Progress in ion source development for on-line separators

Abstract

Abstract Design criteria for ion sources will be discussed in view of the principle requirements of the ISOL-process. The demands for high ionization efficiency (at a level above 1%), for versatility (the ionization of almost all elements) and for at least some selectivity (the ionization of one element at a time) can be met with present-day technology by a set of a high temperature discharge ion source, an ionizer with variable-temperature and variable-material options for positive and negative surface ionization, and a “high temperature cavity ion source”. The present yield limitation is related to the release efficiency rather than to the ionization efficiency. A number of examples will demonstrate to what extent the principal design aims have been realized in existing devices. The fundamental characteristics of thermal ion sources are also dealt with. The boundary conditions-quasi-neutrality and thermal equilibrium-are treated in detail, permitting the calculation of the ionization efficiency. The occurrence of “impossible” ion species in thermoionization mass spectra (e.g. noble gas and multiply charged ions) and “halos” of the mass lines will be explained in terms of the relevant ionizing processes.