EP0362946A1 - Ion extraction and acceleration device limiting reverse acceleration of secondary electrons in a sealed high flux neutron tube - Google Patents

Abstract

Device for extraction and acceleration of ions in a sealed high flux neutron tube containing a deuteriun-tritium gas mixture under low pressure and in which an ion source (12) furnishes several ion beams (3a,3b,...3e) projected onto a target electrode (4) by means of a system for extraction and acceleration, in order to produce thereat a fusion reaction causing an emission of neutrons. According to the invention, an additional electrode (13) is disposed close to and downstream of the latter acceleration electrode (2) in the space of the tube lying between this latter acceleration electrode and the target, and the polarisations of each of these two acceleration and additional electrodes relative to the target are chosen so as to repel the secondary electrons created by ionisation of the gas in the path of the said beams, which makes possible the increasing of the length of the said space in order to greatly reduce the nonuniformity of the ion bombardment on the target, and thus to increase the lifetime of the tube. …<??>Application to neutron tubes. …<IMAGE>…

Description

The invention relates to a device for extracting and accelerating ions in a sealed high-flux neutron tube containing a deuterium-tritium gas mixture under low pressure from which an ion source provides a beam (s) ( x) ionic (s) extracted and accelerated (s) at high energy by passing through an extraction and acceleration system, to be projected (s) on a target electrode and to produce there a fusion reaction resulting in an emission of neutrons.

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 entire surface offered by the target for bombardment. ions.

One of the ways to reduce this maximum density is to use the divergence of the beam in the sliding space between the point of convergence and the target. Any increase in this space by a factor x of the ion path results in a reduction of the type in 1 / x² of the maximum bombardment density.

In a sealed neutron tube, the pressure of the deuterium-tritium mixture necessary to obtain the current of ions is first order, the same throughout the tube. Now the ions extracted and accelerated towards the target will react with the molecules of the gas to produce ionization, dissociation and charge exchange effects resulting on the one hand in a reduction in the average energy of the ions 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 source of ions or the electrodes of the tube.

This results in energy deposits which will increase the temperature of the materials of the electrodes such as molybdenum or stainless steel, or pyrolytic carbon. 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 accentuation of the erosion.

These effects increase with the operating pressure and the path length of the ions. Thus, a correction to the inhomogeneities of the bombardment of the target which could be brought about by an increase in said path is rendered ineffective due to the increase in ion-gas reactions, greater than or equal to simple proportionality.

The object of the invention is to provide a structure for which these reactions no longer have detrimental repercussions on the operation of the tube. For that, it is enough to avoid that the electrons created by the ions of the beam can "go up" towards the source of ions where they would deposit a significant energy. It is therefore necessary to push them back inside the sliding space where they acquire only a very low energy and to collect them on the electrodes limiting this space.

To this end, the invention is remarkable in that said acceleration system comprises an addition electrode polarized so as to limit the re-acceleration towards the source of the secondary electrons created by ionization of the gas on the path of said beam (s) inside the space between said extraction and acceleration system and said target electrode, which makes it possible to increase said space to greatly reduce the inhomogeneities of the ion bombardment.

An embodiment of the device of the invention consists of a last acceleration electrode brought to the same potential as the target and said additional electrode playing the role of electron repulsion electrode, polarized negatively with respect to said last electrode acceleration and whose plane is located near and downstream of the exit plane of the last acceleration electrode in the equipotential space accelerator-target electrode.

In another embodiment, said device of the invention comprises a last ion acceleration electrode polarized negatively with respect to the target electrode to play the role of electron repulsion electrode. Said additional electrode, disposed near and downstream of the exit plane of said last ion acceleration electrode, is polarized at the same potential as the target. The electrons are collected by the target and the additional electrode.

