Challenges in achieving hermetic brazing joints in single-insulator multi-stage depressed collector of travelling wave tubes

Abstract

Travelling wave tube (TWT) being a vacuum electronic device, demands achieving hermetic sealing in ~350 braze and weld joints to attain ultra-high vacuum. One such critical brazing joint exists in its multi-stage depressed collector (MDC) which is generally employed to enhance the TWT efficiency. As an alternative to conventional MDC, single-insulator MDC technology has the capability to offer: (i) increased high-voltage breakdown margins, (ii) reduced diameter/weight and (iii) simpler fabrication. Here, all the electrodes and their corresponding radial feedthrough connections are brazed with a single insulator. Although this technology appears attractive, the major challenge has been in attaining hermetic brazing joints, particularly between the insulator and the radial feedthrough pins. Authors have experimented with different configurations of the feedthrough pins to achieve hermetic braze joints and the details of which have been presented in this paper.