Ion instabilities initiated in the gun of a linear beam microwave tube

A theory is presented which offers an explanation for many of the causes of ion-induced instabilities of the phase and amplitude of the RF output of a solenoid-focused 10-kW continuous-wave coupled cavity traveling-wave tube (TWT). This theory is based on the effects of ions in the electron gun. Some of these ions enter the gun from the interaction region of the TWT. Ions generated in the gun can also have a significant effect on the instabilities. The characteristics (in particular, the timing and shapes) of the waveforms of the instabilities are discussed in detail. The sawtooth waveform often observed is explained by the rates of ion generation in the beam and then the rapid discharge of ions into the gun. When gas pressure in the gun is elevated because of relatively high pressure in the TWT, cathode evaporation, or sputtering, the waveforms can take on a complex double exponential shape that sometimes has a double period. Some of the reasons for these effects are given. The effects that are observed of solenoid current, cathode voltage, RF drive level, and collector voltage on the instabilities are explained by the effects of these parameters on an ion barrier at the collector end of the TWT. When these quantities are such that the barrier is small, the drainage of beam ions to the collector is increased and to the gun is decreased. With reduced ion flow to the gun, gas generation in the gun is reduced, and so the causes of the instabilities are suppressed.