Mid-plane PAM launcher study for KSTAR LHCD system

A prototype of a midplane passive active multijunction (PAM) antenna was fabricated for the lower hybrid current drive system of KSTAR. The antenna includes a PAM launcher, bidirectional couplers, power dividing components, and two RF windows. The RF characteristics of each component, such as the splitting ratio, phase shift, transmission, and reflection, were measured using a network analyzer. A taper adapter and matched load, which can be inserted into the active waveguide of the PAM mouth, were designed and fabricated for the phase shift and power coupling measurement. The maximum error and the standard deviation of measured phase shifts between adjacent active waveguides of the PAM were 6.5 degrees and 2.28 degrees, respectively. The design improvement of the mid-plane PAM launcher is suggested. Multiple round trips of the reflected field in a multijunction provides the advantage of lowering the reflection but, at the same time, it causes the deterioration of directivity and an increase of the electric field in the waveguide. Adding a hybrid splitter at the end of the multijunction structure reduces the reflection by redirecting the reflection to plasma as in multijunction, however, contrary to the multijunction, it does not switch the toroidal direction of phase progression and thus improves the antenna directivity. The directivity of the hybrid PAM increases by more than 10% and the reflection decreases to 1/3-1/100 in comparison with the prototype PAM. (authors)