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Colliding beam fusion , or colliding beam fusion reactor , is a class of fusion energy 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 known as accelerator based fusion or beam-target fusion, but the physics is the same as colliding beams.
CBFRs face a number of problems that have limited their ability to be seriously considered as candidates for fusion energy. When two ions collide, they are more likely to scatter than to fuse. Magnetic fusion energy 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 do not give the ions much time to interact before the beams fly apart. This limits how much fusion energy a beam-beam machine can make.
CBFR does offer 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 anuetronic fuels. A number of designs have sought to address the shortcomings of earlier CBFRs; these include Migma, MARBLE, MIX and other beam-based concepts. These concepts attempt to overcome the fundamental challenges of CBFR by applying radiowaves, bunching the beams together, increasing re-circulation or applying some quantum effects - none of these approaches have succeeded yet.