Impact of Thermal State on Backward-Wave Oscillation in Helix TWT Under Operation

The impact of thermal state on backward-wave oscillation (BWO) in a helix traveling-wave tube (TWT) under operation is explored, considering total thermal losses induced by high-frequency electromagnetic field power dissipation on helix, output port, carbon-coating attenuators, support rods, inner wall of the barrel, and the electron beam collision on the helix. By numerically solving the Maxwell equations, the electron motion equation, the heat conduction equation, and the thermal expansion equation, we analyze integrally the beam-wave interaction process and the resulting thermal distribution as well as thermal deformation of both slow wave structure and the output port. The simulation results for a broadband (8-18 GHz) helix TWT show the temperature on the last 20 turns of the helix is the highest, with a peak value of 347 °C at the end of helix. The greatest thermal deformation (0.0151 mm) is located on the inner conductor of the output port. The experimental result for the saturated output power of the TWT prototype shows a BWO power hole at 11 GHz, which is not found in simulation without considering thermal deformation. We find also that, considering the thermal deformation of helix and output port, BWO occurs at 22.4 GHz with the amplitude of 0.0708 V, which is over 5 times of that without considering thermal deformation, and it induces the same power hole as the experimental one at 11 GHz. Thermal deformation is one of the contributing factors for BWO in the helix TWT.