The devices according to the invention do not cause any appreciable disturbance in terms of the operation of the tube when the sliding space is increased.
- The energetic ions lose very little energy during ionizing shocks (of the order of 10⁻⁴) and, during charge exchanges, they transform into fast neutrals with the same energy as the incident ion.
- The electrons and the ions created in the sliding space consequently have only a weak energy and taking into account the polarizations of the electrodes are captured by them and the deposited energies are reduced (of the order of 1% of the energy dissipated on the target). Increasing the length of the sliding space will simply have the effect of increasing the inter-electrode currents (target-acceleration electrode or repulsion electrode-acceleration electrode and target), which will result in slight heating. These electrodes will therefore be made of refractory material.

• 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 des ions.
• La figure 3 représente schématiquement une première variante de la structure des éléments d'optique ionique du dispositif de l'invention.
• La figure 4 montre la répartition du potentiel sui­vant l'axe du faisceau ionique dans le cas du dispositif de la figure 3.
• La figure 5 représente schématiquement une deuxième variante de la structure des éléments d'optique ionique du dispositif de l'invention.
• La figure 6 montre la répartition du potentiel sui­vant l'axe du faisceau ionique dans le cas du dispositif de la figure 5.

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 deep erosion and the ion bombardment radial profile.
• FIG. 3 schematically represents a first variant of the structure of the ion optical elements of the device of the invention.
• FIG. 4 shows the distribution of the potential along the axis of the ion beam in the case of the device of FIG. 3.
• FIG. 5 schematically represents a second variant of the structure of the ion optical elements of the device of the invention.
• FIG. 6 shows the distribution of the potential along the axis of the ion beam in the case of the device of FIG. 5.

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 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 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.

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 ion bombardment in any radial direction Or, from the point of impact 0 of the central axis of the beam on the surface of the target for standard optics with a single electrode . 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 energy ions represented in dotted lines is effected 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. 3a shows diagrammatically a neutron tube comprising an ion source 12 of the multicellular multi-beam Penning type whose cylindrical anode 6 pierced with juxtaposed multitrous 6a, 6b, ... 6e is brought to a higher potential of the order of 4 kV to that of cathode 7 itself brought to a very high voltage of 250 kV for example.

The ion beams 3a, 3b, ... 3e coming from the emission channels 7a, 7b ... 7th practiced in the cathode opposite the corresponding anode holes are projected onto the target 4 by means of the acceleration electrode 2.

The section of beam intercepted by the target depends on the divergence of the trajectories and especially on the distance from the target to the point of convergence.

The diagram of FIG. 3a illustrates this property by a judicious choice of the position of the target.

It can be seen in this figure that for position A, the impact surfaces of the elementary beams on the target are distinct from each other and the density profile J of each elementary beam is presented as shown in FIG. 3b with an axial value high and a rapid decrease on both sides of the axis.

One way to obtain a more homogeneous density distribution at the impact of the global beam on the target is to move the latter away from the source, from position A to position B for example, so that there is overlap elementary beams.

It can be seen in FIG. 3c that the density profile J of each beam on the target is more spread out and that its axial value is lower. In addition, the recovery of elementary profiles makes it possible to obtain an almost homogeneous resulting profile.

Unfortunately this ideal result cannot be achieved in practice due to the increase in the phenomena of ionization of the gas by the ions of the beam when the length of the trajectories in the target space-accelerating electrode is increased for a structure according to the prior art. Indeed, the electrons thus created are re-accelerated towards the source and the electrodes of the tube, the heating of which results in a desorption effect of the impurities and the creation of impurity ions such as Co⁺ whose spray coefficient is greater than 10² to 10³ times that of deuterium ions, which seriously disturbs the quality of the tube atmosphere. Furthermore, the secondary electrons emitted by the target at the rate of several electrons emerging per incident ion and re-accelerated in the same way towards the source also contribute to the increase in its heating and ultimately to its destruction.

