respectively on the corresponding axes of the emission channels in the cathode.

It should also be noted that an additional gain in the discharge current may result from the increase in length of the structure of the multicellular ion source. This gain can go up to a factor of 2.

The increase in current resulting from the new source configuration can then be used to reduce the operating pressure of the neutron tube and thus limit the harmful impact of ion-gas reactions.

The feasibility of the multicell structure assumes that a magnetic field is suitable for the proper functioning of a Penning structure, in particular at the level of the relationship between the magnetic induction and the hole radius of the multi-hole anode.

The variation of the magnetic field in level and according to the shape of the lines of force can be corrected by an increase in said radius, which amounts to making structures with a variable anode radius. In addition, better adaptation of the shape of the anode to the main lines magnetic can be obtained by replacing the cylindrical structures with circular or square section by frustoconical structures so as to make the generators of truncated cones coincide with the lines of force which rest on the contours of the holes.

The ions of the various structures are emitted through channels made in the cathode acting as an emission electrode. These channels, the number of which is identical to that of the elementary cells, are arranged respectively along the same axes of symmetry. In the case of circular section structures, the diameter is a function of the applied electric field and the thickness of the electrode.

A variant of this system consists in introducing an expansion chamber below the cathodes in order to standardize the densities in the vicinity of the emission which then takes place through orifices the arrangement of which can be almost independent of that of the elementary cells.

In a neutron tube of the invention, the extraction-acceleration electrode can be constituted by an electrode provided with n orifices having axes corresponding respectively to those of the n elementary cells, or with j orifices smaller than the number of n elementary cells and therefore diameters greater than those of the emission channels and whose arrangement avoids any interception of the beams.

The thickness of this extraction-acceleration electrode can be increased in order to improve the mechanical strength and to allow cooling by forced circulation of fluid.

The following description with reference to the appended drawings, all given by way of example, will make it clear how the invention can be implemented.

Figure 1 shows the block diagram of a sealed neutron tube according to the state of the prior art.

Figure 2 shows the effects of target erosion at depth and the radial profile of ion bombardment density.

FIG. 3 represents the diagram of a neutron tube of a known type, compatible with the invention, provided with a multicellular ion source of the Penning type and with an extraction-acceleration electrode comprising as many orifices than cells.

FIG. 4 represents a neutron tube of a known type compatible with the invention, provided with a multicellular ion source and with an extraction-acceleration electrode comprising a number of orifices different from the number of cells.

FIG. 5 represents a first variant of the neutron tube of the invention, provided with an ion source whose anode holes are of variable radii.

FIG. 6 represents a second variant of the neutron tube of the invention provided with a source whose anode holes are of frustoconical shape.

FIG. 7 represents a third variant of the neutron tube of the invention according to the first variant or according to the first and second variants, provided with a source with expansion chamber.

Identical elements in said figures will be indicated by the same reference signs.

The diagram in FIG. 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 extraction-acceleration electrode 2 between which there is a very high potential difference allowing the extraction and acceleration 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 for the passage of the THT supply connector (not shown) is a Penning type source for example, consisting of a cylindrical anode 6, a cathode cavity 7 to 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 cavity opposite the anode

The diagrams in Figure 2 represent the effects of erosion on the target as the phenomenon increases.

FIG. 2a shows the profile of the density J of bombardment of the ions in any radial direction Or, from the point of impact O of the central axis of the beam on the target surface. The shape of this profile highlights the inhomogeneous nature of this beam whose very high density in the central part decreases rapidly when one moves away from it.

In FIG. 2b, erosion takes place as a function of the bombardment density and the entire layer of hydrurable material of thickness e deposited on a substrate S is saturated with a deuterium-tritium mixture. The depth of penetration of the deuterium-tritium energetic ions represented in dotted lines takes place over a depth which is a function of this energy.

In FIG. 2c, the erosion of the layer is such that the penetration depth l₂ is greater than the thickness e in the most bombarded part; a part of the incident ions is implanted in the substrate and very quickly the atoms of deuterium and tritium are in supersaturation.

