The downstream energy and impurity content of the plasma produced by a pulsed crossed‐field source is investigated. With a positive center electrode and an optimum neutral gas loading, the source ionizes and accelerates 7×1017protons containing 140 J of energy to the end of a 2‐m‐long axial magnetic guide field. This energy is distributed equally between axial motion and gyromotion. The dominant acceleration method is the−&mgr;·∇Bforce resulting from a gradient in the axial magnetic field within the source. Impurities, in the form of C2+and O2+ions, account for about 10% of the plasma. Reversing the source polarity results in a lower energy efficiency and the appearance of Fe2+ions in the downstream plasma.