Theory of long term gain growth in traveling wave tubes

The properties of the attenuator materials used in traveling wave tubes (TWT) are a major factor in determining the gain of the tube. Carbon is a commonly used attenuator material which is typically deposited on the dielectric rods used to support the helix slow wave structure in TWTs. The deposited carbon layers are susceptible to ion bombardment induced damage during operation and outgassing of the tube, which increases the electrical resistivity of the layer and reduces the rf loss in the tube. The total gain in the tube grows during this process until the bombardment ceases or the layer damage saturates. A simple theory is developed that relates this gain change to the gas evolved in the tube that results in the particle bombardment of the layers during operation. The model accurately describes the observed time dependent behavior of the gain, and the time constant for outgassing the tube is found for several tubes ranging from S to Ka band. The time constant increases significantly for smaller, high frequency tubes with a low conductance and poor pumping speeds. The model shows that stabilization of the gain occurs after several time constants have passed, which may take hundreds to thousands of hours.