In FIG. 2d, the atoms of deuterium and of tritium have gathered to give bubbles which, when they burst, formed craters and very quickly increased the erosion of the target at depth l₃.

This last process just precedes the end of the tube's life by causing either a drastic increase in breakdowns (presence of microparticles resulting from the bursting of bubbles), or pollution of the target surface by atomized atoms absorbing the energy of the ions. incidents.

FIG. 3 shows a neutron tube provided with a multicellular ion source of the Penning type consisting of a cathode cavity 7 and a multi-hole anode 6, brought to a potential 4 to 8 kV higher than that of the cathode cavity itself brought to a very high voltage of 250 kV for example.

The magnet 8 provides a magnetic field for confining the ionized gas of the order of a thousand gauss.

The invention consists in exploiting, in a first known step, the property of multicellular discharge structures with confinement of magnetic type, namely that for the same anode section, the discharge current as well as the current of the ion beam extracted from this discharge are respectively greater in the case of a multicellular source structure than the same currents obtained in the case of a single-cell structure. Similarly, it is more advantageous to use a multicell structure with n anode holes than a multicell structure with m holes if n> m. Each section of the structure with n holes is then smaller than each of the sections of the structure with m holes; but the aforementioned advantage is only ensured if the anode section remains equivalent for said structures, which makes it possible to reduce the pressure of the gaseous mixture and therefore the probability of ion-gas reactions.

A new structure with n cells has thus been constituted in known manner comprising the multi-hole anode 6 having n holes 6₁, 6₂, ... 6 _n and the cathode 7 in which the anode holes have been practiced. emission channels 7₁, 7₂, ... 7 _n from which n ion beams are extracted. These multi-beams 3 are projected onto the target 4 by means of the extraction-acceleration electrode 2 having the same number of orifices 2₁, 2₂, ... 2 _n as that of said beams and arranged along the same axes.

In another neutron tube device shown diagrammatically in FIG. 4, the number of orifices made in the extraction-acceleration electrode is less than that of the beams coming from the source: for example each orifice 13 of this electrode 2 delivers passage to two beams from the source as shown in the figure.

In a multicellular ion source structure, the divergence of the lines of force of the magnetic field shows that it is very high in the central area and gradually decreases to a very low value on the periphery. To compensate for this variation, the anode holes 6′₁, 6′₂, ..., 6 ′ _n are, according to the invention, constituted as shown in FIG. 5 with radii varying in opposite directions to the magnetic field so that the product of magnetic induction by the anode radius remains substantially constant. This arrangement tends to standardize the density of ion current.

The device shown in Figure 6 provides a significant improvement in that the anode holes 6˝₁, 6˝₂, ..., 6˝ _n have frustoconical shapes which approximate the lines of force of the magnetic field.

In FIG. 7, an expansion chamber 14 is arranged below the cathodes in order to standardize the ion densities. The emission is carried out through orifices 15 the number of which can be independent of that of the holes of the multi-hole anode.

• A parcours ionique identique, les créations de paires ions/électrons sur le trajet du faisceau d'ions sont moins nombreuses et l'énergie déposée dans la source d'ions par les électrons réaccélérés est moindre ; l'échauffement de la source d'ions est plus faible et par conséquent le dégazage des matériaux constitutifs est réduit. Les ions lourds résultant de ce dégazage sont moins nombreux et leur contribution à l'érosion de la cible plus faible. Par ailleurs l'énergie moyenne des ions deutérium-tritium est accrue, ce qui peut permettre de réduire le courant tube.
• A courant de faisceau identique, il est possible d'accroître les distances interélectrodes et de diminuer ainsi le champ électrique afin de réduire les phénomènes d'émission froide.
• A courant de faisceau intégré (sur l'unité de temps) identique, on peut accroître le courant maximal en mode pulsé dans le rapport des pressions Pmax/P, Pmax étant la pression maximale de fonctionnement n'entraînant pas un changement du régime de fonctionnement du tube (passage de la décharge en régime d'arc).
• En outre la répartition du courant sur la cible est beaucoup plus homogène en raison d'une part de l'homogénéité de la décharge au niveau des canaux d'émission et d'autre part de la multiplication du nombre de faisceaux élémentaires. Il en résulte une décroissance de la densité ionique maximale et à courant de faisceau identique un accroissement de durée de vie.

