A "hybrid"-type system for an achromatic focusing of ions
  It is known, that at high energies of ions (more than 10 MeV for light ions) the focusing by magnetic quadrupole fields is most effective, whereas at small energies (less than 1 MeV) - the focusing by electrical quadrupole fields. At intermediate energies of ions (from 1 to 10 MeV) the efficiency of each of these types of a focusing is reduced. The authors offer to utilize for this purpose the "hybrid" type of focusing systems consisting of sequence of electrical and magnetic quadrupole lenses, which are localized in the same space. For making fields of electrical quadrupole lenses it is offered to utilize RF quadrupoles. The RF electrical quadrupoles are created due to the relevant configuration of drift tubes in the vicinity of an accelerating gap. As a result of the general theoretical analysis and calculation of concrete versions of practical realizations it is shown that the given system has advantage as contrasted to by known analogs in the field of medium energies. The velocity band of ions, in which the stable motion is ensured, is essentially changed, and the focusing becomes "achromatic". The calculations have been performed for protons with velocities 0.5-1.0 % of light velocity, at the gradients of electrical field up to 50 kV per square cm, and the gradients of magnetic field up to 1 T/m. Achromaticity of this system allows to lower a radiative activation of accelerator and to reduce damage of electrode surfaces by lost ions. 
B.Yu. Bogdanovich, A.V. Nesterovich