Design of a high power water load for LHCD system of SST-1 tokamak

Water loads are traditionally used in lower hybrid current drive (LHCD) system to condition klystrons at full power or used in conjunction with a circulator to protect the tube from high VSWR. Various designs of water load are available in the literature. However the availability of indigenous design of such water loads is limited. This paper presents a novel indigenous structure of S Band water load capable of absorbing 250 kW CW power at 3.7 GHz for LHCD installed on SST1 machine. Further this paper also presents generalized equations through which the design can be reproduced at different frequencies without much effort. As water is an excellent absorber at S band (loss tangent=0.0036), it is used as an absorber in this design. Conventional designs of water load usually use a quarter wave transformer to match the impedance of water to the air filled waveguide. However this matching along with the issue of bonding the ceramic to the waveguide is often critical and if not done properly can lead to arcing and breakdown. In this design, water flowing in Teflon/polypropylene/glass channels arranged in a tapered configuration is used to absorb the incident microwave power. The tapering provides impedance matching. The flow is regulated and maintained at a rate so as to avoid localized boiling of water. The water load is designed and tested in COMSOL and ANSYS and exhibits a VSWR of < 1.1 and a bandwidth of 10%. (author)