Analyses of bombardment traces on the tube head of a relativistic backward wave oscillator

Abstract

In the repetitive experiments of relativistic backward wave oscillators operating at either low or high magnetic field, we found that the spiral traces appeared on the surfaces of the tube heads (entrance to the reflector and slow wave structure). Three possible causes: electrons emitted from the cathode base, cathode plasma, and cathode negative ion, are analyzed. Among them, the cathode negative ion is the most likely factor leading to the traces. The negative ions are generated by electron impact-ionization and ion impact-ionization of molecules that are desorbed from the cathode by the Ohmic heating of explosive electron emission. Due to the larger mass of negative ion, it cannot be restricted effectively by the magnetic field and will basically move along the electric field line. Many negative ions bombard the tube head with a Larmor radius of several centimeters, and the traces are connected to form spiral lines. The viewpoint of cathode negative ion current is further confirmed by the comparative experiments that show that a graphite cathode after being immersed in water for 12 h results in heavier traces, while a larger anode and cathode gap leads to slighter traces. Some suggestions are provided to reduce the production and the effect of cathode negative ion current.