Acceleration of a microwave‐energized plasma by expansion in a diverging magnetic field is studied. Results of a theoretical collisionless particle analysis are reviewed. An experimental accelerator is described and its pertinent conditions are considered. Measurements of plasma beam shape and potential are compared with the theoretical predictions. The conclusions are that, while some experimental characteristics (beam shape and potential profile) agree with theory, collisions in the experimental device prevent it from attaining operating conditions predicted by collisionless theory.