Gain stability of traveling wave tubes

The long-term gain stability of traveling wave tubes (TWT's) with helix slow-wave structures is examined. A major variable in the gain of TWT's is the stability of the attenuator material that is placed in the tube to damp oscillations and improve input-to-output isolation. Thin carbon layers are often used for this purpose in TWT's and are deposited onto the helix support rods by several different techniques that produce a variability in the material structure and properties. The carbon layers are also susceptible to physical damage due to the environment in the tube during conditioning and long-term operation. Modification of the electrical conductivity of the layer by energetic particle bombardment and chemical erosion decreases the net RF loss in the tube and causes the gain to increase with time. The presence of impurity gases and rapid conditioning procedures produce gain increases due to the lattice damage of the attenuator material of up to 10 dB in a TWT in the first several hundred hours of operation. Properly designed attenuator loss-patterns and minimization of the gas evolution in the TWT causes these effects to saturate and the gain to stabilize quickly. Techniques to ensure long-term stability of these layers are discussed.