The dependence of the virtual cathode in a Polywell™ on the coil current and background gas pressure

Floating potential measurements have been carried out on a Polywell™ inertial electrostatic confinement device that uses magnetic cusps to trap electrons and establish a virtual cathode. In particular, the dependence of the floating potential on the coil current and background gas pressure was studied. The magnetic field coils were driven by a pulsed current supply and it was found that the virtual cathode could only be established within a narrow range of currents. In addition, it was shown that the magnitude of the floating potential increased with decreasing background gas pressure. It is conjectured that the depth of the virtual cathode and its lifetime are dependent on the magnitude of the injected electron current.