Thermal limitation of the mean power rating of coaxial transmission lines for ICRH

A limitation of the mean power rating of a coaxial transmission line arises from the coupling elements, which connect adjacent sections of the inner conductor by means of contact springs that are subjected to thermal degradation. This paper is focused on the design applied in a commercially available transmission-line coupling element. The main scope is to demonstrate the possibility to extend the working range of its spring contacts up to 200 deg. C. Presently this component is extensively used in several transmission lines of fusion experiments working with short pulses and operating with a maximum temperature of 150 deg. C at the inner conductor. Test components are successively exposed to a well controlled temperature of 200 deg. C over certain time periods and are characterized afterwards with regard to the resilient forces of the contact lamellae with an accurate experimental setup. The weakening of the spring force in dependence of the cumulative annealing time is studied and the remaining spring force is compared to the minimum required force to ensure proper electrical contact, which is also determined experimentally within this work.