A large-volume microwave electron cyclotron resonance discharge in athree-dimensional multicusp trap can be used for neutralization of fastnegative ions in the ITER neutral beams injection system. The requirements of a plasma neutralizerare discussed. Experiments on the PNX-U installation, constructed for thestudy of plasma confinement in a neutralizer, are described. Improvedpotential confinement of the plasma at the centre of a trap is found. Theimprovement is caused by the formation of the hot electrons on the plasmaperiphery.