Experimental data are presented on the production of field-reversed deuterium plasma by a modified coaxial plasma gun. The coaxial gun is constructed with solenoid coils along the inner and outer electrodes that, together with an external guide field solenoid, form a magnetic cusp at the gun muzzle. The net flux inside the inner electrode is arranged to be opposite the external guide field and is the source offield-reversed flux trapped by the plasma. The electrode length is 145 cm, the diameter of the inner (outer) electrode is 15 cm (32 cm). The gun discharge is driven with a 232-/lF 40-k V capacitor bank. Acceleration of plasma through the magnetic cusp at the gun muzzle results in entrainment offield-reversed flux that is detected by magnetic probes 75 cm from the gun muzzle. Field-reversed plasma has been produced for a variety of experimental conditions. In one typical case, the guide magnetic field was Bo = 4.8 kG and the change in axial magnetic field .::iB, normalized to Bo was .::iB,/Bo = -3.1. Total field-reversed flux (poloidal flux) obtained by integrating .::iB, profiles is in the range 2 X 10 3 kG cm 2 • Measurement of the orthogonal field component indicates a sizable toroidal field peaked off axis at r~ 10 cm with a magnitude of roughly one-half the poloidal field component that is measured on magnetic axis. Reconnection of the poloidal field lines has not been established for the data reported in the paper and will be addressed in future experiments which attempt to trap and confine the field-reversed plasma in a magnetic mirror.
Variations of the nature and pressure of the background gas used for neutralizing the charge of a relativistic electron beam have been observed to alter the current waveform of the propagating beam, the time delay before propagation begins, and the voltage and current waveforms of the diode which accelerates the beam. This interaction between the beam region and the diode occurs during the delay phase before beam propagation begins and can be explained by (1) the reflection of electrons back into the diode from a virtual cathode which exists in the beam region during the delay phase and (2) the flow of a significant positive-ion current in the diode during this same period. A theory describing positive-ion flow in the presence of multiply reflected electrons is developed and compared to experimental observations.
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