Patent Number: 047939613
Section: summary

FIELD OF THE INVENTION This invention relates to a method and a source for producing positively charged molecular ions of hydrogen (H.sub.2.sup.+) or deuterium ((D.sub.2.sup.+). BACKGROUND OF THE INVENTION Neutral beam injection has proven to be an effective way to heat plasmas in tokamaks as well as mirror devices. Multi-amperes of neutral atoms have already been obtained from deuterium ions for energies as high as 120 keV. In some future fusion reactors, such as the Mirror Fusion Test Facility at the Lawrence Livermore Laboratory, high currents of lower energy (40 keV) deuterium atoms are required in certain neutral beam lines. In that respect, it is more advantageous to form the neutral atoms from the molecular D.sub.2.sup.+ ions and accelerate them to twice the energy (80 keV). In passing through the gas neutralizer, these D.sub.2.sup.+ ions will first dissociate and will then be neutralized to form two atomic particles with half the original D.sub.2.sup.+ ion energy However, this technique is useful only if ion sources that can generate a high percentage (&gt;70%) of D.sub.2.sup.+ ions are available. SUMMARY OF THE INVENTION One principal object of the present invention is to provide a novel method and apparatus for generating a high concentration of H.sub.2.sup.+ or D.sub.2.sup.+ ions by using a new and improved multicusp ion source. The basic principle in achieving a high percentage of H.sub.2.sup.+ or D.sub.2.sup.30 ions is to extract them from the source as soon as they are produced. Otherwise they will react with background gas molecules to form tri-atomic ions H.sub.3.sup.+ or D.sub.3.sup.+ or be dissociated by electrons. The former reaction H.sub.2.sup.+ + H.sub.2 .fwdarw.H.sub.3.sup.+ +H have a very short mean free path length .lambda.. Assuming a background neutral gas density of approximately 3. 3.times.10.sup.13 cm.sup.-3 and a cross-section .sigma. of approximately 6.times.10.sup.-15 cm.sup.2, 80=(n.sub.o .sigma.).sup.-1 is estimated to be about 5 cm. Thus the distance traversed by the H.sub.2.sup.+ ion before it arrives at the extaactor electrode cannot exceed this value. This in turn sets a limit on the length of the source chamber. Among the novel features of this invention is the provision of a short ion source, both physically and electrically. This is accomplished by placing the filaments close to the plasma grid which is connected electrically to the ion source shell. The plasma grid thus becomes part of the anode. Also, the length and width of the source is approximately 40 cm.times.10 cm, for example, but the depth is only about 6 cm, for example. Because the physical distance from the filaments to the plasma grid is substantially less than the mean free path for neutralization or capture of a H.sub.2.sup.+ ion, the ratio of the desired ion to total ions is about 80%. The same applies to D.sub.2.sup.+ ions. The positive ions are extracted from the source by a negative voltage which may vary widely, according to the application, from 300 volts or lower to 40 keV, or higher, for example. The source utilizes a multicusp magnetic field, produced by rows of magnets, for example, which may be of the samarium-cobalt type. Such magnetic field on the back plate reduces the effective path length and loss of the positive ions before they are extracted from the source. In turn, it increases the primary electron path length by making it difficult for these electrons to be lost to the surfaces protected by the magnetic field.