FROM CURRENT LITERATURE

CONDITIONS FOR OBTAINING INTENSE ION BEAMS¹

The production of powerful ion beams at the present time, in connection with work in nuclear physics, is of such great importance that this question has been subjected to detailed theoretical¹ and experimental investigation.

The requirements that a good ion source must satisfy are, generally speaking, numerous and partly contradictory. Above all, it is necessary that the source be powerful. At the same time it is not at all necessary that the resulting ion flux be approximately parallel. If it is possible to obtain an ion beam with a constant cross section in the region where ionization occurs, then it is already very difficult to obtain a beam with high density outside this space. Another important condition is constancy of the beam, which must not change its intensity over a period of at least several intermediate intervals of time. On the other hand, the source must permit smooth and reproducible adjustment of the beam intensity over wide limits. Further, it is extremely necessary that in the beam the ratio of the number of ions to the number of atoms be as high as possible. This, generally speaking, requires intense ionizing electron streams. In the case where one is working with protons, it is extremely important that the ratio of the number of protons to the number of molecular hydrogen ions be as large as possible. Finally, it is very desirable to have ion beams homogeneous with respect to particle velocity, to have, if possible, a smaller spread of velocities in the ion source, and to have as low a voltage as possible extracting the ions from the ionization region.

In the paper under review¹, consideration is given first of all to the ionization process, since it determines, on the one hand, the concentration of ions in general and, on the other, the concentration of ions of the required kind. In this connection only ionization by electron impact is considered, as the most effective method. Here, of course, the dependence of the effective ionization cross section on the electron energy must be taken into account, as well as the details of the process, since, for example, in the case of molecular hydrogen the formation of \(H_1^+\) ions is predominantly a secondary process, while the primary process consists in the formation of atomic hydrogen (direct formation of \(H_1^+\) from \(H_2\) is unlikely, although possible).

The most probable primary reaction is the following:

\[ H_2 + e \to H_1 + H_1 + e, \]

for which, according to the calculations of Massey and Mohr², the maximum ionization cross section corresponds to \(\sim 15\) V. However, since at such low voltages in most cases it is not possible to extract all the ions from the ionization region, a compromise has to be made here.

The problem of extracting the greatest possible number of ions into a directed beam is a problem of positive space charge.

charge. For the potential difference \(V_e\) between the electrodes bounding the ionization region, which is necessary for complete extraction of the ions, one obtains, in the first approximation, the expression

\[ \sigma (V_b+V_e)=\left(\frac{m}{M}\right)^{1/2}\cdot \frac{2v}{\tau_1pl}, \]

where \(V_b\) is the energy with which the ionizing electrons enter this space, \(m\) and \(M\) are the masses of the electron and ion, \(p\) is the gas pressure, \(l\) is the distance between the electrodes, and \(\sigma\) and \(v\) are constants. The quantity \((V_b+V_e)\) is of the order of hundreds of volts.

On the basis of the analysis carried out of the operating conditions of ion sources and the study of the operating characteristics of the newest sources using a low-pressure arc discharge, the authors come to the conclusion that these sources operate under conditions that do not permit the use of all ions for the formation of a directed ion beam. On the other hand, this work has served as a guide in the development of new types of ion sources.

N. Khlebnikov, Moscow

REFERENCES

1. L. P. Smith and G. W. Scott, Phys. Rev., 55, 946, 1939.
2. N. Massey and F. Mohr, Proc. Roy. Soc., A 135, 258, 1932.