Some aspects of circuit power dissipation in high power CW helix traveling-wave tubes, part I: General theory

This paper describes the basic theory underlying calculations of the thermal behavior of helix-type slow wave circuits used in high power traveling-wave tubes. Typically, the helix is supported by means of three ceramic rods in a metal shell with the helix brazed to the rods. At the highest power levels the support rods are also brazed to the tube shell. The results of calculations on specific structures are presented which show the use of different support rod and helix materials, of beam interception, and of brazing the support rods to the shell. Of particular interest is the conclusion that the temperature difference between the hottest and coldest parts of the helix can be significantly larger than the temperature drop across the support rods when using high thermal conductivity ceramic materials brazed to the tube shell.