Thus, the increase in the ratio of the intensity of the beam to the pressure in the neutron tube, resulting from the multicellular source structure of the invention can be exploited in various ways:

• With an identical ion path, the creations of ion / electron pairs on the path of the ion beam are less numerous and the energy deposited in the ion source by the re-accelerated electrons is less; the heating of the ion source is lower and consequently the degassing of the constituent materials is reduced. The heavy ions resulting from this degassing are less numerous and their contribution to the erosion of the target weaker. In addition, the average energy of the deuterium-tritium ions is increased, which can make it possible to reduce the tube current.
• At identical beam current, it is possible to increase the interelectrode distances and thus reduce the electric field in order to reduce the cold emission phenomena.
• With the same integrated beam current (over the time unit), the maximum current can be increased in pulsed mode in the pressure ratio Pmax / P, Pmax being the maximum operating pressure not causing a change in the operating regime of the tube (passage of the discharge in arc mode).
• In addition, the distribution of the current on the target is much more homogeneous due on the one hand to the homogeneity of the discharge at the level of the emission channels and on the other hand to the multiplication of the number of elementary beams. This results in a decrease in the maximum ion density and at the same beam current an increase in lifetime.

Claims (6)

1. A sealed neutron tube which contains a low-pressure gaseous deuteriumtritium mixture wherefrom an ion source comprising two electrodes, an anode and a cathode, forms an ionised gas which is guided by a magnetic confinement field produced by magnets (8) or any other means for producing said field, which source emits, via emission channels formed in said cathode, ion beams which traverse an extraction-acceleration electrode (2) and which are projected with high energy onto a target electrode (4) in order to produce therein a fusion reaction which causes an emission of neutrons, said ion source being a multi-cell type which is formed by a structure of Penning-type elementary cells comprising, for said cells together, a cathode cavity (7) in which there is arranged a multi-hole anode (6), the axes of said holes being aligned with the corresponding axes of said emission channels, characterized in that the number of said holes is optimised so as to enlarge the extracted ion beam for equivalent coverage of said ion source, and the shape and/or the dimensions and/or the position of said holes are adapted to the topology of said magnetic field, in such a manner that the radius of said anode holes (6′₁, 6′₂, ..., 6′_n) of said ion source progressively increases from the centre towards the periphery of the structure in order to take into account the topology of the magnetic field.
2. A neutron tube as claimed in Claim 1, characterized in that said holes (6˝₁, 6˝₂, 100, 6˝_n) of said ion source have a truncated shape so as to be adapted to the topology of the magnetic field.
3. A neutron tube as claimed in Claim 1 or 2, characterized in that an expansion chamber (14) is arranged underneath said ion source in order to enhance the uniformity of the densities at the area of the ion emission, formed in the wall of the chamber which faces the target, via orifices (15) whose disposition and number may be quasiindependent of those of said elementary cells.
4. A neutron tube as claimed in the Claims 1 to 3, characterized in that said extraction-acceleration electrode (2) comprises a number of orifices (2₁, 2₂, ..., 2_n) equal to the number of anode holes, which orifices are disposed along the axes of said emission channels.
5. A neutron tube as claimed in the Claims 1 to 4, characterized in that said extraction-acceleration electrode (2) comprises a number of orifices which is smaller than the number of anode holes, the disposition of said orifices on the extraction-acceleration electrode precluding interception of the beams.
6. A neutron tube as claimed in the Claims 4 and 5, characterized in that the thickness of said extraction-acceleration electrode (2) is increased in order to improve its mechanical strength and to enable cooling of the electrode by forced circulation of liquids.

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