The device of the invention makes it possible to repel the secondary electrons emitted by the target as well as those created by ionization of the gas. In a first variant of this device, this is achieved by placing an additional electrode 13 suitably polarized near the acceleration electrode in the sliding space between this electrode and the target, which then makes it possible to fully benefit from the effect of distance from the target. This additional electrode is brought to a negative potential (-5 kV for example) compared to those of the acceleration electrode and the target grounded and made of a refractory material to prevent its heating by the inter-electrode currents in the target space-accelerating electrode.

FIG. 4 shows the potential distribution along the axis of the ion beam for the device of FIG. 3.

Instead of moving the target away from the ion source, it has been assumed (which amounts to the same thing) that the target is fixed and that the ion source electrode assembly is moved in reverse

The positions of the target C, the suppressor electrode ES1, the accelerating electrode EA1 and the source S1 were plotted on the abscissa on the one hand for a certain configuration of neutron tube and on the other hand the positions of the suppressor electrode ES2, accelerator electrode EA2 and source S2 for another configuration of neutron tube corresponding to a doubling of the sliding space. The level of potential VS of the suppressor electrode has been indicated on the ordinate. The curves in solid line and in broken lines represent respectively the difference between the potential V along the axis of the ion beam and the potential Vc of the target for the two configurations. The variations in this potential difference in the C-ES1 and C-ES2 zones and the resulting electric fields produce the "repelling" effect of the additional electrode by which the electrons emitted by the target and those created by ionization are collected by the target. The same potential variations in the acceleration zones of the ES1-S1 and ES2-S2 ions, identical in the two configurations, show that the operating regime of the tubes remains unchanged, the flow of electrons created in this region, accelerated towards the source of ions, remaining identical.

FIG. 5 represents a second variant of the device of the invention in which a target-carrying electrode 14 in the form of a well, or which may have a structure with holes, at the same potential as the target 4 is arranged in the vicinity of the electrode d acceleration 2 in the space between this electrode and the target. The electron repulsion effect is achieved by polarizing the acceleration electrode at a slightly negative potential Va relative to the target.

A graph similar to that of FIG. 4 illustrates in FIG. 6, for this second variant of the device, the variation of the potential V-Vc along the axis of the ion beam. The positions ER1 and ER2 of the rim of the target-carrying electrode placed near the acceleration electrode were plotted on the abscissa. The above considerations for the graph in Figure 4 remain valid.

Claims (6)

1. Device for extracting and accelerating ions in a sealed high-flux neutron tube containing a gaseous mixture of deuterium-tritium at low pressure from which an ion source provides an ion beam (s) (s) extracted and accelerated (s) at high energy by passing through an extraction and acceleration system, to be projected (s) on a target electrode and to produce there a fusion reaction involving an emission of neutrons, characterized in that said extraction and acceleration system comprises an additional electrode polarized so as to limit the re-acceleration towards the source of the secondary electrons created by ionization of the gas on the path of said (or said) beam (s) to the inside the space between said extraction and acceleration system and said target electrode, which makes it possible to increase said space to greatly reduce the inhomogeneities of the bombardem ionic ent. 2. Device according to claim 1, characterized in that said extraction and acceleration system comprises a last acceleration electrode communicating to the ions the nominal energy brought to the same potential as said target electrode and said additional electrode playing the role of electron repulsion electrode, polarized negatively with respect to said last acceleration electrode and the plane of which is located near and downstream of the exit plane of the last acceleration electrode in the equipotential space accelerating electrode- target. 3. Device according to claim 1, characterized in that said acceleration system comprises a last acceleration electrode and said additional electrode brought to the same potential as said target electrode and whose plane is located near and downstream of the plane of output of the last acceleration electrode in the accelerator-target electrode space, said acceleration electrode performing the role of electron repulsion electrode being polarized negatively with respect to the additional electrode-target electrode assembly. 4. Device according to claims 1 to 3, characterized in that it makes it possible to form several elementary beams originating from the same or from several ion sources but irradiating the same target electrode. 5. Device according to claims 2 and 3, characterized in that said acceleration electrode is made of refractory conductive material. 6. Device according to claim 2, characterized in that said additional repulsion electrode is made of refractory material.

Images