The concept of a direct energy recovery system that applies a varying magnetic field is proposed for a negative-ion-based neutral beam injection system (NNB) to heat a plasma and/or drive a plasma current in a fusion reactor. The output beam energy and power of such an NNB will be ∼1 MeV and ∼ 10 MW/beamline, respectively, and nearly the same amounts of positive- and negative-ion beams remain unneutralized in an NNB by using a gas-neutralizing cell. Therefore, the output of a beam direct converter in an NNB is a bipolar direct current (dc) electric power with close to ±1 MV and several amperes if a conventional electrostatic or magnetostatic field is applied for ion beam separation. However, such high-voltage dc power is difficult to handle at the point of the regeneration of the power back to a commercial electric line because a very high voltage inverter tough enough to withstand occasional sparkdowns at recovery electrodes is required. If residual positive- and negative-ion beams are introduced to two or more electrodes in turn by a varying magnetic field, an alternating current (ac) electric power can be produced directly. The ac voltage can be easily lowered by a stepdown transformer, and a conventional, low-voltage inverter can be used. Such a beam direct converter will greatly reduce the technological difficulty involved in the regeneration of a recovered electric energy. The total efficiency of an NNB will be improved from ∼45 to ∼70% with a beam direct converter.