The main conclusions of our theoretical studies on the conditions that optimise the efficiency of accelerator driven subcritical reactors, performed as part of the ADSR Project are presented. A new approach based on the possibility to use low energy ion beams instead of protons is proposed. It was shown that the ADSR can represent not only a solution for the problem of nuclear waste, but also an efficient source of energy. With beam intensities above 1016 p/s, energy gain G higher than 15 is achieved and the use of light ion beams, especially 7Li with energy 0.25-0.3 AGeV interacting in Be converter instead of 1-1.5 GeV protons, ensures G>20, and allows a significant reduction of the accelerator length. The analysis of possible experiments meant to check the results of theoretical work about the perspectives of ADSR demonstrates that the most reliable comparison of the energy efficiency of different beams is obtained by measuring the fission distribution inside an extended target. Based on these conclusions, an extended lead target, LETASUR, was designed. The target is designed for the comparative study of the energy efficiency of proton and ion beams and of the influence of the shape of the neutron spectrum on the breeding capability of the ADSR. The target consists in a lead block with dimensions 80x80x150 cm, surrounded by a stainless steel blanket 10 cm thick as substitute for the lead-bismuth coolant. The target has a central hole with radius 10 cm and length 120 cm. Converters from various materials, with total length 110 cm are placed in the central hole, ensuring a beam window with length 10 cm, and holes with length 150 cm in horizontal and vertical directions, at different radii for the placement of the detectors. Expected results predicted by simulation for different beams and converter materials are presented.