Patent Number: 051046105
Section: description

DESCRIPTION OF THE INVENTION FIG. 1 shows the basic elements of a sealed neutron tube 11 which contains a low-pressure gaseous mixture to be ionised, for example deuterium-tritium, and which comprises an ion source 1 and an acceleration electrode 2 wherebetween a very high potential difference exists which enables the extraction and acceleration of the ion beam 3 and its projection onto the target 4 where the fusion reaction takes place which causes an emission of neutrons of, for example, 14 MeV. The ion source 1 is integral with an insulator 5 for the passage of the high-voltage power supply connector (not shown) and, is for example a Penning-type source which is formed by a cylindrical anode 6, a cathode structure 7 which incorporates a magnet 8 with an axial magnetic field which confines the ionised gas 9 to the vicinity of the axis of the anode cylinder and whose lines of force 10 exhibit a given divergence. An ion emission channel 12 is formed in the cathode structure so as to face the anode. The diagrams of FIG. 2 illustrate the target erosion effects. FIG. 2a shows the density profile J of the ion bombardment in an arbitrary radial direction Or, starting from the point of impact 0 of the central axis of the beam on the surface of the target. The shape of this profile illustrates the inhomogeneous character of this beam where the very high density in the central part rapidly decreases towards the periphery. FIG. 2b shows the erosion as a function of the bombardment density and the entire hydride layer having a thickness e and deposited on a substrate S is saturated with the deuterium-tritium mixture. The penetration depth of the energetic deuterium-tritium ions, denoted by a broken line, equals a depth l.sub.1 as a function of this energy. In FIG. 2c the erosion of the layer is such that the penetration depth l.sub.2 is greater than the thickness e in the most heavily bombarded zone; a part of the incident ions propagates in the substrate and the deuterium and tritium atoms are very quickly oversaturated. In FIG. 2d the deuterium and tritium atoms collect and form bubbles which form craters upon bursting and which very quickly increase the erosion of the target at the depth l.sub.3. The latter processes immediately precede the end of the service life of the tube, causing either a drastic increase of breakdowns (presence of microparticles resulting from the bursting of bubbles) or pollution of the target surface by the pulverised atoms which absorb the energy of incident ions. In the Penning-type ion source 1 shown in FIG. 1, the cylindrical anode 6 is connected to a potential which is approximately 4 kV higher than that carried by the cathode 7 which is connected to a very high voltage of, for example 250 kV. The magnet assembly 8 produces a strong magnetic field in the order of a thousand gauss. This magnetic field serves to limit the transverse movement of the charges formed inside the anode by ionisation of a gaseous mixture of deuterium and tritium. This ionised gas is thus confined to the vicinity of the axis of the anode and has a much higher density along the axis. This results in a substantial inhomogeneity inside the discharge. The ions are extracted from an emission channel 12 formed in the cathode, thus acting as the emission electrode, by means of the acceleration electrode 2 which is connected to ground potential 0 whereto as the target electrode 4 is also connected. At the level of the ion extraction the inhomogeneity of the ionised gas will have more repercussions at the axis than at the periphery of the beam. Thus, this type of inhomogeneity largely contributes to the erosion of the target and hence to the limitation of the service life of the tube. In order to make the ion density more homogeneous at the extraction level, the idea of the invention is to modify the confinement of the ionised gas by influencing the arrangement of the magnets of the assembly 8 so that the divergence of the magnetic field is greater. The resultant reduction of the discharge current can be attractively compensated for by means of the solutions illustrated by the FIGS. 3 and 4. In FIG. 3 the circular anode has been replaced by a truncated anode 13 whose generatrices tend to take the shape of the lines of force of the magnetic field 10. The ionised gas 9 is wider because of this modification of the confinement. The diameters of the truncated anode will have to be increased in order to avoid the interception of electrons. In FIG. 4 the height of the circular anode 14 has been reduced and the anode has been shifted nearer to the zone where a strong field prevails in the vicinity of the upper part of the cathode in order to avoid the interception of electrons. These modifications ensure a substantial compensation for the discharge current and at the same time improve the homogeneity of the beam.