From Current Literature

Measurement of the Compensation Time of the Space Charge of a Beam of Positive Ions

Recently, significant progress has been achieved in the development of sources of positive ions. With the aid of the available sources it is possible to obtain beams of positive ions of several tens of milliamperes.^1,2 Intense ion beams are used not only for isotope-separation purposes. In accelerators at comparatively low energies, nuclear reactions are carried out, for example, of the type:

\[ {}^{2}\mathrm{H}(d,n){}^{3}\mathrm{He} \quad \text{and} \quad \mathrm{Li}(p,\gamma)\mathrm{Be}. \]

Thanks to the availability of intense ion sources, these reactions can serve for obtaining intense sources of neutrons and \(\gamma\)-rays. In all these applications of ion beams there arises the problem of compensating the positive space charge inside the ion flow.^3 Compensation of this charge occurs automatically, by the accumulation within the beam of electrons formed as a result of ionization of the residual gas by the primary ions. However, the accumulation process requires time. By simple calculations, neglecting recombination processes, it can be shown that the compensation time is expressed by the formula

\[ \tau=\frac{1}{N_{0}\sigma_{b}}\left(\frac{M}{2eV}\right)^{1/2}, \]

where \(M\) is the mass of the primary ion, \(eV\) is the energy of this ion, \(N_{0}\) is the number of neutral-gas molecules per unit volume, and \(\sigma_{b}\) is the effective ionization cross section.

The paper under review^4 describes an experimental method for measuring the compensation time. The apparatus consists of an ion source, an accelerating electrode, two grids, a magnetic analyzer, and an ion collector. One of the grids was connected to a generator of rectangular voltage pulses, the duration of which could be varied from 150 \(\mu\)sec to 10,000 \(\mu\)sec.

At the moment when a negative pulse of voltage was applied to the grid, ions could pass from the source into the chamber of the magnetic analyzer. An ion beam was then formed in the chamber. The collector was arranged in such a way that, when compensation was present, it collected all the ions of the beam. If decompensation occurred, the beam spread out as a result of the action of electrostatic repulsive forces, and only a part of the ion beam reached the collector. According to the oscill-

from the oscillogram of the collector current it was possible to judge the time required for compensation of the space charge.

Measurements on the apparatus described made it possible to verify the dependence of the compensation time on the primary-ion current, pressure, type of residual gas, and energy of the primary ions (Ar and Mg ions). The experimental data agree with the formula derived for $T$. Under the experimental conditions the value of $T$ varied within the limits of 10–200 μsec.

When primary ions pass through electronegative gases, negative ions are formed. From the form of the oscillogram it was possible to judge the role of negative ions in compensating the space charge of the beam.

The method of measuring the compensation time can be used to determine the effective ionization cross section.

V. S. Anastasevich

References

1. C. R. Keim, J. Appl. Phys. 24, 1255 (1953); Ann. Rev. Nuclear Sci., p. 263, 1952.
2. Bell, Bull. Am. Phys. Soc. 8, No. 5, 24 (1953).
3. Smith, Parkinus and Forrester, Phys. Rev. 72, 989 (1947).
4. R. Bernas, Kaluszynar and J. Druaux, J. phys. et radium 15, 273 (1954).