Influence of anode composition on the electrical properties of relativistic electron-beam diodes

Current, voltage, x-ray, and witness plate measurements of the diode characteristics of the Nereus accelerator (150 kV, 50 kA) have shown that the composition of the anode can have a profound effect on the properties of the electron beam. In particular, if the anode is heated to remove absorbed gases and volatile impurities, the diode impedance holds up longer than in the case of an anode having an unprepared surface or one that has been covered with an easily vaporized low-Z substance (which leads to very rapid diode shorting). The fact that the anode surface can influence the electron current less than 10 ns into the beam pulse, before space-charge-limited electron flow has been established, indicates that ions, plasmas, or photons are generated at the anode and cross the diode gap before the electron beam has deposited more than 1 cal/cm2 into the anode.