Electrical characterization of the JET A(sub 2) antenna: Comparison of model with measurements.

摘要

The JET experiment is replacing its previous (Al) antennas with upgraded designs (A2) for its upcoming ''pumped diverter'' operation. These antennas are more directional than the previous two-strap Al antennas when operated as a phased array. The frequency range is 23 to 57 MHz. A full-scale low power ''flat'' mockup was tested at JET; strap lengths were adjusted to give balanced operation with resonance at 42 MHz. A second mockup module, differing only slightly from the original, was subsequently fabricated and both modules were sent to ORNL for additional measurements and to test the operation of the power compensator circuit. There are benefits to using a transmission line model to characterize coupled antenna systems, primarily in the ease of incorporating the antennas into the overall analysis of the transmission, tuning, and matching system. The characteristics of the array under arbitrary phasing are also needed for the design, analysis, and control of the power compensator. There are aspects of the JET A2 antenna geometry that differ considerably from previously modeled cases. Each transmission line feeds two poloidally-stacked straps connected in parallel. The parallel straps present different electrical loads at the match point due to geometrical differences. Currents in one section of the strap influence other sections of the same strap as well as in neighboring straps due to internal inductive coupling. The lengths of the inner and outer straps differ; moreover, the inner straps are fed from ports located behind the outer straps, resulting in increased coupling between the inner and outer straps due to the long feed lines and in greater disparity between the electrical loads presented at the inner and outer feed ports. The present effort is to determine whether a more general coupled transmission line model can characterize the array response with sufficient accuracy for the purpose of design and analysis.