The propagation of plasmoids (neutralized ion beams) in a vacuum transverse‐magnetic field has been studied in the University of California, Irvine laboratory for several years [Phys. Fluids24, 739 (1981);25, 730, 2353 (1982);26, 2276 (1983); J. Appl. Phys.64, 73 (1988)]. These experiments have confirmed that the plasmoid propagates by theE×Bdrift in a low beta and high beta plasmoid beam (0.01<&bgr;<300), where &bgr; is the ratio of beam kinetic energy to magnetic field energy. The polarization electric fieldEarises from the opposite deflection of the plasmoid ions and electrons, because of the Lorentz force, and allows the plasmoid to propagate undeflected at essentially the initial plasmoid velocity. In these experiments, plasmoids (150 keV, 5 kA, 50–100 A/cm2, 1 &mgr; sec) were injected into transverse fields ofBt=0–400 G. Anomalously fast penetration of the transverse magnetic field has been observed as in the ‘‘Porcupine’’ experiments [J. Geophys. Res.91, 10,183 (1987)]. The most recent experiments are aimed at studying the plasmoid propagation dynamics and losses in the presence of a background, magnetized plasma which is intended to short the induced polarization electric field and stop the beam. Background plasma was generated by TiH4plasma guns fired alongBtto produce a plasma density,np=1012−1014cm−3. Preliminary results indicate that the beam propagation losses increase with the background plasma density; compared to vacuum propagation, roughly a 50% reduction in ion current density was noted 70 cm downstream from the anode fornp∼1013cm−3. Principal diagnostics include magnetically insulated Faraday cups, floating potential probes, calorimeters, microwave interferometer, and thermal‐witness paper.