In order to study the beam power amplification of an accelerator-driven system (ADS), a new parameter, the proton source efficiency * is introduced. * represents the average importance of the external proton source, relative to the average importance of the eigenmode production, and is closely related to the neutron source efficiency [varphi]*, which is frequently used in the ADS field. [varphi]* is commonly used in the physics of subcritical systems driven by any external source (spallation source, (d,d), (d,t), 252Cf spontaneous fissions, etc.). On the contrary, * has been defined in this paper exclusively for ADS studies where the system is driven by a spallation source. The main advantage with using * instead of [varphi]* for ADS is that the way of defining the external source is unique and that it is proportional to the core power divided by the proton beam power, independent of the neutron source distribution.
Numerical simulations have been performed with the Monte Carlo code MCNPX in order to study * as a function of different design parameters. It was found that, in order to maximize * and therefore minimize the proton current needs, a target radius as small as possible should be chosen. For target radii smaller than ~30 cm, lead-bismuth is a better choice of coolant material than sodium, regarding the proton source efficiency, while for larger target radii the two materials are equally good. The optimal axial proton beam impact was found to be located ~20 cm above the core center. Varying the proton energy, */Ep was found to have a maximum for proton energies between 1200 and 1400 MeV. Increasing the americium content in the fuel decreases * considerably, in particular when the target radius is large.