The paper is devoted to the problem of constructing nuclear power reactors based on the synthesis of light elements. This problem is currently subject to assessment under options of thermonuclear reactors operating with high-temperature plasma. In such reactors, plasma is isolated from the walls of the reaction chamber using a magnetic field. Even in advanced designs of thermonuclear reactors, temperature regime is far from the conditions for ensuring optimal interaction cross sections of the source nuclei. Despite many years of work, the task of efficient plasma isolation is difficult to solve. The size of the reaction chambers increases, predicted costs of constructing an effective reactor, even in version with the deuterium-tritium reaction, which has a maximum cross section and minimum energy of interacting nuclei. The article considers a version of the fusion reactor based on the interaction of colliding ion beams in a toroidal magnetic field. This solution provides a cost cutting to trigger the reaction, address the problem of effective retaining of interacting nuclei in the reaction chamber. An example with D–D, D–T and D–3Не reactions shows reactors based on them with low energy losses and small dimensions.