{{short description|Fusion energy concepts based on intersecting beams of fusion fuel ions}}
'''Colliding beam fusion''' ('''CBF'''), or '''colliding beam fusion reactor''' ('''CBFR'''), is a class of [[fusion energy]] concepts that are based on two or more intersecting beams of [[fusion power#fuels|fusion fuel]] [[ion]]s that are independently accelerated to fusion energies using a variety of [[particle accelerator]] designs or other means. This contrasts with conventional approaches that heat a bulk fuel to very high temperatures.
CBFR suffer from a series of problems that have limited their ability to be seriously considered as a candidate for [[fusion energy]]. When two ions collide, they are more likely to scatter than to fuse, because of their relative [[Cross section (physics)|cross sections]]. Fusion reactors overcome this problem byusing havinga bulk plasma, where ions can collidehave many thousands of times.chances to But,collide. twoTwo 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. In addition, two beams collidingdo cannotnot stayremain [[Coherence (physics)|coherent]]focussed. They repulse one another. In the 1950's [[Marshall Rosenbluth]] showed that it takes a great deal more energy to hold the beams together, than what can be expected from their fusion reactions. This fundamentally limits two colliding beams for a fusion power plant.
CBFR do offer more efficient ways to heat plasma. by Theydirectly canaccelerating heatindividual plasma, one particle at a time, making them more efficient than bulk fusion reactorsparticles. CBFR plasma is naturally non-thermal which gives it advantages. A number of designs have been attempted to overcoming the shortcomings of CBFR. These include the [[Migma]], MARBLE, MIX and other beam-based concepts. These concepts attempt to overcome the fundamental problems with CBFR by applying radiowaves, bunching the beams together, increasing re-circulation or applying some quantum effects - none of these approaches have succeeded yet.
