Thermal-structural reliability assessment of helix TWT interaction circuit using finite element analysis

Finite element analysis (FEA) techniques were used to assess the mechanical performance due to thermal loading of a high power, wide band, helix traveling wave tube´s interaction (slow wave) circuit. A steady state heat transfer analysis was performed using calculated heat dissipations and boundary temperatures that were obtained through data supplied by the TWT´s manufacturer. The resulting temperatures from this analysis were used as loading conditions in a linear static analysis of a smaller model which represents a 12 turn section of the circuit. The failure modes investigated were cracking of the helix tape and fracture of the support rods due to excessive thermal stresses. Cracking of the helix tape would cause an open-circuit to occur while fracture of the support rods could cause small mechanical perturbations in the slow wave structure which may reflect the RF signal. Both cases could possibly lead to electrical failure of the TWT. Static stress analysis of the attenuation section indicated that the stress levels in the helix and support rods due to this particular temperature gradient were within acceptable limits and would not fracture if these components were free of initial cracks or flaws. Stresses through the helix were sufficient to nucleate cracks, however the length of these cracks would be minute and would not affect the useful life of the helix