Microwave Attenuation Measurements on Tetrahedral amorphous Carbon coatings for TWT Applications

摘要

In the helix TWT amplifier, a coating of lossy material is done on the helix support rods to absorb reflections and hence to improve the stability of the device. Carbon is a very commonly used material for this type of coating, and is commonly deposited by the pyrolytic deposition method of hydrocarbon cracking. In the present work, coatings of tetrahedrally bonded amorphous carbon (ta-C) have been done on the helix support rods of alumina using the filtered cathodic vacuum arc system. This is a predominantly sp~3 bonded material and a π-electron semiconductor (due to the residual sp~2 bonds). An experimental set up involving a narrow-height wave-guide with a hole was used to measure the attenuation of the coated rods at 6.0GHz frequency. Measurements have been done for the five rods coated with different thicknesses of ta-C. On the basis of the experimental study, it is concluded that ta-C coated rods does not provide significant attenuation and also the thickness of the films that can be achieved with proper adhesion properties is less than that suitable for the significant absorption. Hence, the method and material is not much suitable for this particular application of the microwave absorption.