Electron and plasma flow in a relativistic diode subjected to a crossed magnetic field

The behavior of a cylindrical field emission diode (radial potential ~200kV) is studied in the presence of an externally applied axial magnetic field B ≲ 16kG. Observations made of the space-charge limited electron current (~50kA) compare favorably with theory which takes into account the presence of both the external axial magnetic field and the azimuthal self-magnetic field generated by current flow in the cathode. At magnetic fields exceeding the critical field necessary for cutoff, small but not insignificant current flows persist. The associated microwave emission suggests that this residual current is being driven by an as yet unidentified high frequency instability. Time-resolved measurements of the diode current and voltage lead to a determination of the expansion velocity of the cathode and anode plasmas. It is found that an external magnetic field of ~5kG suffices to stop plasma motion. This is in agreement with magnetohydrodynamic computations for a hydrogen plasma having an initial temperature of a few eV. Spectroscopic measurements of the diode plasma show that hydrogen is indeed the dominant ion species. Stark broadening of the spectral lines yields their number density.