Considerations on accelerator systems requirements and limitations for μ-probe applications

Abstract

It is commonly accepted that the requirements for μ-probe applications on beam brightness and energy spread favour single-ended accelerators. With the Singletron™ systems HVE can offer high beam brightness (74Am−2rad−2eV−1) and low energy spread as well as high terminal voltage stability (∼10ppm). On the other hand, Tandetron™ systems have the advantage that no ion source maintenance is required in the tank as opposed to single-ended systems. Especially for the larger single-ended systems needed to obtain higher terminal voltages, source maintenance is time consuming. Tandem accelerators provide higher beam energies at the same terminal voltage. This offers the possibility of greater probe depths e.g. larger than 1mm for protons (10MeV) or over 300μm for He3 (15MeV) in biological samples, which can be desired in single-cell irradiation experiments.Single-ended and tandem systems exhibit several important differences. First, of course the difference in ion sources, as for a tandem system a negative ion source is needed. The development of these sources has led to the availability of the multicusp ion source that combines low beam emittance with high current. Furthermore, related to the stripping process needed with a tandem accelerator, energy straggling of the ions increases the energy spread and small angle scattering of the ions tends to lower the beam brightness.At HVE, we investigated the influence of the small angle scattering on the beam brightness. Various ion optical approaches were compared and it was concluded that a multicusp ion source provides the possibility to minimise the degradation in the beam brightness due to small angle scattering of the ions. It is expected that a beam brightness of 10–50Am−2rad−2eV−1 can be achieved with the Tandetron™ accelerator systems.