Aperture impedance matching for lower hybrid current drive launchers

Circulating power within a multijunction launcher is both the system’s greatest strength and a fundamental weakness. Although destructive interference of power reflected from the plasma interface at the multijunction results in low return loss to the RF source, the resulting standing wave within the launcher increases peak electric field, thereby placing fundamental limitations on maximum power handling due to the onset of multipactor breakdown. Passive active multijunctions improve impedance matching at low edge densities but suffer reduction in power density and non-optimal directivity due to the sparse array theorem as half of the waveguides are passive elements. An alternative technique of impedance matching a fully active lower hybrid grill to low edge density conditions with internal matching structures within the waveguide apertures is presented. This method of impedance matching reduces circulating power, sensitivity to edge density conditions and ponderomotive effects, and improves both power density and array directivity.