Colliding beam fusion (CBF), or colliding beam fusion reactor (CBFR), is a class of fusion power concepts that are based on two or more intersecting beams of fusion fuel ions that are independently accelerated to fusion energies using a variety of particle accelerator designs or other means. One of the beams may be replaced by a static target, in which case the approach is termed accelerator based fusion or beam-target fusion, but the physics is the same as colliding beams.
CBFRs face several problems that have limited their ability to be seriously considered as candidates for fusion power. When two ions collide, they are more likely to scatter than to fuse. Magnetic confinement fusion reactors overcome this problem using a bulk plasma and confining it for some time so that the ions have many thousands of chances to collide. Two beams colliding give ions little time to interact before the beams fly apart. This limits how much fusion power a beam-beam machine can make.
CBFR offers more efficient ways to provide the activation energy for fusion, by directly accelerating individual particles rather than heating a bulk fuel. The CBFR reactants are naturally non-thermal which gives them advantages, especially that they can directly carry enough energy to overcome the Coulomb barrier of aneutronic fusion fuels. Several designs have sought to address the shortcomings of earlier CBFRs, including Migma, MARBLE, MIX, and other beam-based concepts. These attempt to overcome the fundamental challenges of CBFR by applying radio waves, bunching beams together, increasing recirculation, or applying some quantum effects. None of these approaches have succeeded yet.