Investigations of a technique for build-up of a trapped plasma in a magnetic field by ionization of an atomic beam injected transversely across an arc are reported. The principal attractions of the technique are the high trapping fraction, injection deep into the magnetic field before ionization by the arc and space charge neutralization by photoelectrons produced by arc-ultra-violet radiation. The fraction of an atomic beam ionized by traversal of an energetic carbon arc was experimentally found as high as 21 per cent, depending upon the arc density, multiply-charged ion populations in the arc and beam velocity. Arc-ionization was found compatible with the injection of multi-momentum neutral beams.