EP0362945A1 - Device for treating the Penning ion source in a neutron tube - Google Patents

Abstract

In a high flux sealed neutron tube provided with a Penning type ion source (1) the magnetic field for confining the ionised gas (9) is rendered more divergent in the direction of the emission zone of the ions by action on the system of magnets (8) of the ion source. The ion beam issuing from the plasma is accelerated (2) and projected onto a target (4). The geometry and the position of the anode (13) inside the ion source adapt to the topography of the lines of force, for minimum intercept of the trajectories of the ionising electrons oscillating in the structure, an adaptation obtained in particular by the use of a truncated anode, the generatrices of which follow the lines of force. <IMAGE>

Description

The invention relates to a Penning-type ion source device for a high flux sealed neutron tube in which said ion source with two electrodes (anode and cathode) forms an ionized gas channeled by a magnetic confinement field created by magnets or by any other means of creating said field and from which a high energy ion beam is projected onto a target electrode by means of an extraction and acceleration device to produce there a fusion reaction causing a neutron emission.

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

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

The d (3 _H , 4 _He ) n fusion reaction delivering 14 MeV neutrons is usually the most used due to 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 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 electrode, which results in a constant neutron emission, this improvement will result from a distribution as uniform as possible of the current density over the whole of the surface offered by the target for bombardment. 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), to have a high extraction efficiency (from 20 to 40%) and to be of small dimensions.

On the other hand, this type of source has the drawback of requiring a magnetic field of the order of a thousand gauss which introduces a significant inhomogeneity of density of the current of the ions inside the discharge and at the level of the emission zone. ions.

The object of the invention is to make the ion density more homogeneous at the emission level by modifying the Penning structure according to the prior art.

To this end and in accordance with the invention, said magnetic field is made more divergent in the direction of the ion emission zone, by action on said magnets or on any other means of creating said field, modifying the confinement of ionizing electrons of the discharge and therefore of the ionization which results therefrom, being compensated by the adaptation of the shape and / or of the dimensions and / or of the positioning of the anode in said ion source.

This adaptation can be implemented using the following devices:
the anode is of frustoconical shape with the largest diameter on the side of the low values of the magnetic field to take account of the divergence of the lines of force towards the zone of emission of the ions.
- The circular anode is reduced in height and brought closer to the cathode in the zone of strong gradient of the magnetic field.

• La figure 1 représente le schéma de principe d'un tube neutronique scellé selon l'état de l'art antérieur.
• La figure 2 montre les effets de l'érosion en pro­fondeur de la cible et le profil radial de densité de bombar­dement d'ions.
• Les figures 3 et 4 représentent respectivement les schémas d'une première variante et d'une seconde variante de dispositifs d'extraction des ions selon l'invention.

The following description with reference to the accompanying 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.
• Figures 3 and 4 respectively represent the diagrams of a first variant and a second variant of ion extraction devices according to the invention.

In these figures, identical elements 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 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 the 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, of a cathode structure 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 structure opposite the anode.

• La figure 2a montre le profil de la densité J de bombardement des ions suivant une direction radiale quelconque 0r, à partir du point d'impact 0 de l'axe central du faisceau sur la surface de la cible. La forme de ce profil met en va­leur le caractère inhomogène de ce faisceau dont la densité très élevée dans la partie centrale décroît rapidement lors­qu'on s'en éloigne.
• Sur la figure 2b l'érosion s'effectue en fonction de la densité de bombardement et toute la couche de matériau hydrurable d'épaisseur e déposée sur un substrat S est saturée en mélange deutérium-tritium. La profondeur de pénétration des ions énergétiques deutérium-tritium représentée en traits pointillés s'effectue sur une profondeur l₁ fonction de cette énergie.
• Sur la figure 2c, l'érosion de la couche est telle que la profondeur de pénétration l₂ est supérieure à l'épais­seur e dans la partie la plus bombardée ; une partie des ions incidents s'implante dans le substrat et très rapidement les atomes de deutérium et de tritium sont en sursaturation.
• Sur la figure 2d, les atomes de deutérium et de tritium se sont rassemblés pour donner des bulles qui, en éclatant ont formé des cratères et accru très rapidement l'érosion de la cible sur la profondeur l₃.

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

• FIG. 2a shows the profile of the density of bombardment of the ions in any radial direction 0r, from the point of impact 0 of the central axis of the beam on the surface of the target. 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 penetration depth of the deuterium-tritium energy ions represented by lines dotted lines are made on a depth l₁ 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.

In the ion source 1 of the Penning type represented in FIG. 1, the cylindrical anode 6 is brought to a higher potential of the order of 4 kV than that of the cathode 7 itself brought to a very high voltage of 250 kV for example.

The set of magnets 8 provides a large magnetic field of the order of a thousand gauss.

The role of this magnetic field is to limit the transverse movement of the charges formed inside the anode by ionization of a deuterium-tritium gas mixture. This ionized gas is thus confined around the axis of the anode and in much higher density along this axis. This results in significant inhomogeneity inside the landfill.

The ions are extracted from the emission channel 12 formed in the cathode thus playing the role of emission electrode, by means of the acceleration electrode 2 carried as well as the target electrode 4 at the potential 0 of the mass.

The inhomogeneity of the ionized gas will have repercussions on the level of the extraction of the ions more important on the axis than on the periphery of the beam. Thus this type of non-uniformity contributes to a large extent to the erosion of the target and consequently to the limitation of the lifetime of the tube.

In order to make the ion density more homogeneous at the level of extraction, the idea of the invention consists in modifying the confinement of the ionized gas by acting on the arrangement of the magnets of the assembly 8 so that the magnetic field be more divergent. The reduction in the discharge current which results therefrom can be advantageously compensated by means of the solutions in FIGS. 3 and 4.

In FIG. 3, the circular anode has been replaced by a frustoconical anode 13 whose generators tend to follow the lines of force of the magnetic field 10. The ionized gas 9 is more spread due to said modification of the confinement. The diameters of the frustoconical anode must be increased in order to avoid the interception of the electrons.

In FIG. 4, the circular anode 14 is reduced in height and offset towards the strong field zone near the upper part of the cathode so as to always avoid the interception of the electrons.

These modifications ensure a substantial compensation of the discharge current at the same time as a better homogeneity of the beam.

Claims (3)

1. Penning-type ion source device of a high flux sealed neutron tube in which an ion source with two electrodes (anode and cathode) forms an ionized gas channeled by a magnetic confining field created by magnets or by any other means of creating said field and from which a high energy ion beam is projected onto a target electrode by means of an extraction and acceleration device to produce there a fusion reaction leading to an emission of neutrons , characterized in that said magnetic field is made more divergent towards the ion emission zone by action on said magnets or on any other means of creating said field, modifying the confinement of the ionizing electrons of the discharge and consequently of the resulting ionization, being compensated by the adaptation of the shape and / or dimensions and / or of the positioning of the anode in said source of i ons. 2. Device according to claim 1, characterized in that the shape of said anode is frustoconical with the largest diameter on the side of the weak magnetic field to take account of the configuration of the lines of force of said field. 3. Device according to claim 1, characterized in that said circular anode is reduced in height and brought closer to the cathode in the area of strong magnetic field.

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