Radiofrequency (rf) power couplers are a critical and often troublesome component of superconducting accelerator systems. The couplers primary functions are power transfer and preserving the integrity of the cavitys cleanliness. Additional functions include loading the cavity to decrease the effective quality factor and extracting power that would otherwise excite unwanted resonant modes that could disrupt the beam. Conventional power couplers are based on a waveguide transition into a coaxial transmission line that penetrates the side of the beam tube near the resonant cavity. The coupling factor is increased by increasing the penetration of the coupler into the tube. Unfortunately, increasing the penetration produces a distortion in the electromagnetic fields that can push the beam away from the centerline. The on-axis coupler concept was developed as a means of overcoming these and other shortcomings of conventional side-couplers and has significant advantages compared with the current state-of-the-art couplers. The on-axis coupler replaces a portion of the beam tube immediately upstream of the cavity and the beam passes through the hollow center conductor of the coaxial transmission line. The cylindrical geometry does not perturb the beam and the penetration of the center conductor can be adjusted to optimize the coupling factor. In addition, this geometry couples well to the higher-order modes (HOMs) and propagates this energy into a region where can be safely dissipated. Our approach is to convert the conceptual design of a 1500 MHz on-axis coupler into a mechanical design that can be fabricated and tested at JLAB in Phase II. Translating the conceptual design into a mechanical design involves a large number of considerations including: 1) the efficient conversion of waveguide energy into coaxial energy with minimal reflected power, 2) the electrical and thermal design of the ceramic window, 3) the detailed design of the choke-joint isolating the center conductor, 4) issues associated with cryogenic design and thermal management, 5) the mechanical support and motion control of the center conductor, 6) efficient collection and safe dissipation of HOM energy, and 7) careful analysis to eliminate the propensity for electron multipacting. Addressing these issues involves 3D simulations of the radiofrequency circuit, simulations of the thermal properties, and development of a suitable mechanical design to support the cantilevered center conductor to minimize droop while enabling longitudinal movement for adjusting the coupling factor. Successful development of the on-axis coupler would significantly improve the operation of superconducting accelerator cavities. Phase III involves modification of this design to produce couplers suitable for operation at other frequencies
