The propagation of plasmoids (neutralized ion beams) in a vacuum transverse magnetic field has been studied in the UCI laboratory for several years. These experiments have confirmed that the plasmoid propagates by the tcg drift in a low p and high p plasmoid beam (0.01 < β < 300) , where p is the ratio of beam kinetic energy to magnetic field energy. The polarization electric field, t, arises from the opposite deflection of the plasmoid ions and electrons, due to the Lorentz force, and allows the plasmoid to propagate undeflected at essentially the initial plasmoid velocity. In these experiments we used plasmoids (150 keV, 5 kA, 50-100 A/cm1, 1 'Is) injected into itifansverie fields of Bt = 0-400G. Anomalously fast penetration of the transverse magnetic field has been observed as in the "Porcupine" experiments. Our 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 p2sma w4Ã‚Â§ gensrated by TiH4 plasma guns fired along Bt to produce a plasma density, n = 10 - 10 cm . , Preliminary results indicate that the beam propagation losses 1.1crease with the background plasma density; compared to vacuum propagation, roughly a 50%,educOon in ion current density was noted 70 cm downstream from the anode for n, - 10 cm '. Principal diagnostics include: magnetically insulated Faraday cups, floating potential probes, calorimeters, microwave interferometer, and thermal-witness paper. Experiments in the near future will use an improved accelerator and transverse-field coil system which allows higher energy, 500 keV, higher current density plasmoids to be studied; this generator will improve the beam uniformity and angular divergence to allow beam propagation for up to 5 meters and permit the study of losses from surface erosion.