The present work has been developed within the frame of the EFDA task “HCD-08-03-01:
LH4IT, EU contribution to the ITER LHCD Development Plan”
The use of rectangular oversized waveguides in the Main Transmission Lines (MTLs) of the
Lower Hybrid Current Drive (LHCD) system of ITER, requires to investigate the problem of
bends. The high number of involved waveguides (from 24 to 48) must be also taken into
account. Thus, it has to consider not only the best choice in terms of curved framework, but
also the proper allocation of all the waveguides. In this context, the principal specifications
that characterize the design of the bends are: a) to minimize the reflection of the fundamental
TE10 mode; b) to maximize the transmission of the fundamental TE10 mode; c) to minimize
the coupling between the TE10 mode and other spurious modes that propagate at 5 GHz.
This paper presents an overview about the bend options, and it compares the performances of
several frameworks analyzed by using the Finite Element Method (FEM) commercial
software, HFSS®.
First of all, simple circular trajectory curves with different angulations, are considered. Then,
the so called Mitre Bends alternatives are deeply analyzed. These curves are studied by
several authors in the mono-modal configuration, with different techniques but the
propagation in an oversized environment is a topic not much attended in literature.
The only design parameter of the simple circular trajectory bend is the bending radius, so that
the design is not flexible; the Mitre Bend structure is at least more flexible than the previous
one and it is of great interest to study this type of bend to check the possible advantages.
Finally an innovative modified Mitre Bend solution based on a cascade of trapezoidal
elements is proposed