Long term gain stability of traveling wave tubes

Long-term gain changes in traveling wave tubes (TWT) with helix slow-wave structures have been investigated. The major variable in the gain of the TWTs is the stability of the electrical resistivity of the carbon attenuator material deposited onto the helix support rods in the tube to damp oscillations and improve input-to-output isolation. The thin carbon layers used for this purpose are deposited onto the ceramic support rods by several different techniques that produce a variability in the material structure and electrical properties. These carbon layers are 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. It was previously reported that a modification of the carbon resistivity due to lattice damage by energetic ion bombardment in the tube produced gain increases in S-band space-TWTs of up to 10 dB in the first several hundred hours of operation. This undesirable behavior was found to be caused by impurity gases in the tube from poorly cleaned parts. Proper cleaning of these parts eliminated the large gain changes during early life operation of these tubes.