A novel stacked combline antenna was fabricated for driving plasma current in order to control the rotational transform profile in the Large Helical Device. The antenna has ten elements facilitating excitation of fast-wave traveling in the toroidal direction.

Each antenna element has an electrical length of a half-wavelength and is supported at the midpoint from the back plate by a metallic block. Such an antenna has two modes: even and odd. A mixed excitation of these modes will reduce the current drive efficiency. The electrical properties of this antenna were studied in an attempt to find ways of exciting a traveling wave of pure even mode. A matching section was used in combination and proved to be a good measure to improve the directionality over that of a bare combline antenna. It is confirmed in this paper that the fabricated real antenna has fairly good even-mode purity keeping the odd-mode intensity at a tolerable level. An antenna with insulating supports instead of the metallic supports is also examined, and it is found that even-mode purity is further improved. For practical uses, an entire system including impedance matching and power circulation is proposed, and sensitivity to a change in plasma loading is analyzed. Finally, the power-handling capability is discussed including estimations of plasma loading and driven current reaching an assertion of consistency with the experimental goal.