Simulation of rod charging in TWT helix structures

Spontaneous beam defocusing can occur in a helix-type TWT (traveling-wave-tube) when stray beam electrons cause the dielectric helix support rods to become electrically charged. A test configuration was developed that simulates this effect and allows measurement of the resulting surface voltage. The helix voltage was simulated by varying the energy of the arriving focused electrons, and the surface voltage was measured over a range of currents and angles of incidence. Tests were made on beryllia and boron nitride samples. Measurements on boron nitride showed that this surface voltage can be as much as 3 to 4 kV below a 10-kV simulated helix voltage, using a 10-nA beam at normal incidence. A simplified model explaining the observed phenomena led to the measurement of secondary yield, the ratio of backscattered plus secondary electrons to those incident. It was found that the shift in voltage is largely dependent on two properties of the dielectric surface, namely the secondary yield and the surface conductivity. While secondary yield is shown to be energy dependent, the surface conductivity was also found to exhibit semiconductorlike properties, making it highly energy dependent as well.<>