For a fusion-relevant plasma, the required fast neutral energy gets in the range of 1 MeV. With increasing energy, it is increasingly difficult to obtain fast hydrogen atoms starting from precursor beams composed of positive ions. For that reason, recent and future heating neutral beams will be based on negative-ion beams. In the interaction with background gas, it is much easier to detach the extra electron from a negative ion (H− has a binding energy of 0.75 eV and a very large cross-section for electron detachment in this energy range) rather than to attach one electron to a positive ion.